Radiation and Air Carrier Crew Members
The earth is continuously bombarded from space by
high energy sub-atomic particles. As near as we can tell, this has been the environment with which we humans have
contended for our entire history on the planet.
These particles and rays racing toward our globe originate either from the sun or from other stars and enter
our atmosphere traveling at a very high velocity, in some cases nearly that of the speed of light. As they enter
the upper reaches of our atmosphere, they collide with atoms already present and produce secondary rays which have
considerably less energy than the primary bombardment, but they are still capable of penetrating the lower layers
of the atmosphere. Some of these reach the surface of the earth.
The intensity of this radiation and its power diminish rapidly with decreasing altitude below 50,000 feet, as
further collisions occur with the molecules of the air. Thus, at sea level the effects of cosmic radiation are
only about one-seventieth of that encountered at an altitude of 70,000 feet. It follows that as we climb in altitude,
our exposure climbs as well.
The purpose of this document is to define and quantify these exposures, put them in perspective with other environmental
risks, and address the issue of managing the exposure.
Definitions and Terms of Measurement
Measurements of humans' exposure to radiation are generally given as average dose equivalent rates. The term
dose equivalent is a measure of the biological harmfulness of radiation and takes into account the fact that equal
amounts of absorbed energy from different types of radiation are not necessarily equally harmful.
The present international unit dose equivalent is the sievert. Divisions of the sievert are used as well, so
1,000 millisieverts (mSv) equal one sievert and 1,000 microsieverts (uSv) equal one millisievert (mSv).
Sub-atomic particles and rays traveling at a high velocity which can theoretically cause changes to human tissue
by ionizing (changing the chemistry of) cells in our bodies.
That portion of ionizing radiation which comes from radioactive atoms normally present in our bodies and in the
That portion of ionizing radiation which comes to us through the atmosphere from the sun (solar) and other stars
Air crew are potentially exposed to ionizing radiation in 3 general categories. Exposure varies with a number
of predictable as well as some poorly predictable variables.
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 uSv/Hr.) or 0.6 mSv/Yr. At higher elevations more cosmic radiation reaches the
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. Considerable variation in this figure is due to terrestrial radon.
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.
A typical chest x-ray represents an effective dose of about 0.1mSv.
Galactic Cosmic Radiation
At an altitude of 35,000 feet the dose equivalent rate from galactic cosmic radiation is about 5.0 microsieverts
per hour (5.0 uSv/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 as over the equator (10 uSv/Hr.). Air carrier aircraft may fly these high latitude routes between the
contiguous United States and Europe or Asia.
Solar Cosmic Radiation
Although charged particles are continuously being ejected from the sun, they are usually too low in energy to
contribute to the radiation level at air carrier altitudes. However, on rare and unpredictable occasions, the numbers
and energies of ejected solar particles are high enough to increase substantially the dose equivalent rate at these
altitudes. These are known as solar particle events.
Measurements of exposures as well as guidelines and standards for these exposures generally include all of these
types of exposure to radiation; i.e., terrestrial, galactic, and solar.
Exposures are variable depending upon several factors. These include:
- Flight altitude and duration at that altitude
- Geographic latitude of the flight
- The sun's solar cycle
In general, with a decrease in altitude from approximately 70,000 feet, the amount of galactic cosmic radiation
decreases. As in the discussion above, latitude and its relationship to the protective action of the earth's magnetic
field is an important factor.
Finally, the amount of cosmic radiation particles entering the atmosphere varies inversely with the approximate
eleven year cycle of rise and decline of solar activity known as the solar cycle.
Obviously the first two variables (altitude/duration, and latitude) can be influenced by flight parameters while
we have some predictability and no control over solar events.
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.
Standards and Guidelines
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 which are generally consistent among the groups.
The standard of radiation protection provided for members of the general public is a yearly limit of 1mSv. (ICRP-1990)
The recommended radiation exposure for workers in the nuclear industry is 20 mSv/Yr. averaged over five years
(ICRP - 1991).
During pregnancy, the radiation exposure limit is recommended to be no more than 2 mSv total. In addition, recommendations
include that the exposure of the unborn child not exceed 0.5 mSv in any month (excluding medical exposures) once
pregnancy becomes known (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. A recent FAA Office of Aviation Medicine report agrees
with these British recommendations. (FAA-2000)
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.
Cosmic Radiation Exposure Risks
Although the risks from high levels of radiation are well known, the effects of low level doses of radiation,
such as cosmic radiation or medical tests, are more difficult to predict. These hypothetical risks of low-level
radiation exposure to human beings can be broadly broken down to three general categories: cancer risk, genetic
risk, and risk to the unborn child.
The risk of developing cancer is the principle health concern associated with occupational exposure to radiation.
The average person on the ground has the risk of dying from cancer of about 23 percent in a lifetime. Based on
increased crew member exposure to radiation, models predict that the total risk of dying from cancer for a crew
member from long-haul routes for 20 years would increase from the normal 23 percent to approximately 23.3 percent.
The time for a radiation-induced cancer to manifest itself is normally in the range of 10 to 50 years. We should
note that research projects investigating health effects in flight crews caused by cosmic radiation exposure are
relatively few and while these studies have reported some additional cancers, their results are far from definitive
and are often conflicting.
With respect to genetic risk, in the general population about 2 to 3 percent of children are born with serious
anatomic abnormalities. Models constructed over the past decade predict the total genetic risk of producing such
a child which is attributable to radiation exposure to be approximately 75 in 1 million. Thus, the risk to the
child as a result of work-related radiation exposure of a parent would be slightly less than the current general
population incidence. Whether these risks are additive is not clear.
Another way of looking at risk: 1 mSv equates to a cancer risk of 1 in 20,000; a severe hereditary effects risk
of 1 in 100,000.
For a fetus receiving ionizing radiation during a pregnancy, the risk of harm depends on the stage of development
at the time of exposure as well as the amount of radiation. Estimates of health risks at various stages of prenatal
development indicate a risk to the unborn child of approximately 11 in 10,000 of one or more serious health effects
from radiation exposure above recommended limits.
Although the risks are exceedingly small, the greater the dose equivalent of radiation that a crew member occupationally
receives or is exposed to, the greater the hypothetical risk of potential harmful effects. It would follow then
that managing exposure to cosmic radiation may, at times, be appropriate for the individual crew member.
Table 1 demonstrates how the variables of flight altitude, latitude, and duration affect radiation exposure.
For example, polar flights that are of long duration and flown at higher altitudes and latitudes receive higher
annual equivalent doses than flights that are of shorter duration, lower altitude and lower latitude.
At this time, personal dosimetry (radiation badges) is an inexact science owing to problems of background sea
level radiation and our inability to separate out meaningful radiation at altitude.
Monitoring one's own exposure is now possible utilizing web based calculators such as that previously described
as available from the FAA. Additionally, other related web sites are available for your perusal and education.
With respect to individual consultation, United's Regional Flight Surgeons are aware of the aerospace medical
issues involved and encourage you to discuss specific concerns with them. For the female crew member who may be
pregnant or plans to become pregnant and wants to decrease risk to her offspring, a consultation with your obstetrician
is certainly appropriate, and we are happy to consult with your private physician(s) as well.
With respect to solar particle events, systems are in place to monitor such activity but are likely to be irrelevant
to our everyday operations. In the 100,000 legs flown by British Airways' Concorde, emergency descent procedures
to lower altitudes have never been engaged. Various governmental agencies including the NOAA and NASA have had
aircraft on standby to monitor high radiation dosages during solar events and have failed to find meaningful activity
at flight altitudes.
Because solar particle events can hypothetically produce elevated radiation levels at high altitudes/latitudes,
routine forecasts and alerts are sent through the FAA so that a flight in potential danger could alter its flight
plan and reduce altitude to minimize exposure.
The United Airlines Medical Department would like to thank Dr. Michael Bagshaw of British Airways, Dr. David
McKenas of American Airlines, Dr. Claude Thibeault of Air Canada, and Dr. Wallace Friedberg of the FAA for their
Bagshaw M, Irvine D, Davies DM. Exposure to cosmic radiation of British Airways flying crew on ultra-longhaul
routes. Occup Environ Med 1996; 53:495-8.
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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.
FAA 2000. Office of Aviation Medicine. Gallactic Cosmic Radiation Exposure of Pregnant Aircrew Members II. October
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.
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altitudes. Environment International 1996; 22 (Suppl. 1): S9-S44.
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