air crew radiation exposure - an overview

I T IS AN established fact that an increase inaltitude means an increase in radiation. Amove from a coastal state to the Rocky

mountains, for example, is known to increasean individual’s background radiation dose. Onan airplane, that level of radiation is higher still.

In 1990, the International Commission onRadiation Protection (ICRP) identified air-line flight crews as a group occupationallyexposed to radiation, reversing a policy thatkept any natural source of radiation free fromregulation. Radiation doses received by pi-lots and flight attendants are often greaterthan those received by traditional radiationworkers in the heavily regulated nuclear in-dustry, but, until recently, little attention waspaid to occupationally exposed air crew.Now, sophisticated equipment is allowing re-searchers to undertake studies of such radia-tion exposures, and in some regions of theworld new legislation is in place to monitorair crew exposures. It will be up to the gov-ernment, airlines, and air crew to determinewhat these exposures will mean for the fu-ture of air travel.

Flying is now a common mode of trans-portation, but recent changes in air travelmay mean an increase in dose to exposed in-dividuals. High-powered, high-altitude air-craft such as the Concorde commonly fly at55 000 feet, and future aircraft might fly evenhigher. Galactic cosmic radiation exposureapproximately doubles with every 6000 feetof increased altitude. While cosmic radiationposes little or no risk to the “pleasure” trav-eler, business travelers who log as manyhours as air crew themselves could be la-beled occupationally exposed. Additionally,radiopharmaceuticals being transported ascargo can increase the radiation dose to crewmembers.

The Federal Aviation Administration(FAA) first acknowledged radiation risks in1990, with the publication of Advisory Cir-cular 120-52, “Radiation Exposure of Air Car-rier Crewmembers.” According to that report,

the average dose rate in the contiguous Unit-ed States from cosmic and terrestrial radiationis 0.06 microsieverts (µSv)/hr. At an altitudeof 35 000 feet, which is common for domes-tic air travel, the dose rate from galactic cos-mic radiation alone is 6 µSv/hr.

Advisory Circular 120-61, dated May 19,1994, contains the FAA’s official recommen-dations to U.S. airlines regarding in-flight ra-diation. It simply states, “Air carrier crew-members are occupationally exposed to lowdoses of ionizing radiation from cosmic radia-tion and from air shipments of radioactive ma-

terials.” The two-page document recommendsthat airlines educate crewmembers on the typesand amounts of radiation received during airtravel, with comparisons to other sources of ex-posure; variables that have an effect on theamount of radiation exposure (for example, al-titude, latitude, and solar flares); guidelines re-garding exposure to ionizing radiation; healthrisks to crewmembers from cosmic radiation;special considerations needed to limit the ex-posure of a fetus to cosmic radiation; howcrewmembers can manage their exposure; andradioactive material shipments.

32 N U C L E A R N E W S January 2000

Regulators, airlines, and flight crewsare paying more attention to cosmic radiation.But what is the risk, and how can it be managed?

Air crew radiation exposure—An overviewBY SUSAN BAILEY

Fig. 1. Galactic radiation levels (monthly means) at various altitudes, at the equator and at a highlatitude, January 1958–December 1997. Forty-year mean (minimum-maximum) effective dose rates arealso shown. The effect of the 11-year solar cycle on cosmic radiation levels is apparent at high latitude.

Equator (0°, 20°E)

Photo: Gregg M. Taylor

What exactly is it?Ionizing radiation particles (mostly protons

and alpha particles) enter Earth’s atmosphere,where they collide with nitrogen, oxygen, andother atoms, breaking apart their nuclei. Boththe charged particles entering the solar sys-tem and the secondary radiation they producein the atmosphere are referred to collectivelyas galactic cosmic radiation. Each disruptednucleus can itself yield multiple ionizing par-ticles, which can interact with other nucleiand produce still more particles, until, afterseveral interactions, they have lost the ener-gy to cause disruptions. The sun is also a con-siderable source of radiation; solar radiationand galactic cosmic radiation are commonlyreferred to jointly as cosmic radiation.

Wallace Friedberg, team leader of the ra-diobiology research group at the FAA’s Civ-il Aeromedical Institute (CAMI) in Oklahoma

City, Okla., explainedin a 1998 meeting pa-per that the effect ofgalactic cosmic radia-tion generally declineswith decreasing alti-tude. Radiation levelsare also lower near theequator than towardthe north and southpoles, Friedberg ex-plained, because theEarth’s magnetic field

deflects incoming galactic cosmic radiationparticles (particularly those with low energy).The effect of the Earth’s magnetic field isgreatest at the geomagnetic equator, which islocated near the geographic equator. Fromdata gathered during January 1958–Decem-ber 1997, Friedberg was able to estimate that

an airplane at an altitude of 20 000 feet at 70°North latitude (near the Arctic circle) wouldreceive galactic cosmic radiation a factor of2.0 higher than at the same altitude at theequator (see Fig. 1). He presented the paper,“Guidelines and Technical Information Pro-vided by the U.S. Federal Aviation Adminis-tration to Promote Radiation Safety for AirCarrier Crewmembers,” at the InternationalConference on Cosmic Radiation and AircrewExposure, Implementation of European Re-quirements in Civil Aviation, on July 1–3,1998, in Dublin, Ireland.

At every latitude, the altitude at which thedose rate is highest is different. The initial in-teraction of galactic cosmic radiation with theEarth’s atmosphere can be so intense that aunique phenomenon is observed at high alti-tudes above the equator: The intensity of theradiation is lower at 80 000 feet than at 60 000feet, where particle interactions reach theirpeak.

Friedberg was able to identify the particlesresponsible for the galactic cosmic radiationdose to an aircraft cruising at typical altitudesof 20 000–40 000 feet. At that altitude, andover his 40-year study period, 88–97 percentof the effective dose rate was from neutrons(33–52 percent), protons (21–28 percent), andelectrons and photons (17–41 percent) (seeFig. 2). Muons contribute 2–11 percent, andcharged pions less than 1 percent.

Cosmic radiation levels are never constant.Researchers have been taking measurementsfor nearly 50 years, and have identified an 11-year cycle of galactic cosmic radiation inten-sity, which is influenced by the sun’s activi-ty. Solar particle events (SPEs), also knownas solar flares, can occur at any time, but oc-cur more frequently during a few years of thatcycle, known as solar maximum. When thebasic dipole component of the sun’s continu-ally varying magnetic field reverses direction,solar minimum is reached, and SPEs are in-frequent and less powerful.

The solar wind (a plasma of solar radia-tion—mostly protons and electrons—ejectedfrom the sun) carries a convoluted magneticfield throughout the solar system. This wind

January 2000 N U C L E A R N E W S 33

Fig. 2. Percent contributions to the mean effective dose rate of galactic radiation by its components as related to altitude, at the equator and at a highlatitude, January 1958–December 1997 (Source: W. Friedberg, et al., ibid. Reproduced with permission from Nuclear Technology Publishing.)

High Latitude (70°N, 20°E)Equator (0°, 20°E)

(Source: W. Friedberg, et al. Reproduced from Cosmic Radiation and Aircrew Exposure, pp. 323–328, withpermission from Nuclear Technology Publishing, P.O. Box 7, Ashford, Kent TN23 1YW, England.)

High Latitude (70°N, 20°E)

Friedberg

contains more irregularities at solar maxi-mum, which makes the magnetic field changeand become unusually tortuous and strong.Since the ionized particles making up galacticradiation are electrically charged, they can beaffected by the highly ionized particles in thesolar wind. “Irregularities in the magneticfields carried by the solar winds scatter thelow-energy galactic particles that would oth-erwise enter the Earth’s atmosphere,” Fried-berg told Nuclear News. Because of this,galactic cosmic radiation is at a minimum dur-ing solar maximum, but during solar mini-mum, more of that radiation can reach theEarth. The most recent solar minimum oc-curred in early 1997, and solar maximum isexpected to begin ahead of schedule in May2000, according to Friedberg.

SPEs are usually too low in energy to con-tribute to radiation levels at the altitudes com-monly reached by a standard airplane. Occa-sionally, however, during solar maximum, thenumbers and energies of these solar radiationparticles increase enough to affect the cosmicradiation dose to air travelers. SPEs are short-lived: They commonly rise to a peak radiationlevel and then drop to near normal levels with-in two–three hours. Figure 3 illustrates solarproton levels, as measured by the NationalOceanic and Atmospheric Administration’sSpace Environment Center, on November 7,1997, when a measurable solar flare occurred(Fig. 4, by contrast, shows typical solar protonlevels).

The strongest SPE ever recorded occurredon February 23, 1956. At the time, no mea-surements of cosmic radiation at high altitudescould be made, and researchers have usedmodels to estimate the dose that a person fly-ing at a high altitude during the SPE wouldhave received. Paul Goldhagen, a physicistwith the Department of Energy’s Environ-mental Measurements Laboratory, in NewYork, N.Y., explained that researchers haveperformed extrapolations to determine that anSPE with the strength of the 1956 event couldmean a dose equivalent of more than 10 mil-lisieverts (mSv)/h to the passengers and crewof a high-latitude supersonic flight.

SPEs could have more of an effect on fu-ture high-flying jets. A hypothetical super-sonic transport (SST) would contain an on-board radiation monitor, as the Concorde doesnow, that could warn pilots of the onset of anSPE if air traffic controllers were not able todo so. If this monitor indicated an SPE wasoccurring, the plane could evade the radiationby lowering its altitude from 65 000 feet to ap-proximately 45 000 feet, or by moving to alower latitude. “That’s fine when you haveone plane flying,” said Goldhagen. “If you’vegot a hundred SSTs in the year 2050, it’s sud-denly more dangerous for them all to try to godown to a lower altitude at once than it is to sitthere and accept the radiation.” By that time,he theorizes, it may be possible to predictwhen an SPE is likely.

An SPE could also affect the SST of thefuture by interfering with the avionics on-board. A single event upset, or SEU, couldoccur if ionizing radiation damages a com-puter chip, and could put the aircraft in jeop-

ardy. It is possible, Goldhagen theorizes, thatif no steps are taken to prevent radiation fromupsetting avionics, the biggest health threatto passengers and crewmembers from radia-tion may be from resulting malfunctions inthe aircraft.

How large a dose?Friedberg has developed a computer pro-

gram that can estimate the galactic radiationdose received on a flight between any two

airports in the world. The latest version ofthat program, CARI-5E, is available on theWeb at <www.cami.jccbi.gov/AAM-600/610/600radio.html>. The program takes intoaccount the location of an airplane from take-off to touchdown, including the altitudesreached during the flight and the time spentat each altitude, as well as latitude and lon-gitude changes. Using the date of flight en-tered by the software user, CARI-5E adjustsits dose estimation to reflect the 11-year so-


A I R C R E W R A D I A T I O N E X P O S U R E

Fig. 4. Data acquired under normal solar conditions, by the GEOS-9 satellite, showing the intensity ofsolar protons in three energy ranges (>10 MeV, >50 MeV, and >100 MeV) (Source: NOAA)

Fig. 3. Data acquired during the solar particle event of November 4–7, 1997, by the GEOS-9 satellite,showing the intensity of solar protons in three energy ranges (>10 MeV, >50 MeV, and >100 MeV)(Source: NOAA)

lar cycle. The program can estimate dose onany flight from January 1958 to the present,providing a useful tool for epidemiologiststrying to assess possible health effects causedby long-term cosmic radiation exposures toair crew.

Table I illustrates typical mean, minimum,and maximum dose rates received on flightsover the last 40 years. According to Friedberg,a flight attendant working 700 block hours an-nually (measured from the time the aircraftleaves the blocks before takeoff to when itreaches the blocks after landing) flying be-tween Athens, Greece, and New York Citywould receive an annual occupational expo-sure of 4.2 mSv. Friedberg explained in his pa-per that based on the data in the table, the av-erage annual radiation dose to crewmembersfrom occupational and nonoccupationalsources can vary. For some, it can be close tothe amount received by the general population,and for others it can be twice that amount.

At the request of the National Aeronauticsand Space Administration (NASA), the Na-


NASA ER-2 taking off. Radiation sensors for the AIR measurements were carried in the nose, thefuselage behind the cockpit, and the front third of both wing pods. (Source: DOE EnvironmentalMeasurements Laboratory, presented at the 1998 NCRP Annual Meeting, “Cosmic Radiation Exposureof Airline Crews, Passengers, and Astronauts”)

TABLE I. EFFECTIVE DOSES OF GALACTIC COSMIC RADIATION RECEIVED ON AIR CARRIER FLIGHTS

Single nonstop one-way flight

Highest MillisievertsAltitude, feet Air time, Block Effective dose, per 100

Origin – Destination in thousands hours hours microsievertsa block hoursb

Seattle WA – Portland OR 21 0.4 0.6 0.14 (0.11 – 0.15) 0.02Houston TX – Austin TX 20 0.5 0.6 0.14 (0.12 – 0.15) 0.02Miami FL – Tampa FL 24 0.6 0.9 0.34 (0.28 – 0.36) 0.04St. Louis MO – Tulsa OK 35 0.9 1.1 1.57 (1.20 – 1.74) 0.14San Juan PR – Miami FL 35 2.2 2.5 4.84 (4.16 – 5.18) 0.19Tampa FL – St. Louis MO 31 2.0 2.2 4.31 (3.35 – 4.74) 0.20New Orleans LA – San Antonio TX 39 1.2 1.4 3.11 (2.54 – 3.31) 0.22Los Angeles CA – Honolulu HI 35 5.2 5.6 12.9 (11.5 – 13.3) 0.23

Denver CO – Minneapolis MN 33 1.2 1.5 3.54 (2.56 – 4.05) 0.24New York NY – San Juan PR 37 3.0 3.5 9.20 (7.52 – 10.1) 0.26Honolulu HI – Los Angeles CA 40 5.1 5.6 15.2 (13.4 – 15.8) 0.27Chicago IL – New York NY 37 1.6 2.0 6.09 (4.33 – 7.10) 0.30Los Angeles CA – Tokyo JP 40 11.7 12.0 38.0 (31.8 – 40.4) 0.32Tokyo JP – Los Angeles CA 37 8.8 9.2 30.0 (24.6 – 32.2) 0.33Washington DC – Los Angeles CA 35 4.7 5.0 17.2 (13.2 – 19.1) 0.34New York NY – Chicago IL 39 1.8 2.3 8.42 (5.93 – 9.85) 0.37

Minneapolis MN – New York NY 37 1.8 2.1 7.91 (5.54 – 9.26) 0.38London GB – Dallas/Ft. Worth TX 39 9.7 10.1 38.8 (27.6 – 45.1) 0.38Lisbon ES – New York NY 39 6.5 6.9 27.3 (20.5 – 31.1) 0.40Dallas/Ft. Worth TX – London GB 37 8.5 8.8 35.3 (24.8 – 41.4) 0.40Seattle WA – Anchorage AK 35 3.4 3.7 15.1 (10.4 – 17.8) 0.41Chicago IL – San Francisco CA 39 3.8 4.1 17.7 (13.2 – 19.8) 0.43Seattle WA – Washington DC 37 4.1 4.4 20.4 (14.3 – 23.8) 0.46London GB – New York NY 37 6.8 7.3 34.0 (23.8 – 40.0) 0.47

San Francisco CA – Chicago IL 41 3.8 4.1 19.5 (14.2 – 22.1) 0.48New York NY – Seattle WA 39 4.9 5.3 25.6 (17.7 – 30.1) 0.48New York NY – Tokyo JP 43 13.0 13.4 67.1 (48.3 – 77.7) 0.50London GB – Los Angeles CA 39 10.5 11.0 55.2 (38.5 – 64.9) 0.50Chicago IL – London GB 37 7.3 7.7 38.7 (26.6 – 45.8) 0.50Tokyo JP – New York NY 41 12.2 12.6 63.5 (44.3 – 74.8) 0.50London GB – Chicago IL 39 7.8 8.3 43.3 (29.6 – 51.6) 0.52Athens GR – New York NY 41 9.4 9.7 58.2 (42.3 – 67.0) 0.60

a Mean (minimum–maximum) effective dose, January 1958–December 1997. (Source: W. Friedberg, et al., ibid. Reproduced with b Based on the mean effective dose for the one-way flight. permission from Nuclear Technology Publishing.)


tional Commission on Radiation Protectionstudied radiation exposure and high-altitudeflight, and in July 1995 published Commen-tary 12, which recommended that “averageabsorbed dose rates and their uncertainty inthe altitude range of 30 000 to 80 000 feet re-quire greater specification, and additionalmeasurements utilizing currently flyinghigh-altitude aircraft should be made withadequate instrumentation” to eliminate thatuncertainty.

Since then, the NASA High Speed Re-search Project Office, working out of theagency’s Langley Research Center under thedirection of John W. Wilson, has attempted tocharacterize radiation conditions at high-alti-tude flight. Assisting in the mission were theDOE’s EML, NASA’s Johnson Space Center,the Canadian Defense Research Establish-ment and Royal Military College, GermanAerospace Research Establishment, U.K. Na-tional Radiological Protection Board, BoeingCompany, and several researchers from do-mestic and foreign universities.

Galactic cosmic radiation measurementswere made using a converted military spyplane, the ER-2, which can reach altitudes of75 000 feet. Behind the mission, known asthe Atmospheric Ionizing Radiation (AIR)project, was the need for accurate character-ization of radiation levels at the high alti-tudes that would be reached by a hypotheti-cal SST.

The ER-2 plane was used on a series offive missions flown from NASA’s Ames Re-

54Fig. 5. Flight paths ofthe AIR measurement

ER-2 flights (Source:DOE Environmental

MeasurementsLaboratory, presented

at the 1998 NCRPAnnual Meeting, ibid.)



search Center at Moffett Field, Calif., in June1997 (during the last solar minimum), whengalactic cosmic radiation exposures could beexpected to be at their highest (see Fig. 5).Fourteen different instruments were con-tained within the plane, including a multi-sphere neutron spectrometer, ionizationchamber, and scintillation counters fromEML, two spherical tissue-equivalent pro-portional counters, and two particle tele-scopes. Data from the flights were collectedto allow further refining of the AIR model ofthe galactic cosmic radiation environment,and to permit precise dose estimates for highaltitudes.

Regulators have their sayIt has been acknowledged that the radiation

doses crewmembers receive constitute occu-pational exposure, but should this exposure belimited? Governments worldwide are now de-ciding how to address the risk of radiation toair crew.

In the United States, the FAA has publisheddocuments discussing air crew radiation ex-posure, and has issued recommendations toairlines on educating air crew about therisks—but it has not issued dose limits. U.S.airlines have not voluntarily adopted trainingor dose monitoring programs similar to thosein the nuclear power industry. Friedberg toldNuclear News that his research group atCAMI “will continue providing informationand making recommendations. We don’t haveany regulations, however, at the moment. Thatis something that may or may not happen inthe future. . . . If there is enough of a reactionin the aviation community [the FAA] mightfeel compelled to do it.”

As shown in Table II, air crew typically re-ceive more radiation exposure than radiationworkers at a nuclear facility. Most groups of“terrestrial” radiation workers include a num-ber of people whose occupational exposure isnear zero, which lowers the average effectivedose. All air crew, however, are exposed to un-avoidable radiation for the duration of a flight,so their average effective dose is relativelyhigh.

According to Robert Barish, of New York,N.Y., a medical physicist and certified healthphysicist who has worked in radiation oncol-ogy, and who speaks and writes about cosmicradiation, considering that crewmembers areoccupationally exposed to radiation, “they area legitimate, regulated radiation cohort thatshould be told about the risks. Unfortunately,the airlines have never told them.” In Europe,by contrast, airlines must begin informing aircrew of their radiation doses in May 2000.The European Union’s radiation protectionpolicy is contained in the EU Directive on theprotection of workers and members of thepublic against the hazards of ionizing radia-tion (96/29/Euratom), which is binding on allmember states and is revised every 10–12years.

The Directive generally matches ICRP rec-ommendations, and requires that the dose re-ceived by any crewmembers who may re-ceive more than 1 mSv per year should beassessed. In addition, airlines will be required

to organize the schedules of crewmemberswith the objective of reducing the doses ofhighly exposed air crew, educate the crewabout health risks, and give special protec-tions to women who have declared pregnan-cy. Flight attendants at British Airways, forexample, are “grounded” immediately afterdeclaring pregnancy, and are given othertasks until they take maternity leave, accord-ing to Michael Bagshaw, head of MedicalServices at the airline. Individuals who mayreceive more than 6 mSv per year may besubject to more stringent measures after May2000, such as warning signs or individualdosimetry.

What do crewmembers think?The International Federation of Air Line Pi-

lots Associations (IFALPA) is trying to makeits concern about radiation exposures known.IFALPA represents approximately 100 000pilots from pilot associations in about 90 dif-ferent countries, including the United States.The organization’s Human PerformanceCommittee held a meeting in October 1998,and produced a policy proposal similar to theEU Directive that states, among other things,that long-range airplanes normally operatedabove 8000 m (26 000 ft) should carry equip-ment to measure and indicate continuously thedose rate of total cosmic radiation being re-ceived, the cumulative dose on each flight,and the presence of any solar flares. Allcrewmembers, the policy said, should be al-lowed to adjust their flying schedules so thatthey do not exceed an annual threshold limitof 6 mSv/year.

The policy was approved by IFALPAmember associations at a conference held inApril 1999. Herbert Meyer, senior technicalofficer at IFALPA headquarters in the UnitedKingdom, admitted, “this issue has been thesubject of extensive debate within IFALPAand the Human Performance Committee inparticular, and as is the case in the entire sci-entific community, no conclusive findingshave been reached.” The policy approved inApril was revised by the IFALPA Human Per-formance Committee at an October 1999meeting. The revised policy, not yet approved

by the IFALPA member associations, readsas follows:

“IFALPA policy recognises 20 mSv/yr asthe cosmic radiation limit for airline flightcrews as established by the National Councilof Radiation Protection and Euratom. It is fur-ther recognized that airline flight crewsshould be categorised as occupationally ex-posed radiation workers, likely to receivemore than 1 mSv/yr. As cosmic radiation im-poses a potential health risk to airline flightcrews, it is highly recommended that nation-al authorities make provisions for exposureassessment verification. . . . Crew membersshould be made aware through extensive ed-ucational programs that high altitude flyingexposes them to significantly higher ionisingradiation levels, with carcinogenic potential,than the general population and the scope ofradiation protection legislation. . . . Flightcrew members should be warned that radia-tion exposure above 1 mSv during the courseof the entire pregnancy may cause an in-creased risk to the fetus. Operators shouldhave provisions in place to adjust flight du-ties (low altitude flights that minimise expo-sure/ground duties) so that this limit is not ex-ceeded after declaration of pregnancy by theflight crew member.”

Emily Carter, national health coordinatorfor the Association of Professional Flight At-tendants (APFA), says that most flight atten-dants on U.S. carriers have an “underlyingknowledge” that cosmic radiation could posea threat, but that they currently don’t feelthere is anything they can do about it. Thismay change, however. U.S. flight attendantshave the opportunity to interact with their in-ternational counterparts, and as Europeanflight attendants begin to monitor their expo-sures in May 2000, those from the UnitedStates might begin to wonder why they don’tget dose estimates as well, said Carter. “Theflight attendants on U.S. carriers are going tobe very angry that they’re exposed to this, butthey’re not going to want to cut down theirlifestyles,” she predicted. “They’re just go-ing to get angrier.” Although the mitigationor monitoring of radiation doses has not beenan issue in contract negotiations between

TABLE II. ANNUAL EXPOSURE, QUALITY OF EXPOSURE, AND RISK COEFFICIENT

UNCERTAINTY OF U.S. RADIATION WORKERS

Exposure Quality Distribution Risk uncertaintyGroup (mSv) Low LET, % High LET, % factor

Terrestrial occupations 2.2 93 7 2.1-3.8

Aircraft:subsonic 5-9 32 68 3.4-10.8supersonic 8-17 32 68 3.4-10.8hypersonic 14-21 28 72* 3.4-11.6

Low earth orbit:low inclination 17 62 38 2.8-7.6high inclination 144 34 66* 3.3-10.9

Deep space 500 14 86** 3.7-13.3

*Significant exposures to HZE (highly charged, energetic) ions.**Exposure dominated by HZE ions (Source: NASA, presented at the 1998 NCRP Annual Meeting)

Although the ER-2 plane did not leave the Earth’s atmosphere, its datais still useful to the NASA researchers pondering a mission to Mars.

According to Robert Singleterry, a research scientist at NASA’s LangleyResearch Center (LRC) who participated in the high-speed research proj-ect, the radiation environment 70 000 ft above the Earth is very similarto that on the surface of Mars.

The concept of aerial density, measured in g/cm2, allows NASA re-searchers to make comparisons between different atmospheres, even ifthey are composed of different elements, Singleterry explained. Aerialdensity is the sum of the mass of all the matter, in this case molecules ofatmospheric gas, above any given cm2 area. The measurement is signifi-cant because it identifies the amount of mass between an incoming cos-mic radiation particle and a planet or other object, according to Sin-gleterry. “[The number of] molecules the cosmic ray would see as itcomes in to penetrate the atmosphere and either hit the Martian surfaceor hit the airplane would be the same,” he said.

Astronauts face a greater likelihood of suffering health effects fromoccupational exposure to cosmic radiation than do air crew (see TableII). They are exposed to more radiation, and a large percentage of the ra-diation is high-LET (linear energy transfer) particles. If astronauts wereinadequately shielded at the time of a large solar particle event, life-threat-ening injuries could occur, as John Wilson, senior research scientist atLRC, explained in “Overview of Radiation Environments and Human Ex-posures,” a paper he presented at the 1998 NCRP Annual Meeting.NASA’s researchers cannot guess when a large SPE might happen; theymust therefore design equipment that can help astronauts withstand thelargest possible solar radiation exposure.

Extensive experimentation must still be conducted, because, as Wil-son told Nuclear News, “space exposures are quite different than any-thing that is experienced either at high altitude or on the surface [ofEarth].” Even on missions to the Moon, spacecraft receive some pro-tection from the Earth’s magnetic field, but there will be no such pro-tection in deep space.

NASA has moved away from the traditional radiation protection prac-tice of setting a radiation dose limit and regulating to keep exposures be-low that limit, Wilson said. Instead, the focus is starting to shift to con-trolling the excess fatal cancers astronauts could develop later in life as aresult of a deep space mission. Currently, NASA has the goal of limiting theprobability of an astronaut’s developing fatal cancer to less than 3 percent.

“We’re willing to limit the risk to 3 percent,” said Wilson, “but we don’tknow how to limit it, and we don’t know what kind of shielding materialsare going to be the best way to help out.” If NASA is to achieve its officialgoal of amassing enough technical information by 2004 to make an edu-cated decision on whether to attempt a manned mission to Mars in 2014,those problems will have to be solved soon, according to Wilson.

Singleterry, a former DOE contractor, explained the difficulties of at-tempting to shield astronauts. While terrestrial radiation workers can lim-it their time near a radiation source, distance themselves from the source,and erect thick shielding, an astronaut cannot. Because any additionalweight added to a spacecraft increases its cost, NASA engineers are forcedto be creative. Aluminum, which is commonly used in spacecraft, doesnot shield people from damage as well as water or liquid hydrogen can,for example. “It’s really hard to build stuff with liquid hydrogen,” said Sin-gleterry, “but it’s a great fuel.” If astronauts were surrounded by their liq-uid hydrogen fuel, they would be better protected. The effectiveness ofthat shield, however, would decrease as the mission wore on. NASA hasconsidered providing small, heavily shielded “storm shelters” onboard towhich the astronauts could retreat during an SPE.

NASA must also worry about protecting equipment from single eventupsets. NASA’s existing space shuttle experiences approximately 400computer upsets during a two-week mission, according to Singleterry.For that reason, the shuttle has five redundant computers. The comput-ers “vote” on critical decisions; any computer that has been damaged andproduces faulty information is overruled by the others. NASA is able totest the ability of some equipment to withstand tough radiation environ-ments by irradiating it with an electron beam. “For deep space, we’re go-ing to have to start radiating some of these parts in front of heavy, high-energy ion beams,” said Singleterry.

NASA researchers currently use Brookhaven National Laboratory’s Al-ternating Gradient Synchrotron (AGS) to test spacesuit and spacecraft ma-terials in high radiation environments similar to those in space. Biologistsuse the same beam to irradiate cell cultures, mice, and rats, according toSingleterry. They are trying to learn what damage high-LET radiation cando to individual cells, and how a biological system reacts to that damage.NASA is building a permanent structure at BNL to perform a large vol-ume of similar tests in support of a possible future manned mission to Mars.The Booster Application Facility, due to open in 2002, will be an adjunctto BNL’s AGS.—SB

Far out!

Artist’s conception of a manned Marsmission, by Pat Rawlings, of SAIC, forNASA. The radiation environment 70 000 ftabove the Earth is very similar to that onthe surface of Mars. (Source: NASA)


flight attendant unions and airlines, Carter ex-pects that within six years it will become anegotiating point.

It is not easy for concerned flight attendantsto estimate their own exposures. “Dosimetrybadges were pretty much discouraged becauseit’s going to give them a false sense of secu-rity,” said Carter. Traditional dosimetrybadges cannot accurately measure the typesof radiation found at high altitudes. However,“the company would never let you [wear dos-imetry badges],” Carter said, “because of theperception that the passengers would have.”Flight attendants could use a computer pro-gram such as the FAA’s CARI-5E to make es-timates, but the most accurate measure of thedose received on any flight would come froman onboard radiation monitor.

Although some U.S. airlines permit flightattendants to continue flying until the 26thweek of pregnancy, according to Carter,APFA distributes materials to its membersfrom the FAA’s Civil Aeromedical Institutewhich suggest caution during pregnancy.Flight attendant unions such as the APFAare “the only people that are going to [edu-cate the flight attendants about cosmic radi-ation] because the airlines aren’t going to doit. . . . They’re not made to, so why shouldthey?” said Carter. The airlines have notchosen to educate their employees, in part,Carter believes, because they do not wantthe traveling public to become alarmed. Al-though cosmic radiation has not receivedmuch attention yet, “the minute we get helpwith it, it’s going to become a major con-cern. And the help is going to come from theEuropean group.”

Radiation protectionBut is the regulation really necessary? Af-

ter all, crewmembers are at no risk of receiv-ing an extremely high dose such as could re-sult during an accident at a nuclear facility.The primary health effect is radiation-inducedcancer. But if that cancer risk is practically un-detectable, given the normal risk everyonefaces, is there any cause for concern?

Researchers who study cosmic radiation,with its increased amounts of high-LET(linear energy transfer), particles originat-ing in deep space, have not been able toidentify conclusively the health effects thatmay result from the exposure an averagecrewmember receives. Most of the data thatradiation protection specialists have at theirdisposal is from research on low-LET radi-ation. Friedberg, whose group at CAMIprovides the research behind the FAA’s rec-ommendations, explained that “our grouphas made risk estimates, but I do it withgreat caution in that I tell people that theyshouldn’t take it overly seriously. We don’tknow enough about the cosmic radiationenvironment.”

The most recent cancer risk estimates pub-lished by the FAA, in 1992, appeared in “Ra-diation Exposure of Air Carrier Crewmem-bers II.” Friedberg warned that while thegeneral discussion in the document is stillcorrect, the dose figures and risk estimateswithin would be different if he recalculated

using current, improved data. The documentstates, for example, that “if a crewmemberworked 700 block hours a year for 30 years,exclusively on flights with average en routealtitudes of 33–40 thousand feet between thecontiguous United States and Europe, thenthe estimated risk of radiation-induced fatalcancer would be between 1 in 250 and 1 in120.” One in five U.S. adults in the generalpopulation will likely die of cancer, the doc-ument continues, so “the likelihood of de-veloping fatal cancer because of occupation-al exposure to galactic radiation is a smalladdition to the general population risk.” Anincreased risk of childhood cancers and birthdefects, particularly mental retardation, canresult from extended radiation exposures topregnant crewmembers, according to thedocument.

The 1992 document acknowledges the an-nual occupational radiation exposure limit of20 mSv suggested by the ICRP, and states thatestimated doses to crewmembers are “con-siderably lower” than the limit. For pregnantflight crewmembers, however, “once a preg-nancy is known . . . the dose equivalent to theunborn child from occupational exposureshould not be more than 0.5 mSv in anymonth.” For radiation protection purposes, thedose equivalent to a fetus is considered to bethe same as that received by the mother.

Monitoring all flight crew members is nosmall task: In the United States alone, thereare more than 160 000 people who make a liv-ing on airplanes. Beginning in May 2000,however, airlines in Europe will have to dojust that. “I think the European Union is ontrack to realize its deadline of May 2000 forhaving in place a way to determine the dosesfor air crew and I think that’s an appropriatething to do,” said the DOE’s Goldhagen.

Airborne ALARARadiation protection specialists employed

at nuclear power plants continually try to re-duce the dose their workers receive, and theindustry has benefited from successfully low-ering exposures. The principle of ALARA,keeping dose “as low as reasonably achiev-able,” is encouraged by regulators. But wouldthe same principle work onboard airplanes?Goldhagen said, “Merely paying attention toALARA constantly makes the allowable lim-its tend to creep downward, which is a goodthing on average. But the airplane case is dif-ferent, because you can’t put up more shield-ing or spend less time near the source [as youcan in a nuclear power plant], which is whatthey had in mind.”

Richard Killick, a director of the consultingfirm Sage Safety and a former director of safe-ty and quality for Scottish Nuclear before itsprivatization, believes that poor public rela-tions and secrecy in the early days of the nu-clear industry created a climate of distrust. Ex-pensive lawsuits and claims for compensationfrom alleged radiation-induced illnesses haveincreased. In a paper presented at the AviationHealth Institute meeting in June 1999, in Lon-don, “ALARA—‘As Low As ReasonablyAchievable’: Applying Lessons from the Nu-clear Industry to Aviation,” Killick advised

airlines to take preventive actions. “A verycost-effective lesson that the airline industrycan learn from the nuclear industry,” he said,“is to put in place an industry-wide procedurefor the practical and quantifiable applicationof the ALARA principle.” Airlines should,Killick said, be prepared to offer comparisonsbetween cosmic radiation and radiation fromother natural and manmade sources. Also, air-lines should establish a legal procedure to cal-culate the appropriate reimbursement for dif-ferent exposure levels before compensationclaims are made, Killick suggested.

Killick discussed dose-reduction measuresthat could effectively lower the dose expo-sure of air travelers. Flying at lower altitudes,or a different latitude, would certainly resultin a lower dose, but is impractical. Cuttingback on the flying times for air crew wouldrequire more trained staff, at a huge expense.While heavy lead shields could not be in-stalled in airplanes, the structure of the plane,the bulk of luggage, and the bodies of otherpassengers all offer shielding, which meansthat radiation levels are different in differentparts of the plane. The occupants of windowseats, for example, receive more dose thanthose in the aisle. Killick suggested theunique idea of moving the air crew, pilots in-cluded, to areas of the plane with lower radi-ation levels.

In an April 1999 article published in theU.K. NRPB’s Radiological Protection Bul-letin (which was reproduced in the HealthPhysics Society Newsletter of June 1999),“Equalising the Window Seaters: PracticalControl of Cosmic Ray Doses During AirTravel,” Gerald M. Kendall of the NRPB out-lined a recent design study by a consortium ofaircraft manufacturers that involved the in-stallation of a “modified mechanical convey-or” in an airplane cabin. In this concept, pas-senger seats would move slowly, takingapproximately 45 minutes to travel throughthe cabin, spending equal time in “windowseats” and the center of the plane. Thearrangement, Kendall suggests, has some sec-ondary benefits. Passengers could disembarkin an orderly fashion, as their seat passes thedoor of the aircraft. Drinks and food could bedistributed self-service, as passengers pass acafeteria area. The result would be a cut instaffing needs. The U.S. Marines, Kendall re-ported, are testing the idea in modified troopcarrier aircraft.

Of course, concerned individuals couldlower their exposures by simply not boardingan aircraft, but flying is a practical means oftravel, and cosmic radiation poses no harm tothe ordinary traveler. But what if a pregnantwoman, for example, wanted to avoid the riskof flying during an SPE? Robert Barish hasbegun a telephone service, which, for theprice of $3, the traveler can call from the air-port. If radiation levels are higher than nor-mal, she will be advised that she may wish todelay her flight for a few hours. “If you’repregnant, particularly at an early stage, andyou can avoid an exposure by waiting a fewhours until the radiation has subsided, I thinkthat’s in conformity with ALARA,” Barishsaid.



air crew radiation exposure - an overview

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