**Section II, 2.14, Exposure to Radioactive Materials**

Ionizing radiation exposure is an occupational hazard for aerospace
industry workers. From mechanics and baggage inspectors to pilots and
flight attendants, all are subject to exposure to radiation from many
sources. These include radioactive materials in cargo and airframes,
x-ray inspections of baggage and aircraft structures, and cosmic
radiation in flight. While background radiation from affects all humans,
medical x-rays remain the largest single source of radiation exposure
for the average person, including aerospace workers.

A number of radioactive materials may be present in the aviation
environment. These may be carried as cargo, comprise part of aircraft
structure, or be used as detector sources while the aircraft is on the
ground.

+-----------------------------------------------------------------------+
| Radiation Terminology                                                 |
|                                                                       |
| 1 Gray (Gy)=100 Roentgens (Rads)=1 Joule/Kg absorbed dose             |
|                                                                       |
| 1 Sievert (Sv)=100 Roentgen Equivalent Man (REM)=1 Gray biological    |
| effective dose                                                        |
|                                                                       |
| 1 Sv = 1000 milliSievert (mSv) = 1,000,000 microSievert (μSv)         |
+-----------------------------------------------------------------------+

Natural background radiation sources are divided into terrestrial
radiation and cosmic radiation. Terrestrial radiation originates from
radioactive atoms normally present in the soil, our bodies and the air.
Many naturally occurring rocks of the earth's surface contain
radioactive elements, particularly those of volcanic (igneous) origin.
These can find their way into the air we breath (as radon and carbon-12)
and the food and water we consume.

Cosmic radiation has extraterrestrial sources consisting of the solar
wind and galactic cosmic rays. Solar cosmic radiation originates as
particles ejected from the sun. These are primarily protons and
electrons which form the solar wind. Most are trapped by the earth's
magnetic fields, forming the Van Allen Belts. Relatively little
penetrates deeply into the earth's atmosphere. Galactic cosmic rays are
high energy particles, mostly nuclei of atoms ranging from helium to
iron, which originate outside our solar system.

There is considerable variation in the amount of cosmic radiation which
reaches the earth and in particular the amount which can affect flight
crews. The main variables are the flight duration and altitude,
geographic latitude and the solar cycle. In general, radiation increases
with altitude above the earth up to about 70,000 feet. At an altitude of
35,000 feet the dose equivalent rate from galactic cosmic radiation is
about 5.0 microsieverts per hour (5.0 ųSv/Hr.). Spaceflights above 300
nautical miles enter the Van Allen belts with a dramatic increase in
radiation levels, an item of concern for astronauts. An astronaut in the
Van Allen belt without shielding could be exposed to over 500 ųSv/Hr.

The earth\'s magnetic field deflects many charged particles of both
solar and galactic origin that would otherwise enter the atmosphere.
This shielding is most effective at the equator where the earth\'s
magnetic lines of force are essentially parallel to the surface of the
earth. This deflection decreases with increase in latitude and
disappears almost entirely over the poles where the magnetic lines of
force are nearly perpendicular to the surface of the earth. Therefore,
at air carrier cruise altitude, the galactic radiation dose equivalent
over the poles is approximately twice that over the equator (10
ųSv/Hr.). Air carrier aircraft may fly these high latitude routes
between the contiguous United States and Europe or Asia. The geographic
latitude is the result of the earth's magnetic fields. At latitudes
greater than 50^o^ north or south, a flight can leave the protection of
these fields and increase the exposure.

The eleven year solar cycle follows the rise and fall of solar activity
and sunspots. During a "Quiet Sun Year", solar eruptions, flares and
sunspots are at a minimum and the solar wind output also is at a
minimum. This allows an increase of Galactic Cosmic Rays to reach the
inner solar system and earth. During solar maximums, the solar wind
blows the Cosmic Rays out of the solar system, but there are higher
levels of protons and electrons in the Van Allen belts, the source for
aurora. The effect near the equator is minimal, but at high latitudes,
radiation exposures to cosmic rays can more than double during solar
minimums.

At sea level the minimal amount of cosmic radiation which has reached
the earth\'s surface is equivalent to approximately 0.06 microsieverts
per hour (.06 ųSv/Hr.) or 0.6 mSv/Yr. At higher elevations more cosmic
radiation reaches the ground. When all contributions to exposures from
natural sources of radiation are taken into account, the average annual
sea level dose is closer to 3.0 mSv/Yr. This varies considerably due to
local geography and altitude, with residents in a city like Denver
receiving twice the dose of Miami but half the dose of Manhattan (a
massive, radioactive granite intrusion) in New York.

Radioactive material transported in aircraft consists mostly of
pharmaceuticals used in medical diagnosis and treatment. D.O.T.
Regulations are specific as to packaging and storage of such cargo in
order to limit radiation levels in areas occupied by people or animals.
Exposure from this sort of radiation is very low. An early Nuclear
Regulatory Commission study of passenger aircraft that transport
radioactive cargo in the U.S. estimated that the average annual
radiation dose to crew members from that cargo amounts to less than 10
percent of the estimated total annual sea level radiation dose or about
0.3 mSv. A typical chest x-ray represents an effective dose of about
0.1mSv while a gastrointestinal series can be as high as 20mSv.
Friedberg estimated that on U.S. commercial aircraft, flight attendants
received doses of 0.06 mSv, and pilots less than 0.01 mSV, due to
transported radioactive cargo. This is a relatively small contribution
to total body exposure.

Some aircraft use depleted uranium ore (from which the unstable isotopes
U233, U234, and U235 have been removed), and built into the control
surfaces or used for balance. The most notable aircraft include military
transports such as the C141B Starlifter and Navy fighter F14A Tomcat.

Depleted uranium is even denser than lead, it also has been used in
military warheads. The material may be carried in the form of cannon
shells, particularly by such aircraft as the A10 Thunderbolt 2, which
potentially exposes ground crew and pilot to radiation.

Additional radioactive material may be introduced as tracers or during
nondestructive inspection of the aircraft and its structures during
manufacture and ground maintenance. Similar to medical radiography,
X-rays and other isotopes are used to inspect and evaluate structural
integrity. Additional radiation exposure occurs during baggage screening
and security screening. The bulk of this exposure, however, is an
occupational hazard to the security personnel, and not to passengers or
air crew. A review of baggage radiographic machines reveals as many as 8
percent exceeded the emission limits and posed an occupational hazard to
the operators.

Given what we know about radiation exposures in the aviation
environment, there are several means of obtaining estimates of the
amount of cosmic radiation received by crew members during particular
flight segments. Approximations can be garnered by multiplying block
hours by average exposures as described above, and for most crew members
this will suffice. The average air crew dose will probably lie in the
range of three to six millisieverts per year (3 to 6 mSv/Yr.), with the
amount of individual radiation depending on number of flight hours,
flight altitude and latitude, and solar activity. Recent British
Airways\' research looking at high altitude, long duration flights gave
an effective dose rate of 3.5 mSv/Yr. Slightly higher doses are recorded
for Concorde crews; slightly lower, for short-haul crews.

The FAA has developed a computer software program for public use,
entitled \"CARI-6\" that provides an estimated equivalent dose for a
particular flight when certain parameters of the flight are supplied to
it. A free download is available through the Internet at
http://www.cami.jccbi.gov/research/610/600radio.html.

There are a number of national and international organizations that
provide guidelines for ionizing radiation exposure limits. The Nuclear
Regulatory Commission (NRC), the Environmental Protection Agency (EPA),
the International Commission on Radiological Protections (ICRP), and the
National Council on Radiation Protection and Measurement (NCRP), have
all weighed in with standards and guidelines. The recommended radiation
exposure for workers in the nuclear industry is 20 mSv/Yr. averaged over
five years (ICRP - 1991). Occupational exposure limits are far below the
level required to produce acute syndromes. The FAA-recommends the NRC
limits for air crew: a five-year mean effect of dose of 20 mSv per year,
with no more than 50 mSv in a single year. For pregnant air crew, the
limit should be 1 mSv, with no more than 0.5 mSv per month. If an
employee\'s annual exposure exceeds 6 mSv per year, medical surveillance
is recommended (NCRP - 1993).

The National Radiological Protection Board (NRPB) of the U.K. recommends
that the exposure of pregnant women should be \"as low as reasonably
achievable\" and such as to make it unlikely that the equivalent dose to
the fetus will exceed 1 mSv during the remainder of the pregnancy.

The effect of these two different approaches to pregnancy results in
different occupational treatment of pregnant aircrew. The US standard
allows pregnant aircrew to fly during the first and second trimesters
while the UK standard has been used to ground aircrew on discovery and
declaration of pregnancy.

These recommended exposure limits have been established based on an
attempt to keep the risk of adverse effects at a minimum. In general,
exposure limits for both the individual member of the public and the
occupationally exposed worker in the nuclear industry have continued to
be reduced over time.

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Baggage is screened by xray or neutron scanner with potential
occupational exposure to TSA employees. These workers must be part of a
radiation safety program.

The biological effects of radiation exposure are attributable to the
amount and quality of the radiation. The highest exposure of most
individuals will be to medical radiation in the form of diagnostic
x-rays. Workplace exposures include diagnostic x-rays, alpha-emitting
isotopes, the occasional beta and gamma isotopes, as well as solar and
galactic cosmic radiation.

Unlikely to be encountered in aviation are acute tissue radiation
sickness, tissue damage and fatally high levels of radiation. However,
flight crew members aviation workers and security personnel exposed to
low doses of ionizing radiation over a lifetime of employment are at
risk for radiation effects such as cancer, genetic defects induced in
sperm and ova, as well as fetal radiation injury in utero.

With regard to cancer risk, Freidberg has estimated that a crew member
exposed to a 25-year career of flying between New York and Chicago would
have a 0.3 percent increased chance of developing cancer, and this would
need to be compared to the background 24 percent risk of dying from
cancer in the general population. This can be attributed to the
generally healthy lifestyle of most air crew members, who smoke at a
lower rate, and exercise at a higher rate, in addition to having
frequent medical evaluations. It is, however, difficult to derive
epidemiological data at low levels of increase and relative risk.
Genetic defects are also difficult to estimate. The National Research
Council has estimated that the incidence of a severe genetic defect will
be increased by 0.004 percent if one parent was exposed to radiation
prior to conceiving a child. However, the general population risk of
congenital abnormalities is in the 2-3% range.

High doses of radiation lead to acute radiation syndrome. These effects
are well known and derived from atomic bomb survivors. An acute dose of
1.0 to 2.5 Sv (100-250 REM) can lead to a hematopoietic syndrome that
occurs with suppression of the bone marrow. This leads to susceptibility
to infection and hemorrhage. When aggressively treated, (administering
platelets, preventing infection, replacing bone marrow, and transfusing
blood, the syndrome is still 50 percent lethal. At a higher dose,
ranging from 2.5 to 5.0 Sv (250-500 REMS), a gastrointestinal syndrome
appears within a week. In this syndrome, there is cell death within the
lining of the gastrointestinal tract due to generation of cytoplasmic
free radicals. A massive loss of fluids and electrolytes occurs,
resulting in an electrolyte imbalance and hypotension. Treatment focuses
on replacing lost fluids. This syndrome is typically 90 percent fatal.
At doses above 5 Sv (500 REMS), a neurovascular syndrome occurs within a
few days or even hours. This is caused by massive and irreversible
damage to the central nervous system. This syndrome is 100 percent
fatal. Levels such as these are not possible in the aerospace
environment with the possible exception of astronaut exposure during a
major solar flare. Research to prevent and predict such events is
ongoing.

References:

Bagshaw M, Irvine D, Davies DM. Exposure to cosmic radiation of British
Airways flying crew on ultra-longhaul routes. Occupational and
Environmental Medicine, 1996; 53:495-98.

Bartlett DT. Cosmic radiation fields at aircraft altitudes and their
measurement. Proceedings of Royal Aeronautical Society Symposium on
In-Flight Cosmic Radiation. London: RAS, 1997.

Beir 1990. Committee on the Biological Effects on Ionizing Radiations.
Health Effects of Exposure to Low Levels of Ionizing Radiation. BEIR V.
Washington, D.C.: National Academy Press.

DOT 1990. Federal Aviation Administration. Advisory Circular: Radiation
Exposure of Air Carrier Crewmembers. March 5, 1990. AC No.: 120-52.

EPA 1987. Environmental Protection Agency. Radiation Protection Guidance
to Federal Agencies for Occupational Exposure. Federal Register 52(17)
Tuesday, January 27, 1987, pp. 2822-2834.

Freidberg, W., Copeland, K., Duke, F., Nicholas, J., Darden, E., and
O'Brien, K. Radiation exposure of air crews, Occupational Medicine State
of the Art Reviews", Volume 17, No. 2, April, June, 2002, 293-309.

Geeze DS. Pregnancy and in-flight cosmic radiation. Aviat Space Environ
Med 1998; 69:1061-4.

International Commission on Radiological Protection. Recommendations of
the International Commission on Radiological Protection. ICRP
Publication 60. Annals of the ICRP 21 (1-3). Oxford: Pergamon Press,
1991.

National Council on Radiation Protection and Measurement. Limitation of
exposure to ionizing radiation. Bethesda, MD: National Council on
Radiation Protection. Report No. 116. 1993.

O\'Brien K, Friedberg W, Sauer HH, Smart DF. Atmospheric cosmic rays and
solar energetic particles at aircraft altitudes. Environment
International 1996; 22 (Suppl. 1): S9-S44.

Oksanen PJ. Estimated individual annual cosmic radiation doses for
flight crews. Aviat Space Environ Med 1998; 69:621-5.

Walker, R., Cerveny, T., "Medical Consequences of Nuclear Warfare",
Armed Forces Radiobiology Research Institute, Pages 15-36, 1989.

Thibeault, C. "The Impact of the Aerospace Industry on Environment and
Public Health", Chapter 30, 645-668, and Fundamentals of Aerospace
Medicine, 3rd Edition, DeHart and Davis, 2002.