Space Tech

equatorial geomagnetic

field at Earth’s surface (14). During geomagnetic

storms, charged particles injected into the Van Allen

radiation belts from the outer magnetosphere cause a

sharp increase in the ring current, with a corresponding

decrease in Earth’s equatorial magnetic field (18). The

injected particles precipitate out of the magnetosphere

into the upper atmosphere at high latitudes, causing

auroral activity and interference with electromagnetic

communications.

GALACTIC COSMIC RADIATION

Composition and Sources

Earth is continuously irradiated from all directions by

high-energy charged particles of GCR. GCR particles lose

kinetic energy principally by ejecting orbital electrons

from the atoms with which they interact. The particle

intensity in free space varies from 1.5 particles/(cm2 ·

second) near sunspot maximum to about 4 particles/(cm2 ·

second) near sunspot minimum. All the natural elements

in the periodic table are present in GCR. Outside the

geomagnetic field, the composition of the GCR is 98%

hydrogen nuclei (protons) plus heavier nuclei stripped

of orbital electrons and 2% electrons and positrons (10).

The nuclei component of the radiation, in the energy

range 100 MeV to 10 GeV per nucleon (where intensity

is highest), consists of 87% protons, 12% helium nuclei

(alpha particles), and 1% nuclei with an atomic number

(Z) higher than helium. GCR nuclei with a Z higher than

helium are called HZE particles (high Z and high energy)

(27). Iron-56 is an important HZE particle because of

its significant contribution to the GCR dose and its high

LET (described in the section entitled Expressing Amounts

of Ionizing Radiation) (27). HZE particles are a concern

for space travelers because the particles can injure the

central nervous system (26). HZE particles are not a

concern at airline flight altitudes; they are broken apart

at higher altitudes in the atmosphere.

The principle source of GCR in our galaxy is stellar

material and surrounding interstellar gas, accelerated by

exploding stars (supernovae).

Star formation is initiated by the gravitational attraction

of particles from a molecular cloud, which is a dense

concentration of cold gas (mostly hydrogen) and dust in

space. As the mass of the core of a future star (protostar)

increases, the increased pressure fuses hydrogen into

helium, releasing thermal energy. The thermal energy

increases the temperature of the gases, and they expand

and create an outward pressure that balances the gravitational

forces. After the hydrogen is exhausted the star

begins to contract. What happens next depends on the

mass of the star. There are two ways a star can become

a supernova.

A star at least 8 times the mass of the Sun can evolve directly

into a supernova by the following procedure. As the star

contracts, the pressure in its core created by gravitational forces

increases. When the pressure becomes great enough, fusion of

helium to carbon occurs and thermal energy is released. This

raises the temperature of the gases, temporarily halting the

collapse of the star. When the helium runs out, the star begins

to collapse again, and increasingly heavier elements are fused

but with progressively less thermal energy produced. Fusion

processes culminate with production of nickel-56, which

8

produces iron-56 by radioactive decay. Iron and elements of

higher atomic weight do not emit heat on formation. When

nuclear sources are almost exhausted and outward pressure

by the gases is not sufficient to balance gravitational forces,

the core of the star suddenly collapses. This is followed by

collapse of the outer layers of the star, and the star explodes

as a supernova.

A supernova can also evolve from a white dwarf, which

is what remains of a low-mass star after fusion has stopped

and the star has begun to cool. If the white dwarf and

another star orbit each other and the white dwarf accretes

enough matter from the other star to start uncontrolled

fusion of carbon and oxygen, it will explode as a supernova.

A white dwarf may also accumulate enough mass

and explode as a supernova by colliding with another star.

A supernova shines for weeks to months. The star’s

luminocity increases by as much as 20 magnitudes. Li,

Chornock, Leaman, et al. (29) estimated that there are

about 2.8 supernovae in the Milky Way galaxy every

hundred years. This estimate is based on a telescopic

survey of more than 1,000 supernovae in nearby galaxies.

Shock waves of a supernova surge out through space,

occasionally initiating new star formations. These shock

waves are thought to be the primary accelerators of most

GCR particles. However, supernovae are not thought to

be powerful enough to generate the highest-energy GCR

particles. Such particles are believed to come from nearby

galaxies with central black holes. The black holes eject

jets of plasma (a gas with a portion of its components

ionized) into intergalactic space (30).

Hydrogen and helium were probably the most abundant

elements in the early universe, with most of the

rest made inside stars and during supernovae explosions.

Galactic Cosmic Radiation in Earth’s Atmosphere

GCR levels in Earth’s atmosphere vary with latitude

(because of Earth’s magnetic properties), with altitude

(because of Earth’s atmosphere), and with solar activity.

Figure 2 (calculated with CARI-6P [23]) shows effective

dose rates in the atmosphere during solar activity cycles

from January 1958 through December 2008, at several

altitudes at geographic coordinates 0o N, 20o E (where

geographic and geomagnetic equators overlap), and at

80o N, 20o E.

At the high-latitude location, cyclic variation in the

radiation level is evident at 30 kft, which indicates that

low-energy GCR particles reach Earth’s atmosphere

and penetrate down to this altitude. At the equator, the

geomagnetic field repels most low-energy GCR particles

that might otherwise enter the atmosphere during low

solar activity. Consequently, there is little variation in

dose rate at equatorial altitudes.

With increasing depth in the atmosphere, the dose

rate from particles that enter the atmosphere (primary

particles) decreases, whereas the dose rate from particles

created in the atmosphere (secondary particles) increases.

This complex situation results in a maximum dose rate

at the so-called Pfotzer maximum, discovered in the mid-

1930s by the German scientist for whom it is named.

From the altitude of the Pfotzer maximum to the Earth’s

Figure 2. Effective dose rates from GCR in the atmosphere during

several solar activity cycles.

9

surface, the dose rate decreases continuously. Using effective

dose rates calculated by LUINNCRP (31): at the

equator, during both the July 1989 solar maximum and

the May 1996 solar minimum, the Pfotzer maximum

was at 56 kft; at the high-latitude location, the altitude

of the Pfotzer maximum varied with solar activity and

was 68 kft during the July 1989 solar maximum and

77 kft during the May 1996 solar minimum.

The next two figures (calculated with CARI-6P [23])

show the proportional contribution to the mean effective

dose rate of each of the principle types of cosmic radiation

particles as related to altitude, at the geographic equator

(Figure 3) and at the high-latitude location (Figure 4),

from January 1958 through December 2008.

The equatorial and high-latitude data both show

that at 20-40 kft, where subsonic air carrier aircraft

commonly cruise, 88-97% of the mean effective dose

rate was from neutrons, protons, and electromagnetic

showers (electrons, positrons, and photons). At sea level,

at the equator, and at the high latitude, more than 67%

of the mean effective dose rate was from muons.

Figure 3. At the geographic equator (0o, 20o E), contribution of the

principal GCR particles to the mean effective dose rate as related to

altitude.

Figure 4. At a high geographic latitude (80o N, 20o E), contribution of

the principal GCR particles to the mean effective dose rate as related

to altitude.

10

THE SUN AND ITS EMISSIONS

Layers of the Sun’s Atmosphere

The photosphere is the Sun’s atmospheric layer visible

in white light. Above the photosphere is the chromosphere,

a major source of UV radiation. Above the chromosphere

is the corona, a region of very hot gas.

Sunspots

A sunspot is an area on the photosphere that is seen

as a dark spot in contrast with its surroundings. Sunspots

appear dark because the area is cooler than the

surrounding photosphere.

Sunspots occur where areas of the Sun’s magnetic field

loop up from the surface of the Sun and disrupt convection

of hot gases from below. The sunspot number is

an index of solar activity which is based on the number

of sunspots and groups of sunspots visible on the Sun

(19). For the last 280 years, the sunspot cycle has been

about 11 years, with solar activity increasing for about

4.8 years and decreasing for about 6.2 years (19).

The Sun initiates disturbances in the geomagnetic

field, and disturbances are more common and more

intense when sunspots can be seen. However, intense

geomagnetic disturbances have occurred when no sunspots

were observed, and high sunspot numbers have

accompanied a low level of geomagnetic disturbances

(32).

Solar Wind

The solar wind boils continuously off the Sun,

producing an interplanetary magnetic field. The mass

of the solar wind is about 80% protons, 18% alpha

particles, and traces of heavier charged particles (33).

Discontinuities in the magnetic field carried by the solar

wind (called scattering centers) deflect away from Earth

some low-energy GCR particles that might otherwise

enter the atmosphere (21). During the active phase of

the Sun’s activity cycle, the solar wind is at its most

intense, and this reduces GCR in Earth’s atmosphere to

its lowest levels. At its highest intensity, the solar wind

can adversely affect telecommunication systems, but

particle energies are too low to increase radiation levels

at aircraft flight altitudes (34). Any environmental effect

from increased intensity of charged particles from the

Sun is called space weather.

Coronal Mass Ejection and Solar Flare

Occasionally a magnetic disturbance in the Sun results

in an explosive ejection of huge amounts of matter and

embedded magnetic fields from the solar corona, and

this is called a coronal mass ejection (CME). A CME usually

originates in a magnetically active region around a

visible sunspot group. A large CME can blast billions of

tons of charged particles into space at speeds of 1,700

km per second (33). When a fast CME plows through

the slower moving solar wind, it produces interplanetary

shock waves, which are responsible for showers of highenergy

particles impacting Earth’s atmosphere (19). These

particles impact the atmosphere from all directions (flying

at night is no benefit), and they interact with air atoms

in the same way as GCR particles.

The term solar flare refers to the electromagnetic energy

and particles released suddenly from a relatively small

volume of the Sun (19). CMEs and solar flares have often

been associated (19), but the relationship between them,

if any, is not known. The amount of energy and matter

released during a solar flare is relatively small, compared

to the amount released during a CME. Most CMEs and

solar flares are not directed at the Earth (33). Particles

associated with a solar flare may not have enough energy

to increase radiation levels in Earth’s atmosphere. Photons

from a solar flare begin arriving about 8 minutes after

departing the Sun. The most energetic charged particles

from a CME or from a solar flare reach Earth in 15-20

minutes. The difference in arrival time between photons

and charged particles is due to the longer path the particles

must follow to reach Earth.

Solar Proton Event

A surge of subatomic particles from the Sun is defined

as a solar proton event (SPE) by the Space Weather

Prediction Center of the U.S. National Oceanic and

Atmospheric Administration (NOAA) if instruments on

a Geosynchronous Operational Environmental Satellite

(GOES) measure in three consecutive 5-minute periods

an average solar proton flux ≥10 particles/(cm2 · steradian

· second) with all proton energies >10 MeV (35).

A particle surge that meets these characteristics is most

likely the result of a CME.

RECOMMENDED IONIZING

RADIATION DOSE LIMITS

U.S. Federal Aviation Administration

The FAA accepts the most recent recommendations

of the American Conference of Government Industrial

Hygienists (ACGIH) (36, 37). For a non-pregnant air

carrier crewmember, the FAA-recommended limit for

exposure to ionizing radiation is a 5-year average of 20

mSv per year, with no more than 50 mSv in a single

year. For a pregnant air carrier crewmember, starting

when she reports her pregnancy to management, the

FAA-recommended ionizing radiation exposure limits

for the conceptus are 0.5 mSv in any month and 1 mSv

during the remainder of the pregnancy.

11

Radiation exposure as part of a medical or dental procedure

is not subject to recommended limits. However,

any radiation exposure of a pregnant woman should

consider the conceptus.

International Commission on Radiological

Protection

For a non-pregnant, occupationally exposed person, the

ICRP 2007-recommended limit for exposure to ionizing

radiation is a 5-year average of 20 mSv per year (100 mSv

in 5 years), with no more than 50 mSv in a single year.

Annual equivalent dose limits are recommended for the

lens of the eye, 150 mSv; for skin, 500 mSv (averaged

over a 1 cm2 area); each hand, 500 mSv; and each foot,

500 mSv (11).

For a pregnant worker, starting when she reports

her pregnancy to management, the working conditions

should be such that the additional dose to the conceptus

would not exceed about 1 mSv during the remainder of

the pregnancy (11).

Radiation exposure as part of a medical procedure

is not subject to recommended limits. However, before

any pregnant woman is irradiated, the exposure of the

conceptus should be considered.

National Council on Radiation Protection and

Measurements

For a non-pregnant, occupationally exposed person,

the NCRP-recommended limit for exposure to ionizing

radiation is 50 mSv per year, with no more than 10 times

the person’s age in mSv cumulative dose. Annual equivalent

dose limits are recommended for the lens of the eye, 150

mSv; for skin, 500 mSv (averaged over a 1 cm2 area); for

each hand, 500 mSv; and for each foot, 500 mSv (24).

For a pregnant worker, starting when she reports

her pregnancy to management, the working conditions

should be such that the dose to the conceptus does not

exceed 0.5 mSv per month during the remainder of the

pregnancy (24).

For space missions in low-Earth orbit, NCRP recommends

a career limit excess-lifetime-risk of fatal cancer

of 3% and uses sex and age-dependent risk coefficients

(10). NASA follows NCRP recommendations.

The risk coefficients for chronic exposures to low-

LET ionizing radiation (lifetime percent increased risk

of fatal cancer per 100 mSv) are (10): for males (% per

100 mSv) = 0.895 + (-0.0177 x age) + (0.0000750 x age2)

for females (% per 100 mSv) = 1.60 + (-0.0339 x age) +

(0.000172 x age2).

Thus, for a male at least 25 years old but not yet 26,

the risk coefficient would be:

0.895 + (-0.0177 x 25) + (0.0000750 x 252) = 0.499

% per 100 mSv 0.499 % per 100 mSv = 0.00499 per

100 mSv.

If exposed to 32 mSv during that year, his increased

lifetime risk of fatal cancer would be:

32 mSv x 0.00499 per 100 mSv = 1.60 in 1,000 =

160 in 100,000 = 1,600 in 1,000,000.

For acute or high-LET radiation exposure, the risk is

doubled (3,200 in 1,000,000).

Table 8 shows recommended organ dose limits for

space missions in low-Earth orbit.

European Union

According to a Directive issued by the Commission

of the European Communities (39) and an associated

document regarding its implementation (40), assessments

of occupational radiation exposure should be made for

crewmembers likely to be occupationally exposed to more

than 1 mSv in a year. These assessments should include

radiation received on the job from natural sources. Work

schedules for crewmembers should be arranged to keep

annual exposures below 6 mSv. For workers whose annual

exposure exceeds 6 mSv, medical surveillance and record

keeping is recommended. For a pregnant crewmember,

starting when she reports her pregnancy to management,

her work schedule should be such that the equivalent

dose to the child to be born will be as low as reasonably

achievable and unlikely to exceed 1 mSv, either for the

remainder of the pregnancy or for the whole pregnancy,

according to how Article 10 is implemented in national

legislation.

Table 8. Organ dose limits in Sv a recommended by the

NCRP for space missions in low-Earth orbit (38).

Bone Marrow Eye Skin

career Men: 1.5 - 4 x (200 + 7.5 x (age - 30))

Women: 1 - 3 x (200 + 7.5 x (age - 38))

4.0

4.0

6.0

6.0

1 year 0.50 2.0 3.0

30 days 0.25 1.0 1.5

a NASA uses the same numerical limits for dose equivalent in Sv and gray

equivalent in Gy-Eq (10).

12

IONIZING RADIATION EXPOSURE

DURING AIR AND SPACE TRAVEL

Radiation Doses Received During Air Travel

The effective dose of GCR to an adult on an aircraft

flight at altitudes <60 kft can be estimated using a computer

program such as CARI-6, which can be run on

the Internet (jag.cami.jccbi.gov/cariprofile.asp). Two

DOS versions of CARI-6 (CARI-6 and CARI-6M) may

be downloaded from the FAA’s Radiobiology Research

Team Web pages (www.faa.gov/data_research/research/

med_humanfacs/aeromedical/radiobiology/). CARI-6P

and CARI-6PM are research-oriented versions available on

request from the authors. CARI-6 and CARI-6P assume

a geodesic flight path; CARI-6M and CARI-6PM follow

user-entered waypoints. The “P” designation indicates

that in the output, the doses from the most important

particles are listed individually, and the effective dose is

reported two ways for protons, using wR

= 2 (current

ICRP and NCRP recommendation) (10, 11) and wR

= 5

(old ICRP recommendation) (12). Information required

for CARI-6 and CARI-6P:

(1) Date of flight;

(2) ICAO code of origin airport;

(3) ICAO code of destination airport;

(4) Number of en route altitudes;

(5) Minutes climbing to first en route altitude;

(6) First en route altitude, in feet;

(7) Minutes cruising at first en route altitude;

(8) Additional en route altitudes (if any), in feet;

(9) For each en route altitude (if any) after the first:

Minutes changing to the en route altitude plus

minutes cruising at the en route altitude;

(10) Minutes descending to destination airport.

Table 9 shows the effective dose (sieverts) from GCR

received on single nonstop one-way air carrier flights

calculated with CARI-6P, using wR

(protons) = 2. Data

in Table 9 and risk coefficients in Tables 10-12 can be

used to estimate cancer risk and genetic effects from

GCR exposure during a career of flying.

From 1 January 1986 through 1 January 2008, there

were 170 solar proton events. During 169 of these events,

Copeland, Sauer, Duke, et al. (22) estimated doses of SCR

and GCR received on simulated high-latitude aircraft

flights. To estimate SCR, they used GOES measurements,

near sea-level neutron-monitor data, and Monte

Carlo calculations. For GCR, they used CARI-6P, with

wR

(protons) = 2.

For each event, they calculated the highest combined

GCR + mean SCR dose received by an adult and the

highest received by a <3-month-old conceptus, in 1, 3,

5, and 10 hours, at altitudes of 30, 40, 50, and 60 kft.

Thus, for each of the 169 events, there were 16 adult

categories and 16 conceptus categories. The highest

value for each of the 32 categories (shown in Table 13)

occurred either during the 29 September 1989 event or

the 16 January 2005 event.

The GCR + mean SCR dose to an adult was always

less than 20 mSv, the annual occupational limit (5-year

average) recommended by the ICRP (12) and the FAA

(36). During 10 of the 169 events, in one or more of the

16 categories for the conceptus, the GCR + mean SCR

dose exceeded the 0.5 mSv monthly limit recommended

by the NCRP (24) and the FAA (36).

The most severe solar proton event for which a record

is available occurred in 1859 (44). Magnetic compasses

went haywire, telegraph systems failed, and auroras were

seen as far south as the Caribbean in the Americas. In

the current high-technology environment, the high levels

of SCR could have damaged electronics on spacecraft,

disabled radio communications, and caused widespread

electrical blackouts. Intense solar activity can heat the

atmosphere, causing it to heat and expand. For a satellite

in low-Earth orbit, such an atmospheric change would

increase the drag on the satellite, causing it to slow

down and change its orbit to lower altitudes. However,

even during the 1859 event, the radiation dose to space

travelers in low-Earth orbit would probably not have

been life-threatening (44).

13

Table 9. Flight data and effective dose from low-LET radiation and from high-LET radiation

(calculated with CARI-6Pa) on single nonstop one-way air carrier flights.

1 2 3 4 5 6

Highest In-air Block b Effective dose (mSv)

Origin – Destination alt.(kft) hours hours low-LET high-LET

Solar Minimum (May 1996)

Houston TX – Austin TX 20 0.5 0.6 0.000058 0.0001

Miami FL – Tampa FL 24 0.6 0.8 0.00012 0.00024

St. Louis MO – Tulsa OK 35 0.9 1.1 0.0005 0.0012

London UK – Los Angeles CA 39 10.5 11. 0.017 0.046

Chicago IL – London UK 37 7.3 7.7 0.012 0.032

London UK – Chicago IL 39 7.8 8.3 0.013 0.036

Athens Greece – New York NY 41 9.4 9.7 0.019 0.041

Average Solar Activity in 51 years (1958-2008)

Houston TX – Austin TX 20 0.5 0.6 0.000055 0.000094

Miami FL – Tampa FL 24 0.6 0.8 0.00012 0.00023

St. Louis MO – Tulsa OK 35 0.9 1.1 0.00047 0.001

London UK – Los Angeles CA 39 10.5 11. 0.016 0.039

Chicago IL – London UK 37 7.3 7.7 0.011 0.027

London UK – Chicago IL 39 7.8 8.3 0.012 0.030

Athens Greece – New York NY 41 9.4 9.7 0.017 0.036

Solar Maximum (July 1989)

Houston TX – Austin TX 20 0.5 0.6 0.000053 0.000088

Miami FL – Tampa FL 24 0.6 0.8 0.00011 0.00021

St. Louis MO – Tulsa OK 35 0.9 1.1 0.00044 0.00091

London UK – Los Angeles CA 39 10.5 11. 0.015 0.032

Chicago IL – London UK 37 7.3 7.7 0.010 0.022

London UK – Chicago IL 39 7.8 8.3 0.011 0.025

Athens Greece – New York NY 41 9.4 9.7 0.016 0.031

a CARI-6P output is the effective dose from muons, electromagnetic showers (photons, electrons, and positrons), protons, pions,

and neutrons. For risk estimates, protons, charged pions, and neutrons were considered high-LET radiation; muons and

electromagnetic showers were considered low-LET radiation. Protons, usually considered low-LET radiation, are more damaging

than other low-LET radiation, causing damage similar to that produced by heavy ions (41). Therefore, we included protons with

high-LET radiation, which may overestimate risk.

b Block hours start before takeoff from the origin airport, after the aircraft door is closed and the brake is released. Block hours

end after landing at the destination airport, the last time the brake is set before the aircraft door is opened.

14

Table 9 continued

Example 1 A crewmember worked 700 block hours per year for 25 years flying between Athens, Greece and New York, NY

(block hours defined in footnote b). Assuming average solar activity for each flight, what is the crewmember's increased lifetime

risk of fatal cancer? Flight data are in Table 9. Risk coefficients for fatal cancer are in Table 10.

Risk from low-LET radiation (doses in column 5):

700 block hours per year x 25 years = 17,500 block hours in 25 years

0.017 mSv per flight / 9.7 block hours per flight = 0.00175258 mSv per block hour

0.00175258 mSv per block hour x 17,500 block hours in 25 years = 30.67015 mSv in 25 years

fatal cancer risk = 3.1 in 100,000 per mSv x 30.67015 mSv in 25 years = 95.077465 in 100,000 = 95.08 in 100,000

Risk from high-LET radiation (doses in column 6):

0.036 mSv per flight / 9.7 block hours per flight = 0.00371134 mSv per block hour

0.00371134 mSv per block hour x 17,500 block hours in 25 years = 64.94845 mSv in 25 years

fatal cancer risk = 6.3 in 100,000 per mSv x 64.94845 mSv = 409.175235 in 100,000 = 409.18 in 100,000

Risk from low-LET radiation + high-LET radiation = 95.08 in 100,000 + 409.18 in 100,000 = 504.28 in 100,000 = 5 in 1,000

****

Example 2 Before conceiving children, one parent worked 700 block hours a year for 5 years flying between Athens, Greece,

and New York, NY, and the other parent did not fly. Assuming average solar activity for each flight, what is the risk of genetic

defects in the first two successive generations? Flight data are in Table 9. Risk coefficients for genetic defects are in Table 11.

Risk from low-LET radiation (doses in column 5):

700 block hours per year x 5 years = 3,500 block hours

0.017 mSv per flight / 9.7 block hours per flight = 0.00175 mSv per block hour

0.00175 mSv per block hour x 3,500 block hours in 5 years = 6.125 mSv in 5 years

genetic risk = 1.2 in 1,000,000 per mSv x 6.125 mSv in 5 years = 7.35 in 1,000,000

Risk from high-LET radiation (doses in column 6):

0.036 mSv per flight / 9.7 block hours per flight = 0.0037 mSv per block hour

0.0037 mSv per block hour x 3,500 block hours in 5 years = 12.95 mSv in 5 years

genetic risk = 2.4 in 1,000,000 per mSv x 12.95 mSv = 31.08 in 1,000,000

Risk from low-LET radiation + high-LET radiation = 7.35 + 31.08 = 38.43 in 1,000,000 = 4 in 100,000

Table 10. Increased lifetime risk of fatal cancer from

ionizing radiation (11).

Whole population a Age group 18-64 years

---- 8.0 in 100,000 per mSv 6.3 in 100,000 per mSv

DDREF = 2 4.0 in 100,000 per mSv 3.1 in 100,000 per mSv

a In the U.S in 1998, cancer caused 23.2% of deaths among persons of all ages and

10.6% of deaths among children ages 1-14 years (42).

Table 11. Increased risk of genetic effects in first two

successive generations (children and grandchildren), from

ionizing radiation exposure prior to conception (11).a

Whole population a Age group 18-64 years

---- 0.4 in 100,000 per mSv 2.4 in 1,000,000 per mSv

DDREF = 2 0.2 in 100,000 per mSv 1.2 in 1,000,000 per mSv

a In the general population, 2-3% of live-born children have harmful abnormalities

at birth (43).

15

Table 12. Increased lifetime risk of cancer (non-fatal or

fatal) from ionizing radiation (11).

Whole population a Age group 18-64 years

---- 34 in 100,000 per mSv 23 in 100,000 per mSv

DDREF = 2 17 in 100,000 per mSv 12 in 100,000 per mSv

Table 13. Highest GCR + mean SCR effective doses to an adult

and highest GCR + mean SCR equivalent doses to a

<3-month-old conceptus, on simulated high-latitude aircraft

flights during 169 solar proton events.

Effective dose to an adult (Equivalent dose to Altitude a conceptus) (mSv)

(kft) 1 h a 3 h 5 h 10 h

29 September 1989 event

30 --- 0.098 (0.11) 0.14 (0.16) 0.20 (0.22)

40 --- 0.27 (0.30) 0.39 (0.43) 0.57 (0.63)

50 --- --- 0.84 (0.90) 1.3 (1.4)

60 --- --- --- 2.6 (2.4)

16 January 2005 event

30 0.048 (0.050) --- --- ---

40 0.16 (0.17) --- --- ---

50 0.42 (0.44) 0.68 (0.72) --- ---

60 0.90 (0.83) 1.5 (1.4) 1.8 (1.7) ---

a 1, 3, 5, or 10 hours of continuous exposure

Radiation Doses Received During Space Travel

Commercial space flights, in the next decade or so,

are expected to be limited to suborbital flights and trips

to the ISS, with suborbital flights the more common.

Travelers on suborbital flights are exposed to trapped

radiation, GCR, and SCR.

It is expected that the suborbital flights will be primarily

like that of SpaceShipOne, a rocket carried aloft

and launched at high altitude from a carrier aircraft, or

else something like the early ballistic flights of the Mercury

program. These kinds of flights usually spend ≤15

minutes at high altitudes and in space. GCR should be

the primary source of ionizing radiation exposure. The

flight path can be chosen to avoid the trapped radiation

of the South Atlantic Anomaly and the timing selected

to avoid SCR hazards. Doses to vehicle occupants are

expected to be quite low during the rocket-powered and

freefall portions of the flight. For example, the effective

dose (calculated with LUINNCRP [31] from the flight

profile) to Alan Shepard on the Mercury 3 mission was

0.00031 mSv.

For tourists going to the ISS, the trip is typically 7-10

days, and doses will be considerably higher than on

suborbital flights. The trapped radiation of the South

Atlantic Anomaly cannot be avoided. Also, the orbit of

the ISS is at a high inclination, so for part of each orbit,

it is outside latitudes well-protected by Earth’s magnetic

field, and ISS occupants receive little protection from

GCR. While of low probability, SCR is also a possible

radiation hazard.

An example of a trip to the ISS orbit is STS-91, a

9.8-day space shuttle mission, during which the effective

dose equivalent to the astronauts, based on in-flight

measurements, was 4.1 mSv (45).

HEALTH EFFECTS OF

IONIZING RADIATION

Deterministic Effects (Non-Stochastic Effects,

Tissue Reactions)

Most tissues of the body can lose a substantial number

of cells without an observable decrease in tissue or organ

function. However, if the number of cells lost is sufficiently

large, harm will be observed. Harm from ionizing radiation

is called deterministic if the harm increases with radiation

dose above a threshold dose (12). The threshold dose is

the dose below which no harm is observed, or the harm

is not clinically significant. For most deterministic effects

16

from low-LET radiation, the threshold dose is higher if

the exposure time required to reach the dose is more than

a few hours (46). Deterministic effects can occur soon

(sometimes minutes) after radiation exposure if the dose

is sufficiently high and delivered at a high rate.

The effect of radiation on the hematopoietic system

is largely dependent on damage to the bone marrow.

The bone marrow contains three cell renewal systems:

erythropoietic, myelopoietic, and thrombopoietic. Normally,

a steady-state condition exists between production

of new cells by the bone marrow and the number of

mature, functional cells in the circulating blood. The

erythropoietic system produces mature erythrocytes (red

blood cells) with a lifespan of approximately 120 days.

Red blood cells carry oxygen from the lungs to the rest

of the body and carry away some of the carbon dioxide.

The myelopoietic system produces mature leukocytes

(white blood cells) with a lifespan of approximately 8

days. White blood cells are important in combating

infection. The thrombopoietic system produces platelets

with a lifespan of 8-9 days. If the endothelial lining of a

blood vessel is traumatized, platelets are stimulated to go

to the site of injury and form a plug, which helps reduce

blood loss. Platelet deficiency causes one to bruise easily

and even hemorrhage.

The intestines are highly vulnerable to radiation

damage. The mucosal layer that lines the intestines is

completely renovated every 72 hours. Injury of the microvasculature

of the mucosa and submucosa, together

with epithelial-cell denudation, results in hemorrhage

and marked loss of fluids and electrolytes. These events

normally occur within 1-2 weeks after irradiation.

Early deterministic effects of ionizing radiation are

called Acute Radiation Syndrome (ARS). Symptoms during

the first stage of this illness (nausea, fatigue, vomiting,

and diarrhea) occur within minutes to days after exposure;

they may come and go for several days, unless the

dose is totally incapacitating or lethal before then. The

irradiated individual usually looks and feels healthy for

short periods of time. During the next stage, there may

be a loss of appetite, nausea, fatigue, vomiting, diarrhea,

fever, seizures, and coma. This seriously-ill stage may last

from a few hours to several months and end with death

from infection and/or internal bleeding. The symptoms

and time to onset of symptoms, and their severity and

duration, generally depend on the absorbed dose. There

are significant differences among individuals. The cause

of death is usually from infections and internal bleeding

because of bone marrow damage. Some late deterministic

effects are cataracts and a decrease in germ cells.

To date, cataract formation is the only deterministic

effect associated with exposure to ionizing radiation in

space (26). Excess cataracts have been seen in former

astronauts who received <2 Gy of high-LET radiation

(47). Table 14 lists deterministic effects in young adults

from an acute whole-body dose of ionizing radiation.

Survivors of deterministic effects are at risk of stochastic

effects (48).

Table 14. Deterministic effects in young adults from a whole-body Gy-Eq a of ionizing radiation received

in <1 day.

Gy-Eq Effects

0.15 Threshold dose for temporary sterility in males (46).

0.35 Within a few hours, some suffer nausea, weakness, and loss of appetite. Symptoms disappear a few hours after

appearing (46).

1-2 After 2-3 hours, nausea and vomiting in 33-50% (48).

1.5 Threshold dose for mortality (27).

2 Permanent sterility in premenopausal females (9). Minimum cataract dose (10).

2-4 Mild headache in about 50%. Almost constant nausea and vomiting in 70-90% (49). There may be initial

granulocytosis, with pancytopenia 20-30 days after irradiation. Possible later effects are infections, hemorrhage,

and impaired healing (49). The latent period for cataracts is normally about 8 years, after 2.5-6.5 Gy (9).

3.5-6 Threshold dose for permanent sterility in males (11).

4 About 50% die within 60 days from hematopoetic failure (11). It has been reported for adult males, that shielding

10% of the active (red) bone marrow will result in almost 100% survival (48). Locations and percent of total bone

marrow in adults are in Table 15.

5-7 Up to 100% vomit within 2 days (27). Mortality about 90% within 60 days (11).

>8 Within minutes, there may be severe nausea, vomiting, and watery diarrhea. After 1-2 hours, there is almost

constant severe headache (49). There may be renal failure and cardiovascular collapse. Mortality 100%, usually

within 8-14 days (49).

>20 Often, burning sensation within minutes. Nausea and vomiting within 1 hour, followed by prostration, ataxia, and

confusion (49). Mortality 100%, usually within 24-48 hours (49).

a Described in the section entitled Ionizing Radiation Terminology.

17

Stochastic Effects

Harm from ionizing radiation is called a stochastic

effect (expressed in sieverts) if the probability (risk), but

not the severity of the effect, is a function of the effective

dose. It is believed that there is no threshold dose for

stochastic effects (9). Stochastic effects include cancer,

genetic disorders in succeeding generations, and loss of

life from such effects. The risk is cumulative and persists

throughout the life of the exposed person. Thus, individuals

exposed to ionizing radiation have an increased

lifetime risk of cancer, and their progeny have an increased

risk of inheriting genetic disorders.

Radiation-induced cancers cannot be distinguished

from cancers of the same type in the unirradiated population,

and it cannot be predicted which individuals in

an irradiated group will develop cancer (43). Regardless

of age when irradiated, radiation-induced tumors tend

to appear when tumors of the same type occur in the

unirradiated population (9).

In Tables 10, 11, and 12 are estimates of the increased

lifetime risk of cancer and of heritable effects from a

whole-body dose of ionizing radiation for a member of

the whole population and for a member of the workingage

population (18-64 years). If the radiation is low LET

(electrons, photons, positrons, or muons) and <200 mSv

and/or <100 mSv/hour, then the estimated risk is half

that found by linear extrapolation from the high-dose

region, and the DDREF = 2 (DDREF is the dose and

dose-rate effectiveness factor) (12, 11).

Thus, for a person in age group 18-64 years who receives

a single acute dose of whole-body radiation <200

mSv: (a) the increased lifetime risk of fatal cancer from

high-LET radiation is 6.3 in 100,000 per mSv (Table

10) and is about 52% of the increased lifetime risk of

cancer (non-fatal or fatal) from low-LET radiation, which

is 12 in 100,000 per mSv (Table 12); (b) the increased

lifetime risk of fatal cancer from low-LET radiation is

3.1 in 100,000 per mSv (Table 10) and is about 26% of

the increased lifetime risk of cancer (non-fatal or fatal),

which is 12 in 100,000 per mSv (Table 12).

Commonly occurring cancers, induced as stochastic

effects of ionizing radiation, are shown in Table 16.

Prenatal Irradiation

Exposure to a high dose of ionizing radiation in the

first 3 weeks after conception may kill a conceptus but

is not likely to cause deterministic or stochastic effects

in a live-born child. However, irradiation in the period

from 3 weeks after conception until the end of pregnancy

may cause deterministic and stochastic effects in a liveborn

child (12).

A dose <100 mGy to a conceptus is not considered a

justification for terminating a pregnancy (48).

RADIOACTIVE CONTAMINATION IN

EARTH’S ATMOSPHERE

Regions of the atmosphere sometimes become contaminated

with radioactive gases and particles released

from a nuclear reactor (as the result of an accident or terrorist

attack) or from a detonated dirty bomb or nuclear

weapon. These radioactive contaminants may travel long

distances in the wind. Radioiodines are the chief gaseous

isotopes of concern after nuclear explosions, and inhalation

problems with fission products other than iodine

are minor (51). In Belarus (formerly part of the Soviet

Union), in the 10-year period before the Chernobyl

reactor accident, 1,342 adult and 7 childhood thyroid

cancers were diagnosed, whereas during the 9-year period

after the accident 4,006 adult and 508 childhood thyroid

cancers were reported (52).

Potassium iodide (KI) blocks iodine uptake by the

thyroid gland. It should be taken daily at the doses recommended

in Table 17, from the first warning of the possible

release of radioactive iodine until the risk of significant

inhalation or ingestion of radioactive iodine no longer

exists. Potassium iodide protects the thyroid gland for

approximately 24 hours. Treatment is most effective if it

begins between 2 days before and approximately 8 hours

after inhalation or ingestion of radioactive iodine (54).

Starting treatment 16 or more hours after radioactive

iodine exposure provides little protection (54). It probably

should not be taken for more than 10 days. After

leaving a contaminated area, potassium iodide should

be taken for at least another day to allow the kidneys

time to eliminate radioactive iodine that is in the blood.

For persons <20 years of age, the risk of thyroid cancer

decreases with increase in age at the time of irradiation.

There is little risk of thyroid cancer after age 40 (52).

However, if the dose to the thyroid is 200-300 Gy or

more, the thyroid parenchyma (functional tissue) will

Table 15. Distribution of active (red)

bone marrow in adults (48).

Percent

sternum 3

cervical vertebrae 4

upper limbs 4

skull 7

thoracic vertebrae 10

lumbar vertebrae 11

lower limbs 11

ribs 19

pelvis 30

18

Table 17. Recommended daily dose of

potassium iodide, if radioactive iodine has been

released into the atmosphere (52).

Persons at risk a

Daily dose of

potassium iodide

(mg)

pregnant or breast feeding 130

>18 years 130

>12 years through 18 years and ≥70 kg. 130

>12 years through 18 years and <70 kg 65

>3 years through 12 years 65

>1 month through 3 years 32

birth through 1 month 16

a Protects for approximately 24 hours.

Table 16. Commonly occurring cancers with ionizing radiation as a risk factor (stochastic effects).

Leukemia Shortest latent period of radiation-induced cancers (9). Minimum latent period 2-3 years (46). Peak

incidence 5-7 years after irradiation, with most cases in first 15 years (9). Acute leukemia and chronic

myeloid (myelocytic) leukemia are the main types in irradiated adults (9). Susceptibility to acute lymphatic

leukemia (stem-cell leukemia, leukemia too premature to classify) is highest in childhood and decreases

sharply during maturation (9). Chronic lymphocytic leukemia (CLL) is not radiation-induced (9).

Lung cancer Most common cancer worldwide and the major cause of death from cancer (7). With increase in age at

exposure, the latent period decreases and the risk increases (10). Dose-fractionation decreases risk (based on

low-LET radiation) (10) Radon causes about 10% of lung cancer deaths in the U.S. (9).

Breast cancer Among women world-wide, the most common cancer and one of the leading causes of death from cancer

(7). Risk highest if irradiated before age 15 years, with little or no risk if irradiated at age 50 or older (9).

Family history is a strong predictor of risk (7). Dose-fractionation data conflicting (10). Risk reduced by

ovariectomy (oophorectomy) or pregnancy at an early age (10).

Gastrointestinaltract

cancer

(esophagus,

stomach, colon, and

rectum)

15-20% of benign colorectal tumors become malignant (10). In the general U.S. population, lifetime risk of

developing gastrointestinal-tract cancer is 2.5-5%, but is 2-3 times higher in persons with a first-degree

relative (father, mother, brother, sister, child) who had colon cancer or an adenomatous polyp (50).

Bone cancer External X-radiation may cause bone cancer, but the numbers are small and the risk estimates are poor (9).

Liver cancer Risk from high-LET radiation, but uncertain if risk from low-LET radiation (10).

Kidney and bladder

cancer

Risk from radiation (10).

Skin cancer Most common cancer in Caucasians in the U.S. Latent period about 25 years. Skin exposed to low-LET

radiation and sunlight (presumably from ultraviolet radiation) is at greater risk than skin protected from

sunlight by hair, clothing, or pigmentation. Ionizing radiation causes basal-cell and squamous-cell

carcinomas, but no unequivocal association has been found for ionizing radiation exposure and melanoma

(the most malignant skin cancer). In fair-skinned individuals, radiation from sunlight or sunlamps is a risk

factor for melanoma and other skin cancers. Skin cancer can develop in areas not exposed to sunlight. Skin

cancer can develop in dark-skinned individuals. Family history is a predictor of risk (27).

Thyroid cancer Risk to a child is significant if dose ≥0.05 Gy. Little or no risk if 30 years or older. Most likely radiation

source is radioactive iodine (I-131) released into the atmosphere from a nuclear reactor, as the result of an

accident or terrorist attack. Radioactive iodine is also a major constituent of detonated nuclear weapons.

Radioactive iodine is incorporated by the thyroid gland and the radiation increases the risk of thyroid cancer

(10).

19

be destroyed (48). A person >40 years of age should

take potassium iodide only if the predicted dose is >5

Gy, to prevent hypothyroidism (53). The risk to a child

is significant if the dose to the thyroid is ≥0.05 Gy (53).

Radioactive iodine easily crosses the placenta (48),

and the fetal thyroid begins to accumulate iodine at

about 10 weeks of gestational age. After 8 weeks postconception,

giving the mother 60-130 mg of potassium

iodide within 12 hours of radioactive iodine intake will

reduce uptake of radioactive iodine by the fetal thyroid

(48). Health risks from stable iodine, a component of

potassium iodide, include sialadenitis (inflammation of

the salivary gland), gastrointestinal disturbances, allergic

reactions, and minor rashes. Persons with iodine sensitivity

should avoid potassium iodide, as should individuals

with dermatitis herpetiformis and hypocomplementemic

vasculitis (rare conditions associated with increased risk

of iodine hypersensitivity) (52).

Because radioactive iodine has a physical half-life of

8.04 days, food affected by radioactive iodine fallout poses

very little risk if stored 2 months or longer.

Other possibly important radioactive substances that

could be released into the atmosphere from a nuclear

reactor, detonated nuclear weapon, or dirty bomb, along

with the annual allowable levels of intake (ALIs) for U.S.

radiation workers, are listed in Table 18. Some possible

therapies are listed in Table 19. The ALI for each isotope

is the intake of that isotope that results in a committed

effective dose approximately equal to the NRC limit for

radiation workers and the FAA recommended limit for

any one year (50 mSv) (55).

If possible, flying through radioactively contaminated

air space should be avoided. If crewmembers are exposed

to multiple isotopes, the ALI for each isotope should

be reduced to keep the total committed effective dose

at or below the FAA-recommended limit. A pregnant

crewmember should avoid any exposure that results

in the dose to the conceptus exceeding recommended

limits (0.5 mSv in any month and 1 mSv total during

her declared pregnancy). The maximum dose rate from

radioactive materials on board the aircraft should not

exceed 0.02 mSv per hour in any occupied space (57).

Table 18. Nuclear Regulatory Commission Annual Allowable Levels of Intake

(ALI)a,b for Selected Radionuclides (55).

Ingestion (μCi) Inhalation (μCi)

Isotope <10 d 10-100 d >100 d <10 d 10-100 d >100 d

Americium-241 0.8 0.006

Cesium-137 100 200

Cobalt-60 ---- 500 200 ---- 200 30

Iodine-125 40 60

Iodine-131 30 50

Iridium-192 900 300 400 200

Palladium-103 6000 6000 4000 4000

Phosphorus-32 600 900 400 ----

Plutonium-239 0.8 ---- 0.006 0.02

Radium-226 2 0.6

Strontium-90 30 20 ---- 4

Tritium (hydrogen-3) 80000 80000

Uranium-233 10 1 0.7 0.04

Uranium-234 10 1 0.7 0.04

Uranium-235 10 1 0.8 0.04

Yttrium-90 400 ---- 700 600

a One ALI gives an effective dose of about 50 mSv. The radiation worker is assumed to be doing light labor

while wearing no protective gear.

b If more than one isotope is involved, the procedure to find whether or not the exposure exceeds the intake

limit is to divide each intake by its respective ALI and then sum the resulting fractions. If the sum is >1, the

ALI has been exceeded.

c The various compounds incorporating these isotopes have different lifetimes in the body. In this table

allowable levels of intake for these isotopes are subdivided into 3 groups, based on their retention times in the

body: “D” < 10 days; “W” 10-100 days; and “Y” > 100 days. If there is no D, W, or Y, then the allowable

level of intake is applicable to all compounds which incorporate that isotope, whatever their lifetime in the

body.

20

Table 19. Drugs used to treat persons contaminated with radioactive materials (56).

Ammonium

chloride

This salt acidifies the patient’s blood, and is useful for the removal of strontium, especially when combined

with intravenous calcium gluconate. Ammonium chloride is given orally, 1-2 gm 4 times per day, for up to

6 consecutive days. While best results occur if given quickly after intake, some effect is seen if used up to

two weeks afterwards. If used promptly with calcium gluconate, strontium levels can diminish 40-75 %.

Nausea, vomiting, and gastric irritation are common. Avoid in patients with severe liver disease.

Calcium (oral) Many oral calcium supplements are available (e.g., Tums®;). Generous doses of oral calcium should be

beneficial, because calcium can interfere with absorption of the other alkaline earths, such as strontium,

barium, and radium, and compete with their deposition in bone.

Ca-DTPA (also

Zn-DTPA)

This is a powerful and stable chelating agent, used primarily to remove plutonium and americium. It

chelates transuranic (Z>92) metals (plutonium, americium, curium, californium, and neptunium), rare earths

(such as cerium, yttrium, lanthanum, promethium, and scandium), and some transition metals (such as

zirconium and niobium). In normal, healthy, non-pregnant adults with normal bone marrow and renal

function, the dose to use is 1 gm in 250 ml normal saline or 5% dextrose in water, intravenously over 1

hour. 1 dose per day should be used, and the dose should not be fractionated. May use for several days to a

week in most cases without toxic effects. Toxicity is due to chelation of needed metals, such as Zn and Mn.

Toxic effects include nausea, vomiting, chills, diarrhea, fever, pruritus, muscle cramps, and anosmia. In

pregnant patients or after a couple of doses, the less toxic Zn-DTPA should be used instead, with the same

dose and dose schedule. These agents are best used as quickly as possible after internal contamination. They

are effective if given later, but therapy may continue for months or years. These agents are only effective if

the metals one wishes to chelate are in ionic form. They are useless for highly insoluble compounds.

Calcium

gluconate

Intravenous calcium gluconate is indicated for Sr-90 contamination, and probably Ra-226 contamination as

well. Five ampoules, each containing approximately 500 mg calcium, may be administered in 0.5 liter 5%

dextrose solution over a 4 hour period daily for 6 consecutive days. It is contraindicated in patients who

have a very slow heart rate and those on digoxin preparations or quinidine.

Dimercaprol

(British

antilewisite,

BAL)

This agent effectively chelates mercury, lead, arsenic, gold, bismuth, chromium, and nickel. It is toxic, and

about 50% of patients given 6 mg/kg intramuscular injections develop reactions. These include systolic and

diastolic hypertension, tachycardia, nausea, vomiting, chest pain, headache, and sterile abscess at the

injection site. The dose to use is 2.5 mg/kg (or less) every 4 hours for 2 days, then twice daily for 1 day, and

then every day for days 5-10. It is available as 300 mg/vial for deep intramuscular injection use (suspension

in peanut oil).

D-Penicillamine This drug chelates copper, iron, mercury, lead, gold, and possibly other heavy metals. The chelated metals

are excreted in the urine. While this drug is relatively non-toxic, it probably has only limited usefulness for

radionuclide decorporation, preventing perhaps only 1/3 of the total radiation absorbed dose that would

have occurred without treatment. The adult dose is 250 mg orally daily between meals and at bedtime. May

increase to 4 or 5 g daily in divided doses. Use caution if patient has a penicillin allergy

Potassium iodide Useful for blocking radioiodine uptake by the thyroid, but needs to be administered within 8 hours of intake

for best effectiveness. See Table 17 for dosing information.

Potassium

phosphate (and

sodium

phosphate)

Used to block uptake of radioactive phosphate. K-Phos® Neutral contains 250 mg phosphorus per tablet.

Usual adult dose is 1-2 tabs orally 4 times per day, with full glass of water each time, with meals and at

bedtime. Pediatric patients over 4y, 1 tab 4 times per day. Contraindicated in hyperphosphatemia, renal

insufficiency, and infected phosphate stones.

Propylthiouracil This drug is useful to decrease the thyroid’s retention of radioiodine, and may be considered if it is too late

for KI to be effective. The adult dose is 50 mg tabs, 2 orally 3 times per day for 8 days.

Prussian blue This drug is indicated for decorporation of cesium, thallium, and rubidium, and has been shown to be highly

effective for Cs-137 contamination. It is benign, with the exception of occasional constipation. Stool turns

blue. Usual dose starts at 0.5 g capsule, 2 caps orally 3 times per day for up to 3 weeks or longer as

required. Doses up to 10-12 g/day for significantly contaminated adults may be used.

Sodium alginate Oral alginates efficiently bind strontium in the gastrointestinal tract, preventing absorption. The dose is 10

gm powder in a 30 cc vial, add water and drink.

Sodium

bicarbonate

Used to protect kidneys from uranium deposition after uranium intake. Oral or intravenous, take as needed

to maintain alkaline urine. The intravenous formulation is 8.9%, 100 or 200 cc vials.

Sodium

phosphate

See potassium phosphate. Also used for radioactive phosphate decorporation.

Zn-DTPA See Ca-DTPA.

21

When an aircraft is contaminated or suspected of contamination

with radioactive materials, it must be removed

from service and not returned to service until the dose

rate from radioactive contaminants at every accessible

surface is <0.005 mSv per hour (58).

Lightning and TErrestrial

Gamma-Ray Flashes

Dwyer, Smith, Uman, et al. (59) recently reviewed

the subject of ionizing radiation emitted from thunderstorms,

citing 66 references. The following is a summary

of the article.

X-rays and gamma rays are emitted by thunderclouds

and are associated with lightning. On average, each commercial

aircraft is struck by lightning about once every

3,000 flight hours. Also, large bursts of gamma rays emanating

from our atmosphere, called terrestrial gamma-ray

flashes (TGFs), have been observed by spacecraft since

1994. In both lightning and TGFs, the high-energy

emissions are believed to occur when electrons in air

traveling at very close to the speed of light collide with

the nuclei of air atoms.

There are many unknowns, including the occurrence

rate of TGFs relative to lightning, the effect of the aircraft

triggering the lightning, and the frequency and lengths

of electron acceleration regions in thunderstorms. The

estimated dose received by an individual in an aircraft

struck by TGF- or lightning-energetic electrons is about

30 mSv. Thus, a significant number of aircraft occupants

may receive a large radiation dose. Direct measurements

have not been made.

CONCLUDING REMARKS

In a Presidential Document, it is recommended that

individuals occupationally exposed to ionizing radiation

be instructed on its health risks (60).

The FAA recommends that air-carrier crews be informed

about their radiation exposure and the associated

health risks (61). The FAA mandates that information on

risks of space travel be provided to space flight participants

prior to their participation on space flights under

its jurisdiction (62).

This report can be used as a source book for instruction

on ionizing radiation exposure of air and space travelers.

It provides information on: (a) its basic characteristics,

(b) the types that will be encountered during air and

space travel, (c) its natural sources, (d) exposure levels

and associated health risks.

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NASA Facts

National Aeronautics and

Space Administration

Lyndon B. Johnson Space Center

Understanding Space Radiation

FS-2002-10-080-JSC

October 2002

• Altitude above the Earth – at higher altitudes the Earth’s

magnetic field is weaker, so there is less protection against

ionizing particles, and spacecraft pass through the trapped

radiation belts more often.

• Solar cycle – the Sun has an 11-year cycle, which culminates in

a dramatic increase in the number and intensity of solar flares,

especially during periods when there are numerous sunspots.

• Individual’s susceptibility – researchers are still working to

determine what makes one person more susceptible to the

effects of space radiation than another person.

Measuring Radiation

The absorbed dose of radiation is the amount of energy

deposited by radiation per unit mass of material. It is measured

in units of rad (radiation absorbed dose) or in the international

unit of Grays (1 Gray = 1 Gy = 1 Joule of energy per kilogram

of material = 100 rad). The mGy (milliGray = 1/1000 Gray) is

the unit usually used to measure how much radiation the body

absorbs. However, because different types of radiation deposit

energy in unique ways, an equivalent biological dose is used to

estimate the effects of different types of radiation. Equivalent

dose is measured in milliSieverts (mSv). The mSv, therefore,

takes into account not only how much radiation a person

receives, but how much damage that particular type of radiation

can do – the greater the possibility of damage for the same dose

of radiation, the higher the mSv value.

Crews aboard the space station receive an average of 80 mSv

for a six-month stay at solar maximum (the time period with the

maximum number of sunspots and a maximum solar magnetic

field to deflect the particles) and an average of 160 mSv for a

six-month stay at solar minimum (the period with the minimum

number of sunspots and a minimum solar magnetic field).

Although the type of radiation is different, one mSv of space

radiation is approximately equivalent to receiving three chest

x rays. On Earth, we receive an average of two mSv every year

from background radiation alone.

Crew members could receive higher doses of space radiation

during space walks while outside the protective confines of the

space station; however, NASA plans space walks to avoid the

trapped radiation belts around the Earth, and doses on previous

space walks have been kept very small.

Protecting Current and Future Space Station Crews

To determine acceptable levels of risk for astronauts, NASA

follows the standard radiation protection practices recommended

by the U.S. National Academy of Sciences Space Science

Board and the U.S. National Council on Radiation Protection

and Measurements.

Aboard the space station, improving the amounts and types of

shielding in the most frequently occupied locations, such as the

sleeping quarters and the galley, has reduced the crew’s exposure

to space radiation. Materials that have high hydrogen contents,

such as polyethylene, can reduce primary and secondary

radiation to a greater extent than metals, such as aluminum.

Space station crew members each wear physical dosimeters, and

also undergo a biodosimtery evaluation measuring radiation

damage to chromosomes in blood cells (see figure above).

Active monitoring of space radiation levels also can help reduce

the levels of radiation an astronaut receives by helping the

astronauts locate the best-shielded locations on the station.

The monitoring also serves as a warning should radiation levels

increase due to solar disturbances. Following a healthy diet and

lifestyle, including the use of antioxidants following radiation

exposure, should also reduce risks.

Radiation Measurements Aboard

the International Space Station

Below, in alphabetical order, are the many radiation measurement

devices and experiments that have flown to the

International Space Station.

Bonner Ball Neutron Detector

March – December 2001

A Japanese Space Agency experiment that measured the amount

of neutron radiation that entered the space station. Neutron radiation

can affect the blood-forming marrow in bones.

Charged Particle Directional Spectrometers – CPDS

2001 – present

There are three units mounted outside on the station’s S0 truss

that are designed to record the direction from which radiation

strikes. There is another unit inside the station.

Dosimetric Mapping – DOSMAP

March – August 2001

A German Space Agency/European Space Agency experiment

that consisted of four different types of radiation detectors

located throughout the space station, These measured the

amounts and types of radiation that entered the ISS.

Study of Radiation Doses Experienced by Astronauts

in EVA – EVARM

February 2002 – present

These sensors are being used to determine the levels of radiation

space walkers receive in their skin, eyes and blood-forming

organs. EVARM consists of three active dosimeters (placed on

Normal chromosome No. 2

and No. 4 in a postflight

metaphase sample

Damaged chromosome No. 2

in a postflight metaphase

sample

the leg, torso and near the eye) that are read before and after a

space walk. The EVARM data could be used to devise methods

of reducing the amount of radiation astronauts are exposed to

during space walks.

Passive Dosimetry

1999 – present

There are several types of radiation detectors aboard the space

station. The radiation area monitor (RAM) is a small set of thermoluminescent

detectors encased in Lexan plastic that respond

to radiation – the amount of radiation they absorb can be revealed

by applying heat and measuring the amount of visible light

released. RAM units are scattered inside the space station and

are returned to Earth for measurement after periodic space

shuttle visits. The crew passive dosimeter is very similar to

the RAM and is carried by each member of the crew. The

AN/UDR-13 radiac Set (a high-rate dosimeter) is a compact,

handheld or pocket-carried device capable of quickly measuring

doses of gamma or neutron radiation. Data readout and warning

messages are provided by a liquid crystal display on the set.

Phantom Torso

March – August 2001

This unique experiment

measured the effects of

radiation on organs inside

the human body by using

a torso equivalent in

height and weight to an

average adult male. The

torso contained radiation

detectors that measured

how much radiation the

brain, thyroid, stomach,

colon, and heart and lung area received on a daily basis. The

data are still being analyzed to determine how the body reacts

to and shields its internal organs from radiation, information

that will be very important during longer-duration space flights.

Tissue Equivalent Proportional Counter – TEPC

2000 – present

This radiation detector consists of a 2"-diameter by 2"-long

cylindrical cell that is filled with low-pressure propane gas. The

gas is used to simulate the hydrocarbon content of a human cell

that is two microns in diameter. A plastic jacket covering the cell

simulates the properties of adjacent tissue cells. Particles passing

through the gas release electrons, which are collected, helping to

identify the energy of the particles.

Measuring Space Radiation Between the Earth

and Mars

As the Mars Odyssey spacecraft made its way to Mars between

April and October 2001, the Mars radiation environment

experiment (MARIE) measured the amounts and kinds of space

radiation the spacecraft encountered along the way. These data

are essential to understanding how much and what kinds of

radiation future space travelers might encounter on a long trip

to explore the red planet.

Now in orbit around Mars, MARIE continues to measure the

amount of harmful radiation at the planet itself. Unlike Earth,

Mars does not have a global magnetic field to shield it from

solar flares and cosmic rays. Mars’ atmosphere is also less

than one percent as thick as the Earth’s. These two factors

make Mars very vulnerable to space radiation.

Aboard the International Space Station and in our own solar

system, NASA researchers continue to quantify the amounts

of space radiation our explorers face every day and will face

in the future. Understanding space radiation will not only protect

the crew currently aboard the International Space Station, but

those first humans who will continue the exploration of our

solar system.

Related Web Sites

http://srhp.jsc.nasa.gov/

http://spaceresearch.nasa.gov/research_projects/radiation.html

http://www.spaceflight.nasa.gov/station/science/bioastronautics/

http://marie.jsc.nasa.gov/main.html