Radiation Exposure Aloft -- Are You Being Nuked?
Have you noticed the headlines lately dealing with galactic cosmic radiation?
- April 7, 2000: Solar shock wave causes surprise aurora display.
- June 9, 2000: Sun storms creating a stir on Earth.
- July 13, 2000: Eruption on sun triggers radio blackouts.
- July 14, 2000: Another strong solar flare heads toward Earth.
This is no coincidence. We're now entering a peak period in the 11-year solar cycle (forecast to reach its maximum sometime in 2001), when the huge magnetic storms on the surface of the sun -- technically known as Coronal Mass Ejections (CMEs) and colloquially as solar flares -- expel eruptions of high-energy ionized gasses through space at speeds of 600 miles per second or more. When these eruptions reach Earth, they often produce eerily beautiful displays of the northern and southern lights in polar regions, and also often cause disruptions in wireless communications. In extreme cases, they have been known to disrupt power grids and cause blackouts.
Such solar particle events can also drastically increase the amount of radiation to which people aboard high-flying jet aircraft are exposed. That's one reason that there seems to be a renewed interest in the possible health hazards of cosmic and solar radiation to professional cockpit and cabin crews, and even frequent-flying passengers.
Measuring radiation exposure
When assessing the health effects of radiation, scientists now measure radiation dose in "sieverts" (abbreviated as "Sv"). This unit replaces an older measuring unit -- the "rem" -- with one sievert equal to 100 rem. Since one sievert is a great deal of radiation, measurements are more often expressed in millisieverts (mSv, one thousandth of a sievert) and even microsieverts (uSv, one millionth of a sievert).
To make things more complicated, scientists use the notion of "effective dose" to reduce the absolute radiation dose in cases where only certain portions of the body are exposed. For example, a typical chest x-ray (posterior-anterior view) involves about 0.09 mSv of radiation exposure, but because that exposure is limited to the chest area, the "effective dose" is considered to be only about 0.03 mSv because the radiation exposure risk includes the lungs and breasts but not the brain or gonads.
Likewise, it is estimated that the average person receives 24 mSv per year of radiation exposure from breathing radon gas in the air. But since that radiation affects only the lungs and not other parts of the body, the average effective dose from radon is estimated as 2.0 mSv per year.
By contrast, the average American receives a dose equivalent to about 0.27 mSv per year from cosmic rays. Since such radiation affects the whole body, the effective annual dose from cosmic background radiation is also 0.27 mSv -- roughly the equivalent of nine chest x-rays per year. On the other hand, if you live in Denver, your annual dose from cosmic background radiation rises to about 0.50 mSv.
Radiation exposure aloft
There are four principal factors that affect the increased radiation dose received by folks who fly a lot: altitude, latitude, hours aloft, and solar activity.
Altitude. On average, the amount of cosmic radiation roughly doubles with every 2,000-meter increase in altitude. We've already seen that it's nearly twice as much in Denver as in New York. At a typical long-haul airline cruising altitude of FL390, cosmic radiation is around 60 times greater than at sea level. For an intercontinental bizjet flying at FL510, the dose is nearly 200 times greater than at sea level.
Latitude. The Earth's magnetic field causes the lion's share of cosmic radiation to be concentrated near the north and south magnetic poles. (That's why we only see radiation-induced atmospheric light shows at very high latitudes.) The exposure rate at 70 degrees north or south latitude is about four times as much as at 25 degrees. Thus, flights over polar routes (e.g., New York to Tokyo or Chicago to London) are exposed to a lot more radiation than those confined to the mid-latitudes.
Hours aloft. The average person doesn't spend enough time aloft for this increased radiation to present a significant risk, but aircrews do. According to FAA estimates, an airline crewmember flying 1,000 block-hours annually between Washington, D.C., and Los Angeles at FL350 would receive 5 mSv per year. Change the route to New York to Athens and the altitude to FL410 and the exposure increases to nearly 10 mSv per year. Although aircrews are at greatest risk from radiation aloft, there are also 400,000 frequent flyers who travel 75,000 miles or more annually, which equates to 200 airborne hours a year.
Solar activity. Solar flares can increase the cosmic radiation level by a factor of 10 or 20 for periods lasting from a couple of hours to a day or two. A one-way 13-hour flight from New York to Tokyo generally involves an exposure of about 0.1 mSv (the dose equivalent of a few chest x-rays), but during a solar storm that exposure could increase to 1.0 mSv or more.
How much is too much?
Government standards for radiation protection are established by the National Council on Radiation Protection and Measurement (NCRP) and its international counterpart, the International Commission on Radiological Protection (ICRP). Both of these organizations offer recommendations for the maximum permissible dose (MPD) of radiation to which people should be exposed, and those recommendations are generally adopted by various government regulatory agencies (e.g., FAA, EPA, OSHA, NRC) as the maximum limits permitted by law. Current MPD limits are shown below:
Maximum Permissible Dose NCRP ICRP
10 mSv x age
MPD During Pregnancy
In making their maximum permissible dose recommendations, both NCRP and ICRP divide the population into two groups: members of the general public, and "radiation workers" who are exposed to radiation through their occupation. Government standards establish limits for occupational exposure that are 20 to 50 times greater than those established for the general public. The rationale is that "radiation workers" presumably accept the increased risk by informed consent as a trade-off in exchange for the benefits of employment.
Note that in addition to its annual MPD for occupationally exposed radiation workers, the NCRP recommends a cumulative lifetime limit (in mSv) equal to 10 times a worker's age. So, for instance, a pilot who retires at age 60 should not be exposed to more than 600 mSv over his entire flying career. Assuming that career lasts 30 years, average annual exposure should not exceed 20 mSv.
Note also that both organizations recommend drastically reduced limits for occupationally exposed workers during pregnancy.
The FAA's posture
For the general public, both the NCRP and ICRP agree that the maximum permissible annual radiation exposure is 1 mSv. Since we've seen that airline aircrews are routinely exposed to 5 to 10 times that much, you might think it was a no-brainer that crewmembers must be classified as occupationally exposed radiation workers. But when a radiation physicist named Edward Bramlitt proposed precisely that to the FAA in 1984, his petition for rulemaking was denied by the FAA (with the support of the airline industry) after two years of study and public comment.
The FAA did agree, however, that airlines should provide education to crewmembers so that they could understand the risks involved and the relationship between radiation exposure, work schedules, altitudes and latitudes. In its 1986 rejection of Bramlitt's petition, the FAA said in part:
"Air carrier crew members in most cases have the unique opportunity to control their occupational scheduling as related to high-altitude, high-latitude flights. This is particularly significant in the case of crew members who are pregnant. ... Crew members who fly at high-altitude, high-latitude flights of long duration should be given the opportunity to make an informed decision whether or not to transfer to short-distance flights in the contiguous United States."
In 1990, the FAA published Advisory Circular AC 120-52, "Radiation Exposure of Air Carrier Crewmembers." In 1994, the agency released another AC outlining a training syllabus for flight crewmembers and their managers about the risks of in-flight radiation. After subsequent scientific studies showed that the in-flight radiation levels used by the FAA to justify its denial of the Bramlitt petition were greatly underestimated, the FAA and EPA agreed that flight crews are indeed occupationally exposed.
Nevertheless, airline crews are not afforded the protections that are routine for ground-bound radiation workers in medicine and industry. Their exposure is not monitored by means of dosimeter badges or other equipment. (There are a few exceptions -- the Concorde cockpit is instrumented for radiation, and FedEx very recently started a program of providing dosimeter badges to their pilots.) They are not monitored for radiation injuries. And in most cases, they don't receive the kind of training that would permit them to assess their occupational risks from radiation.
Global bizjet crews, who often fly above FL500 and don't have the same duty time limits as airline pilots, may be at the greatest risk of all.
Assessing the risk
The principal health concern from radiation is the increased risk of cancer. But what exactly is the risk? No one really knows.
We do have good information on the risks associated with high levels of radiation. For example, extensive studies have been made of cancer and mortality rates among the survivors of Hiroshima and Nagasaki, as well as studies of nuclear accident survivors and radiation oncology patients. But it's not easy to extrapolate such high-level radiation studies to estimate the health risks from low-level cosmic radiation. Doing so requires that two assumptions be made: that cosmic radiation has the same effect as other types of radiation, and that the relationship between dosage and cancer risk is linear. Neither of these assumptions has ever been proven, so risk assessments based on high-exposure studies are speculative at best.
At the same time, it's nearly impossible to do a statistically valid study of the health effects of low-level radiation because the medical problems we're concerned about (primarily cancer and birth defects) occur naturally as the result of a wide range of genetic and environmental factors. To determine the incremental risk from low-level radiation with any sort of statistical accuracy, a huge population would have to be studied. For instance, to determine the effects of 10-mSv radiation doses, a study would have to include five million people (or perhaps five million laboratory animals). There have, however, been several small-scale studies that suggest the radiation risk for aircrews may be non-trivial.
A 1996 paper in the American Journal of Epidemiology examined the incidence of cancer among 2,740 Air Canada pilots. The pilot group had a significantly lower overall incidence of cancer than the general population, which was not surprising because airline pilots as a group are in better shape and tend to live healthier lifestyles (e.g., with regard to smoking, diet and exercise) than non-pilots. In fact, only 125 cancers were found in the Air Canada pilot group, compared with 175 that would be expected in a like-sized group from the general population. What was surprising, however, was that the Air Canada pilot group had a sharply higher incidence of four specific types of cancer compared to the general population: myeloid leukemia, astrocytoma, prostate cancer, and malignant melanoma.
Myeloid leukemia is definitely associated with radiation exposure, and occurred in the Air Canada pilot group four times more frequently than in the general population. Astrocytoma (a type of brain cancer) occurred twice as often in the pilot group as in the general population, and increased incidence has also been found in other studies of airline pilots.
Significantly higher incidences of prostate cancer were found in both the Air Canada study and another study of British Airways pilots. Current thinking is that this might be related to electromagnetic radiation from weather radar and other avionics rather than from cosmic radiation.
Malignant melanoma is a skin cancer normally associated with sun exposure. British Airways pilots were found to have a six times greater risk of melanoma than the general population, and a significantly increased incidence was also found in the Air Canada study. However, increased incidence in these pilot groups might simply be a function of that group's increased opportunity to travel to sunny vacation spots.
A study of female airline flight attendants in Finland and Denmark showed an increased incidence of breast cancer, which researchers have suggested is due to the crewmembers' simultaneous exposure to both magnetic fields and cosmic radiation. It is thought that magnetic fields suppress the function of a gland called the pineal body, which produces the hormone melatonin, and that reduced melatonin increases the risk of developing certain cancers, particularly breast cancer. Another study showed that infection-fighting white blood cells develop more chromosomal aberrations when exposed to both radiation and magnetic fields than under radiation alone.
Finally, a study of male U.S. Air Force pilots showed they had significantly more genital and testicular cancer than non-flying USAF officers. Again, nobody is sure why.
Concern about radiation is greater for female crewmembers who are pregnant.
During the first eight days after conception, high radiation exposure may result in the death of the embryo. Between the ninth and 50th day, organ growth may be affected, with peak sensitivity predicted to occur during the third and fourth weeks of pregnancy. Radiation exposure toward the end of the first trimester of pregnancy may affect the mental development of the fetus, and has been associated with retardation and learning disabilities. In addition, radiation exposure at any time during pregnancy causes an increased risk of childhood cancers.
The risk to a pregnant woman who takes one or two long-haul airline trips during her pregnancy is minimal. However, the risk to a pregnant flight crewmember who flies a full schedule, particularly on high-altitude high-latitude routes, cannot be ignored.
(Pregnant flight crewmembers should also be aware of other issues Dr. Blue discusses here.)
Ongoing research efforts
The Air Line Pilots Association (ALPA) is involved in a number of ongoing research projects aimed at trying to quantify the risks of in-flight radiation exposure.
ALPA's Dr. Gary Butler and Dr. Don Hudson are conducting one such project in conjunction with the Medical University of South Carolina, Department of Biometry and Epidemiology. This study will assess cancer incidence in a large group of U.S. and Canadian airline pilots by means of a health survey sent to approximately 9,000 active and 1,000 retired Delta Air Lines pilots and to 1,300 active and 350 retired Canadian Airlines International pilots. The results, expected by the end of 2000, should provide the most comprehensive database yet assembled on cancer risk assessment and radiation exposure.
Another project co-sponsored by ALPA is intended to measure magnetic field exposure of airline pilots on four different aircraft types (B-737-200, B747-400, B-767-300ER, and A320). As previously mentioned, researchers believe that magnetic fields can increase the susceptibility of the body to certain types of radiation-induced cancer.
What individual crewmembers can do
The risks of in-flight cosmic radiation are still not well quantified, and there are still lots of unanswered questions and no easy answers. Radiation exposure aloft is an unavoidable hazard for those who fly for a living. In case you were wondering, aluminum-foil hats and lead jock straps do not work. (It would take something like 20 inches of lead to block high-energy cosmic radiation.) But there are plenty of things that a prudent crewmember can do, both to assess his or her risk and to limit it to acceptable levels.
Wear a dosimeter. One obvious step is simply to keep track of your own radiation exposure by wearing a dosimeter badge, just as virtually all ground-based medical and industrial radiation workers do. (I find it ironic that airport security folks who screen baggage wear dosimeters but aircrews do not.) In a perfect world, all air carriers and corporate jet operators would provide dosimeters for their cockpit and cabin crewmembers. As of this writing, however, FedEx is the only U.S. carrier that is doing so, and that program started quite recently.
Until programs like this become widely available, individual flight crew members (and even frequent flyers) should consider obtaining and wearing dosimeter badges on their own. I recently discussed this subject with the world's largest dosimeter service company, and learned that the cost is ridiculously low -- less than $50 a year if the badges are exchanged and processed quarterly, and less than $100 a year for monthly service. I'm told that the FedEx program operates on a quarterly basis, and I believe this should be more than adequate for this application.
The only fly in the ointment seems to be that dosimeter service companies are not set up to deal with individual end-users, and are reluctant to do so. Unfortunately, few carriers offer dosimeter programs for their crewmembers, and noone seems to offer such a service for frequent-flyer passengers.
Estimate your exposure. The FAA has created a computer program called "CARI-5E" that you can use to estimate the radiation exposure that you will receive on a particular flight over a particular route at a particular altitude. You can download this program over the Internet at http://www.cami.jccbi.gov/AAM-600/610/600Radio.html#CARI6EXE. Keep in mind, however, that the estimates produced by this program do not take into consideration the increased radiation exposure due to solar flare activity.
Get a preflight briefing on solar flare activity. Until recently, solar storms were considered to be unpredictable events. In recent years, however, astronomers have discovered that certain observable patterns on the surface of the sun are good predictors of upcoming solar flare activity. One radiation physicist has recommended a real-time warning system to alert airlines (and crewmembers and frequent flyers) of forecast solar flare activity that increases in-flight radiation risk. Until such a system exists, you can research the subject yourself by checking the current solar activity reports (updated daily) at http://www.sel.noaa.gov/forecast.html. Unfortunately, these forecasts are written for astronomers, not pilots, and are a bit difficult to decipher. If there's enough interest, however, I'd be willing to try to set up a mailing list of pilots who wish to receive aviation-oriented solar flare warnings via email.
If you're pregnant, minimize your exposure. Female crewmembers need to take special precautions to avoid occupational exposure to radiation during pregnancy. Try to avoid bidding long-haul, high-altitude routes, especially those that involve flights in polar regions. If your employer has difficulty with such requests, point out the reduced maximum permissible radiation dose during pregnancy recommended by both the NCRP and ICRP, as well as the FAA policy statement quoted earlier.
One of the most comprehensive and yet readable sources of information I've found on this subject is a 100-page paperback book titled "The Invisible Passenger -- Radiation Risks for People Who Fly" (ISBN 1-883526-06-X), written in 1996 by Dr. Robert J. Barish (a radiation oncologist and expert on in-flight radiation). I have obtained a limited number of copies of this excellent book, so if you're interested in getting a copy, you can order a copy via my Web page at http://www.aeromedix.com/radiation.html or by toll-free telephone at 1-888-362-7123..
A comprehensive (and rather technical) article on this subject also appeared in the January 2000 issue of Air Line Pilot magazine, written by ALPA researcher Gary Butler, Ph.D.