Wallace Friedberg 

Kyle Copeland

Civil Aerospace Medical Institute

Federal Aviation Administration

Oklahoma City, OK 73125

May 2011

Final Report

Ionizing Radiation in Earth’s

Atmosphere and in Space 

Near Earth 

DOT/FAA/AM-11/9

Office of Aerospace Medicine

Washington, DC 20591

OK-11-0024-JAH

Federal Aviation

Administration
NOTICE

This document is disseminated under the sponsorship 

of the U.S. Department of Transportation in the interest 

of information exchange. The United States Government 

assumes no liability for the contents thereof.

___________

This publication and all Office of Aerospace Medicine 

technical reports are available in full-text from the Civil 

Aerospace Medical Institute’s publications Web site: 

www.faa.gov/library/reports/medical/oamtechreports
i

Technical Report Documentation Page 

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. 

DOT/FAA/AM-11/9 

4. Title and Subtitle 5. Report Date 

Ionizing Radiation in Earth's Atmosphere and in Space Near Earth May 2011 

6. Performing Organization Code 

 

7. Author(s) 8. Performing Organization Report No. 

Friedberg W, Copeland K 

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

FAA Civil Aerospace Medical Institute 

P.O. Box 25082 11. Contract or Grant No. 

Oklahoma City, OK 73125 

12. Sponsoring Agency name and Address 13. Type of Report and Period Covered 

Office of Aerospace Medicine 

Federal Aviation Administration 

800 Independence Ave., S.W. 

Washington, DC 20591 14. Sponsoring Agency Code 

15. Supplemental Notes 

 

16. Abstract 

The Civil Aerospace Medical Institute of the FAA is charged with identifying health hazards in air travel and in 

commercial human space travel. This report addresses one of these hazards – ionizing radiation. 

Ionizing radiation is a subatomic particle of matter or packet of energy (photon) with sufficient energy to eject an 

orbital electron from an atom. Charged subatomic particles from exploding stars (supernovae) are a constant 

source of ionizing radiation in the atmosphere and in space. In space another constant source of ionizing 

radiation is the solar wind from the Sun. The solar wind consists mostly of electrons and protons with energies 

between 10 and 100 keV. The Sun undergoes an approximately 11-year cycle of rise and decline in activity and 

during its active phase there is an increased emission of the solar wind and occasional eruptions of high-energy 

particles (coronal mass ejections). Other sources of ionizing radiation during air travel include radioactive cargo, 

radioactive substances released into the atmosphere as a result of a nuclear reactor accident or terrorist activity, 

lightning, and terrestrial gamma-ray flashes. 

A health effect following exposure to ionizing radiation for which the severity is radiation dose related is called a 

deterministic effect (non-stochastic effect, tissue reaction). Deterministic effects may occur soon after radiation 

exposure. For example, nausea and vomiting might be experienced by a space traveler a few hours after receiving 

a large dose of ionizing radiation while outside a space vehicle during a coronal mass ejection. 

If the probability (risk) of a health effect after exposure to ionizing radiation is dose related, it is called a 

stochastic effect. Such effects seldom occur until years after the radiation exposure. Examples of stochastic effects 

are cancer and genetic disorders. If one or both parents are irradiated prior to conceiving a child, there is a risk of 

genetic disorders in the child and in its progeny. 

The report can be used as a source book for instruction on ionizing radiation exposure of air and space travelers. 

 

17. Key Words 18. Distribution Statement 

 

 

Ionizing Radiation, Cosmic Radiation, Radioactive 

Contamination, Space Weather, Geoomagnetism, 

Radiation Exposure Limits, Flight Doses, Radiation Health 

Risks 

Document is available to the public through the 

Defense Technical Information Center, Ft. Belvoir, VA 

22060; and the National Technical Information 

Service, Springfield, VA 22161 

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price 

Unclassified Unclassified 28 

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized 

iii

ACKNOWLEDGMENTS

For help in preparing this report we thank: Katherine Wade, Roni G. Anderson, Michael E. Wayda, Dr. 

William S. Irving Jr., and Frances E. Duke of the Civil Aerospace Medical Institute; Herbert H. Sauer of the 

National Geophysical Data Center, National Oceanic and Atmospheric Administration; Dr. Donald E. Parker 

(Professor Emeritus) of the University of Oklahoma Health Sciences Center; Dr. Edgar B. Darden Jr. (retired) 

of the Oak Ridge Associated Universities; Daniel Friedberg, Jacqueline Bist, and Anielia M. Bist. 

v

C

INTRODUCTION. 

IONIZING RADIATION DOSE TERMINOLOGY. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Linear Energy Transfer. 

Relative Biological Effectiveness. 

Gray  

Gray Equivalent  

Organ Equivalent Dose (Equivalent Dose). 

Dose Equivalent  

Effective Dose. 

Effective Dose Equivalent. 

Sievert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

EARTH’S ATMOSPHERE. 

Atmospheric Layers. 

EARTH’S MAGNETIC FIELD. 

GALACTIC COSMIC RADIATION. 

Composition and Sources. 

Galactic Cosmic Radiation in Earth’s Atmosphere. 

THE SUN AND ITS EMISSIONS. .

Layers of the Sun’s Atmosphere. .

Sunspots. .

Solar Wind  .

Coronal Mass Ejection and Solar Flare . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Solar Proton Event. .

RECOMMENDED IONIZING RADIATION DOSE LIMITS. .

U.S. Federal Aviation Administration. .

International Commission on Radiological Protection  .

National Council on Radiation Protection and Measurements . . . . . . . . . . . . . . . . . . . . . . . . 11

European Union. .

IONIZING RADIATION EXPOSURE DURING AIR AND SPACE TRAVEL. .

Radiation Doses Received During Air Travel. .

Radiation Doses Received During Space Travel. .

HEALTH EFFECTS OF IONIZING RADIATION. .

Deterministic Effects (Non-Stochastic Effects, Tissue Reactions). .

Stochastic Effects  .

Prenatal Irradiation. .

RADIOACTIVE CONTAMINATION IN EARTH’S ATMOSPHERE. .

Lihtning and

CONCLUDING REMARKS. .

REFERENCES. .

1

Ionizing Radiation in Earth’s 

and in Space Nea

INTRODUCTION

The U.S. Federal Aviation Administration’s (FAA’s) 

Civil Aerospace Medical Institute (CAMI) is charged 

with identifying health hazards in air travel and in commercial human space travel. This report addresses one of 

these hazards – ionizing radiation.

Air and space travelers are constantly exposed to galactic 

cosmic radiation (GCR), which is ionizing radiation from 

exploding stars (supernovae). They are also exposed to 

solar cosmic radiation (SCR), which is ionizing radiation 

from the Sun. Other sources of ionizing radiation include: 

radioactive air cargo, radioactive substances released into 

the atmosphere from a detonated nuclear weapon or from 

a nuclear reactor as the result of an accident or terrorist 

attack, lightning, and terrestrial gamma-ray flashes (TGFs). 

Radiation is energy in transit. The energy travels as (a) 

subatomic particles of matter (e.g., electrons, neutrons, 

protons, alpha particles, pions, muons), and (b) photons, 

which are packets of electromagnetic energy (e.g., visible 

light, ultraviolet [UV] light, radio waves, microwaves, 

gamma radiation, X-radiation).

Ionizing radiation is a subatomic particle or photon sufficiently energetic to directly or indirectly eject an orbital 

electron from an atom. Photons and electrically-charged

particles ionize directly by means of electromagnetism. 

Neutrons cannot ionize directly, but they can ionize 

indirectly. On impacting the nucleus of an atom (e.g., 

atmospheric nitrogen or oxygen), a neutron can (a) induce emission of a gamma-radiation photon by nuclear 

excitation, or (b) break apart the nucleus and release 

protons (which can ionize directly), neutrons (which can 

decay into protons and electrons), and pions (which can 

undergo decay processes shown in Table 1).

Muons are the main contributor to dose at low altitudes. 

Neutral pions decay so rapidly that they are unlikely to impact 

an atom before decaying. Neutrinos and antineutrinos pass 

through matter with almost no effect, and usually they are 

not considered in dose calculations.

Wilhelm Conrad Roentgen in 1895 discovered ionizing 

radiation while experimenting with a Crookes tube (a primitive vacuum tube) (2). Working in a dark room with the tube 

in a carton, Roentgen found that a paper plate coated with 

barium platinocyanide (a chemical that fluoresces when exposed to UV light from the Sun), which was outside the carton 

and 9 feet away from the tube, emitted a fluorescent light 

when the tube was supplied with electric current. Roentgen 

concluded that an invisible radiation from the tube, which 

he called X-rays, penetrated the wall of the carton and traveled to the barium platinocyanide. He could not deflect the 

radiation with a magnetic field, and he found that objects in 

the path of the radiation showed variable transparency. With 

a photographic plate, Roentgen used the device to make a 

picture of the skeleton of his wife’s hand (3).

Antoine-Henri Becquerel in 1896 discovered a natural 

source of ionizing radiation while investigating phosphorescence (4). He observed that a photographic plate covered 

with an opaque paper was fogged when placed near uranyl 

potassium sulfate (a uranium salt). Becquerel demonstrated 

that unlike X-rays, the radiation from uranium could be 

deflected by a magnetic field and therefore consisted of 

charged particles. 

Theodore Wulf measured ionizing radiation levels with 

an electroscope at the bottom and at the top of the Eiffel 

Tower (300 m high) and found that radiation levels at the 

top of the tower were higher than at ground level (5). In 

1912, Victor F. Hess measured ionization rates with an 

electroscope during balloon flights (6). He found that at 

an altitude of 5 km the ionization rate was several times 

the rate at ground level. Hess concluded that a highly 

penetrating radiation enters the atmosphere from above. 

The radiation is now called cosmic radiation. Listed in 

Table 1. Decay of pions and of their muon decay products (1). 

Particle Decay Products 

Half-life 

(seconds) 

positive pion  1 positive muon + 1 muon-neutrino 1.8 x 10-8

negative pion  1 negative muon + 1 muon-antineutrino 1.8 x 10-8

neutral pion  2 gamma-radiation photons 6 x 10-17

 * * * * 

positive muon  1 positron (positive electron) + 1 electron-neutrino + 1 muon-antineutrino 1.5 x 10-6

negative muon  1 electron + 1 electron-antineutrino + 1 muon-neutrino 1.5 x 10-6
2 

Table 2 are radiation levels received by humans on Earth 

from natural sources.

Living material consists of molecules composed of 

atoms held together by electron bonds. Ejection of orbital 

electrons can break the bonds that combine atoms as 

molecules. Particularly harmful to a biological system is 

the breakup of molecules of deoxyribonucleic acid (DNA). 

DNA carries information required for the function and 

reproduction of an organism. Improper repair of DNA 

damaged by ionizing radiation or by free radicals produced by ionizing radiation may lead to cancer (8). Free 

radicals are also believed to have a role in the etiology of 

atherosclerosis, rheumatoid arthritis, and other diseases. 

A free radical is an electrically neutral atom or molecule 

containing one or more unpaired electrons in the valence 

shell, and this makes it very reactive. Ionizing radiation 

particles produce free radicals when they react with the 

water in cells and with some cellular components.

Visible light, UV light, radio waves, and microwaves 

are non-ionizing radiations. A photon of non-ionizing 

radiation or a subatomic particle insufficiently energetic 

to eject an orbital electron may interact with an atom by 

electron excitation. This process forces an orbital electron to a higher energy level. An electron so affected is 

in an unstable energy level and soon falls back to a more 

stable level, releasing energy in the form of non-ionizing 

photons. The photons released include thermal photons 

and light photons of different wavelengths. The aurora 

borealis (northern polar lights) and the aurora australis 

(southern polar lights) result from the excitation of 

atmospheric atoms by charged particles from the Sun 

(solar wind). Auroras are commonly centered over the 

magnetic poles of the Earth, between 60o

 and 70o north 

and south geographic latitudes. Occasionally, during 

high solar activity they are seen at lower latitudes. Also 

during high solar activity, some particles from the inner 

Van Allen radiation belt (described in the section entitled 

Earth’s Magnetic Field) may enter the atmosphere and 

contribute to the aurora display.

IONIZING RADIATION DOSE 

TERMINOLOGY

Radiation dose is the amount of energy absorbed by 

a medium. The medium could be the human body or 

a particular tissue or organ in the body. Various terms 

are used to quantify the absorbed dose and its biological 

impact.

Linear Energy Transfer

Linear energy transfer (LET, Table 3) is the average 

amount of energy per unit track length imparted to a 

medium by ionizing radiation of a specified energy, when 

penetrating a short distance. The energy imparted to the 

medium includes energy from any secondary radiation, 

Table 2. Typical worldwide annual doses of ionizing 

radiation to an adult human at ground level, from natural 

sources (7). 

 Effective dose (mSv a) 

galactic cosmic radiation 0.39 

terrestrial gamma radiation b 0.46 

radionuclides in the body (except radon) c

 0.23 

radon and its decay products d

 1.3 

Total 2.38 

a

 Described in the section entitled Ionizing Radiation Terminology. 

b

 Sources include uranium-238, radium-226, thorium-232, potassium-40, and 

cesium-137. 

c

 Potassium-40 and carbon-14. 

d Radon-222 is a decay product of radium-226, which is a decay product of 

uranium-238. Some areas have high concentrations of uranium-238 in the soil. 

Radon-222, a gas, leaks into homes from the ground underneath. Inhaled radon 

travels through the bronchial tubes to the lungs and decays into polonium-210, a 

solid.

Table 3. Typical LET values of various 

radiations. 

Type of radiation LET (keV/μm) 

cobalt-60 gamma-radiation 0.3 

250 kVp X-radiation 2. 

10 MeV protons 4.7 

150 MeV protons 0.5 

recoil protons from fission neutrons 45. 

14 MeV neutrons 12. 

2.5 MeV alpha particles 166. 

2 GeV Fe nuclei 1000. 
3

such as nuclear particles released from a nucleus impacted 

by a high-energy neutron. LET is usually expressed in 

units of keV/μm (thousand electron volts per micrometer). A radiation with an LET <10 keV/μm is generally 

considered low LET.

Relative Biological Effectiveness

Relative biological effectiveness (RBE) is the ratio of 

absorbed dose of a reference radiation (usually 250 kVp 

X-radiation or cobalt-60 gamma-radiation) to absorbed 

dose of the radiation in question, in producing the same 

magnitude of the same effect in a particular experimental 

organism or tissue.

The RBE is influenced by the biological endpoint and 

the LET of the radiation. With killing human cells as the 

endpoint, the RBE increases with an increase in LET to 

about 100 keV/μm and then decreases with further increase

in LET. At LET 100 keV/μm, the average separation between

ionizing events is close to the diameter of the DNA double 

helix. Therefore, a radiation with LET 100 keV/μm can

most efficiently produce a double-strand break in a DNA 

molecule by a single LET track (9). Double-strand breaks 

in DNA molecules are the main cause of biological effects. 

Gray 

Gray (Gy) is the International System (SI) unit of 

absorbed dose of ionizing radiation. One Gy is 1 joule 

(J) of radiation energy absorbed per kilogram (kg) of 

matter. The rad (radiation absorbed dose) is an older 

unit of absorbed dose of ionizing radiation (1 Gy = 100 

rads). The roentgen (R) is another older unit of ionizing 

radiation. One R is the amount of X-radiation or gamma 

radiation that creates 1 electrostatic unit (esu) of ions 

in 1 milliliter (ml) of air at 00 C and 760 torr (760 mm 

mercury, 1 atm). The effect of 1 R and 1 rad on dry air 

is about the same. 

Gray Equivalent 

Gray equivalent (Gy-Eq) is a measure of the capacity of 

ionizing radiation to cause deterministic effects (described 

in the section entitled Health Effects of Ionizing Radiation).

Gy-Eq is Gy multiplied by a recommended RBE (Table 

4). The RBE takes into account that ionizing radiation 

of different types and energies affects living organisms 

differently. 

Organ Equivalent Dose (Equivalent Dose)

Organ equivalent dose is the total absorbed dose to a 

specific tissue or to a conceptus, after the absorbed dose 

from each radiation incident on the tissue or conceptus is 

multiplied by a radiation weighting factor (wR, Table 5). 

For multiple radiations, the organ equivalent dose is the 

sum of the individual organ equivalent doses. wR takes 

into account the effectiveness of the primary radiation 

(or radiations) and all secondary radiations. wR is an RBE 

for stochastic effects.

Dose Equivalent 

Dose equivalent is the total absorbed dose to a specific 

tissue or a conceptus, after the absorbed dose from each 

radiation incident on the tissue or conceptus is multiplied 

Table 4. Gy x RBE = Gy-Eq (10). 

Type and energy of the radiation RBE 

low-LET radiations (e.g., photons, electrons) 1.0 

protons >2 MeV 1.5 

heavy ions (e.g., helium, carbon, neon, argon) 2.5 

neutrons <5 MeV 6.0 

5 MeV 5.0 

>5 MeV 3.5 

Example Calculation of Gy-Eq: 

A whole-body dose consisting of 4 Gy of 50 MeV protons and 

2 Gy of 50 MeV neutrons, 

(4 Gy x RBE 1.5) + (2 Gy x RBE 3.5) = 13 Gy-Eq. 

Table 5. Radiation weighting factor(wR); takes into account the type and 

energy of the radiation (11). 

Type and energy of the radiation wR

low-LET radiation: photons, electrons, muons 1 

protons,a charged pions 2 

alpha particles,b

 fission fragments, heavy ions 20 

neutrons (energy, En): En <1 MeV 2.5 + 18.2 x exp[-(ln(En))2 /6] 

1 < En ≤50 MeV 5.0 + 17.0 x exp[-(ln(2En))2

 /6] 

En >50 MeV 2.5 + 3.25 x exp[-(ln(0.04En))2

 /6] 

a

 Hydrogen-2 (deuterons) and hydrogen-3 (tritons) have the same LET and track structure as protons, 

therefore the same radiation weighting factor (wR = 2) seems appropriate. b

 Helium-3 (helions) has nearly the same LET and track structure as alpha particles, therefore the same 

radiation weighting factor (wR = 20) seems appropriate. 
4 

by a radiation quality factor (Q, Table 6). For multiple 

radiations, the dose equivalent is the sum of the individual 

dose equivalents. Q relates the LET of the radiation to its 

RBE. Dose equivalent is no longer used by the International Commission on Radiological Protection (ICRP) 

but is still used by the National Council on Radiation 

Protection and Measurements (NCRP) and the National 

Aeronautics and Space Administration (NASA). Dose 

equivalent is used by NASA as a surrogate for organ 

equivalent dose in space radiation applications. 

Effective Dose

Effective dose is the total sex-averaged, whole-body 

absorbed dose, after the absorbed dose to each tissue from 

each radiation incident on the body and/or from each 

internal radiation emitter is multiplied by a radiation 

weighting factor (wR, Table 5) and by a tissue weighting 

factor (wT

, Table 7). wT takes into account the risk of 

stochastic effects from irradiation of a particular tissue. 

Effective Dose Equivalent

Effective dose equivalent is the total sex-averaged, 

whole-body absorbed dose, after the absorbed dose to 

each tissue or organ from each radiation incident on 

the body and/or from each internal radiation emitter 

is multiplied by a radiation quality factor (Q, Table 6) 

and by a wT (Table 7). NCRP considers effective dose 

equivalent as an acceptable approximation of effective 

dose, and they use it as a surrogate for effective dose in 

space radiation applications. 

Sievert

Sievert (Sv) is the SI unit of organ equivalent dose, 

dose equivalent, effective dose, and effective dose 

equivalent. It is a measure of harm to the body from 

stochastic effects of ionizing radiation (described in the 

section entitled Health Effects of Ionizing Radiation). The 

roentgen equivalent man (rem) is an older measure of 

harm (1 Sv = 100 rem).

Table 6. Quality factor (Q); takes into 

account the LET of the radiation (12). 

LET (keV/μm) Q a

<10 1 

10-100 0.32 x LET - 2.2 

>100 300 x LET -1/2

a Q should be rounded to the nearest whole number.

Table 7. Tissue weighting factor (wT); takes into account where 

radiation energy is deposited (11).

Where radiation energy deposited wT ΣwT

 remainder tissues,a red bone-marrow, breast, colon, lung, stomach 0.12 0.72 

 gonads 0.08 0.08 

 bladder, esophagus, liver, thyroid 0.04 0.16 

 bone surface, brain, salivary glands, skin 0.01 0.04 

Total 1.00 

a

 Remainder tissues (males 13, females 13): adrenals, extrathoracic region, gall bladder, heart, 

kidneys, lymph nodes, muscle, oral mucosa, pancreas, prostate, small intestine, spleen, thymus, 

uterus/cervix. For each remainder tissue wT = 0.12/13 = 0.00923. (females do not have a prostate 

and males do not have a uterus/cervix).

Example Estimate effective dose to a Reference Person (adult) from external isotropic exposure to 

heavy ions: 

(1) Estimate absorbed dose in each tissue of an adult female and of an adult male, using a Monte 

Carlo transport code such as MCNPX (from Radiation Safety Information Computing Center 

at Oak Ridge National Laboratory, Oak Ridge, TN), and mathematical models of the bodies, 

such as the ICRP Reference Phantoms (13). 

(2) For each tissue, multiply absorbed dose by the appropriate wR (Table 5). Tissues that received 

only trivial amounts of radiation may be excluded from dose calculations. 

(3) For each tissue, average wR-adjusted absorbed doses to the female and the male. 

(4) For each tissue, multiply sex-averaged wR-adjusted absorbed dose by the appropriate wT 

 (Table 7). 

(5) Effective dose to the Reference Person is the sum of sex-averaged wR-adjusted and wT-adjusted 

absorbed doses to tissues. 
5

EARTH’S ATMOSPHERE

Gravity retains Earth’s atmosphere. The content (percent by volume) of the atmosphere (dry air) is about 78% 

nitrogen, 21% oxygen, 0.93% argon, 0.034% (average) 

carbon dioxide, and trace amounts of other gases (14). A 

variable amount, 0.001-7%, of water vapor is also present 

(14). For aviation, the boundary between the atmosphere 

and outer space is 100 km (328 kft) above Earth’s surface.

Atmospheric Layers

Atmospheric layers, from lower to higher, are: troposphere, stratosphere, mesosphere, thermosphere, and

exosphere. With increase in altitude, temperatures decrease 

in the troposphere, increase in the stratosphere, decrease 

in the mesosphere, and increase in the thermosphere (14). 

Troposphere: extends from Earth’s surface to between 

8-10 km near the poles and 16-18 km in tropical regions, 

with some variation due to weather conditions (14). It 

contains about 80% of the atmosphere’s mass, and it is 

where most daily weather occurs that is observed from the 

ground (15). Traditional subsonic jetliners fly at altitudes 

of 6-12 km (20-40 kft). The Concorde SST cruised at 

14-18 km (45-60 kft).

Tropopause: the boundary between the troposphere 

and the stratosphere (14). The tropopause is where air 

ceases to cool with height, and it is almost completely 

dry (14). 

Stratosphere: extends from the troposphere to about 

50 km (14, 15). The ozone and oxygen in the stratosphere 

absorb much of the UV radiation from the Sun (15). UV 

radiation can be very harmful to living tissues. 

Stratopause: is the level of transition between the 

stratosphere and the mesosphere (14).

Mesosphere: extends from 50 km to 80-85 km (15). 

Most meteors become visible between about 65 and 120 

km above the Earth and disintegrate at altitudes of 50-95 

km (16). Millions enter Earth’s atmosphere every day. 

Mesopause: the level of transition between the mesosphere and the thermosphere (14).

Thermosphere: extends from 80-85 km to more 

than 500 km (15). The International Space Station 

(ISS) orbits Earth at an altitude of 330-400 km, in the 

thermosphere (17).

Exosphere: the highest atmospheric layer. It is where 

Earth’s atmosphere merges with interplanetary space (14). 

In this region the probability of interatomic collisions is 

so low that some atoms traveling upward have enough 

velocity to escape Earth’s gravity (15). 

Ionosphere: contains both ions and neutral molecules 

and extends from about 80 km to 480 km (14). The 

ionosphere typically overlaps the thermosphere and exosphere, and it isthe inner edge of the magnetosphere. The 

magnetosphere is the region around Earth influenced by 

Earth’s magnetic field (geomagnetic field) (18). This region 

is used to reflect radio signals over long distances and it 

is where auroral displays occur (14). On rare occasions, 

known as Polar Cap Absorption (PCA) events, the solar 

radiation increases ionization and enhances absorption 

of radio signals passing through the region, enough to 

absorb most (if not all) transpolar high-frequency radio 

transmissions (19). In extreme cases, PCA events can last 

several days, but they usually last less than one day (19). 

EARTH’S MAGNETIC FIELD

According to the dynamo theory, most of the geomagnetic field is generated by the rotation of liquid iron in 

Earth’s outer core (20). Other sources include the ring 

current (discussed later in this section) and magnetism in 

Earth’s rocks. At Earth’s surface, the geomagnetic field is 

predominantly an axial dipole (like a bar magnet), with 

magnetic field lines radiating between its north and south 

magnetic poles. Magnetic field lines are used to indicate 

the strength and direction of a magnetic field. On a map 

of geomagnetic field lines, the strength of the field is 

represented by the density of the field lines. The denser 

the field lines, the stronger the magnetic field. Thus, the 

field is strongest where the field lines are closest together. 

The direction of the geomagnetic field is the same as the 

field lines. It is usually indicated by arrows drawn on the 

field lines, which run from the north magnetic pole (near 

the south geographic pole) to the south magnetic pole 

(near the north geographic pole).

The geomagnetic field is not uniformly produced, and 

the coordinates of the magnetic poles change frequently. 

The polarity has reversed at irregular intervals of about 

one million years, and the field is becoming weaker at 

such a rate that it will disappear in about 2,000 years. 

However, this may only be a temporary trend (18). If 

the magnetic field does disappear, the radiation belts 

(described below) would disappear and worldwide GCR 

levels would increase to what they are in the polar regions.

The direction of the magnetic force on a moving 

charged particle is at right angles (perpendicular) to both 

the particle’s direction of motion (v) and direction of the 

magnetic field lines (B). The magnitude of the magnetic 

force (F) is proportional to the particle’s electric charge 

(q), the particle’s speed (v), the strength of the magnetic 

field (B), and the sine of the angle between the particle’s 

direction of motion and the direction of the magnetic 

field (∠ v B):

F = q v B sin (∠ v B)

Thus, a charged particle moving parallel to the magnetic 

field (i.e., entering the geomagnetic field at a magnetic 

pole) experiences no deflection from its direction of 
6 

motion (sin[∠ v B = 0o] = 0), whereas a particle moving 

at a right angle to the magnetic field (i.e., entering the 

geomagnetic field at the magnetic equator) experiences 

a maximum in the magnitude of the force deflecting it 

from its direction of motion (sin[∠ v B= 90o

] = 1).

At the geomagnetic equator where geomagnetic 

field lines are parallel to Earth’s surface, only particles 

≥100 MeV can reach Earth’s atmosphere (21). Moving from the geomagnetic equator towards a magnetic 

pole, the field lines gradually become perpendicular to 

the Earth’s surface and therefore more parallel to the 

trajectories of the incoming ions, and more ions enter 

the atmosphere. At the magnetic poles, field lines are 

perpendicular to Earth’s surface and ions of any energy 

can reach Earth’s atmosphere.

Figure 1 shows the effective dose rate at 20o

 E longitude, as related to geographic latitude. Dose rates in 

the figure are calculated for mean solar activity in the 

period January 1958 through December 2008. The 

calculations were made with CARI-6P (23) using NCRP 

Report 116 recommendations (24). If one were to fly 

an aircraft at a constant altitude from the geomagnetic 

equator towards the north or south magnetic pole, the 

dose rate would increase with distance from the equator. 

At high latitudes, shielding by the geomagnetic field 

is minimal. The primary radiation shielding of the 

aircraft would be the atmosphere, with some shielding by magnetic fields carried by the solar wind (the 

interplanetary magnetic field) even when the Sun is 

quiet. The North Atlantic Air Route, which is among 

the busiest in the world, is mostly at high latitudes. 

The geomagnetic field traps charged particles in 

two toroidal (donut-shaped) overlapping radiation 

belts. The belts encircle the planet and are confined to 

an area from about 65o

 N to 65o S of the geographic 

equator (14). The radiation belts are called Van Allen 

radiation belts in honor of Dr. James A. Van Allen, who 

reported the inner belt in 1958 (21). 

The inner radiation belt (nearest Earth) extends 

above Earth’s surface, from about 200 km to 11, 000 

km above the geomagnetic equator and is centered at 

about 3,000 km (18). It consists mostly of electrons and 

10-100 MeV protons. Protons are the most important 

contributor to the dose (10). The flux of high-energy 

protons is relatively constant (14). Protons >50 MeV 

are believed to originate from the decay of free neutrons 

backscattered into the inner belt after being dislodged 

from atmospheric atoms by GCR particles (14). The 

half-life of a free neutron is 10.2 minutes (mean life 

885.3 seconds [25]), and some decay into a proton and 

an electron while in the inner belt. At the geomagnetic 

equator, protons form a continuous distribution, with 

peak fluences of 400 MeV protons at 2,000 km, 4 

MeV protons at 6,000 km, and 0.3 MeV protons at 

13,000 km (10). 

Figure 1. Effective dose rate from GCR, as related to geographic 

latitude at selected altitudes at 20o E longitude. Dose rates are for 

the mean solar activity from January 1958 through December 2008. 

The heliocentric potential used in estimating dose rates is based on 

the average Deep River equivalent count rate for the 51-year period 

reported (22).
7

The dipole axis of Earth’s magnetic field is tilted 

11.1o

 from the rotation axis and offset 515 km from 

the rotation axis (10). As a consequence, the lower 

boundary of the inner radiation belt dips closest to 

Earth in a region off the coast of Brazil called the South 

Atlantic Anomaly, which extends from about 0o to 60o

W longitude and 20o

 to 50o S latitude (geographic 

coordinates). The daily dose of ionizing radiation to 

occupants of the ISS is usually about 0.5-1.2 mSv, with 

approximately 25% of the dose coming from protons 

encountered when passing through the South Atlantic 

Anomaly and the rest from GCR (26).

The outer radiation belt, the belt furthest from Earth, 

consists mostly of electrons (10). It extends from about 

11,000 km to 70,000 km in altitude (27). The center of 

the belt is at about 22,000 km at solar maximum and 

26,000 km at solar minimum (10). The most energetic 

electrons have energies >1.0 MeV and are centered at 

about 19,000-25,000 km (18). The electron flux in 

the outer belt varies considerably during a geomagnetic 

storm because of the fluctuating intensity of the solar 

wind. The main radiation hazard to space travelers in 

the outer belt is soft X-rays, produced when electrons 

decelerate in the shell of the space vehicle (28). 

Individual particles in the radiation belts move towards a magnetic pole in spiral paths along geomagnetic 

field lines (21). As they approach a pole, the particles 

are reflected back towards the opposite pole. As they 

spiral back and forth between the magnetic poles, 

many low-energy protons are lost from the inner belt 

by interacting with atmospheric constituents (10). The 

proton spectrum decreases sharply at energies >500 

MeV, because high-energy particles are not easily held 

by the magnetic field. The flux of trapped particles is 

highest over the geomagnetic equator, and there are 

relatively few trapped particles over the geomagnetic 

poles (21).

As they spiral back and forth between the magnetic 

poles, positive particles drift westward and electrons 

drift eastward. This creates a westward-flowing electric 

current called ring current, which overlaps the outer 

radiation-belt at altitudes of 13,000-25,000 km at the 

geomagnetic equator (18). The magnetic field produced 

by the ring current opposes the 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 mid1930s 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 lowLET 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.

Altitude Effective dose to an adult (Equivalent dose to 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).

Liht

Gamma

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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