Exposure to ionizing radiation, a type of energy released by atoms in the form of electromagnetic waves, such X-ray and gamma waves, or particles, such as electrons or protons, is a regular part of life on earth. Natural sources of ionizing radiation include radioactive material in soil, water and vegetation, and human-made sources include X-rays and medical devices. Another source is cosmic radiation, which comes from the sun (solar) and outer space (galactic) and, according to the International Commission on Radiological Protection (ICRP), comprises about one-sixth of our exposure to natural radiation.
In aviation, ionizing radiation from natural sources is considered “occupational exposure” because of the high levels of galactic cosmic radiation at commercial cruise altitudes, according to U.S. Federal Aviation Administration (FAA) Advisory Circular (AC) 120-61B, In-Flight Radiation Exposure. And flight crewmembers’ exposure to ionizing radiation has received increased scrutiny in recent years because it presents potential physiological challenges.
Research into the health effects of ionizing radiation has been conducted for more than 100 years and has found that even low doses of ionizing radiation can increase the risk of long-term health effects, such as cancer, as well as a smaller risk of genetic effects. “It is 100 percent clear that radiation can even kill,” says Theresia Eberbach of the European Cockpit Association (ECA). “It is more difficult to determine at which level radiation exposure can cause damage to one’s health. This, in turn, is highly dependent on the individual’s health. Similar to tobacco consumption, it is difficult to say how much is too much, so one should try to avoid it as much as possible.”
Joseph Shonka, emeritus fellow of the Health Physics Society, a scientific organization of professionals who specialize in radiation safety, says aircrews’ in-flight exposure is primarily (about 80 percent) due to neutron radiation that is produced from the primary particles. “The neutrons are uncharged and are called indirectly ionizing radiation, since their effect on tissue comes from the charged (ionizing) particles released when the neutrons react with other nuclei,” he said. “Whether directly or indirectly, this ionization can break chemical bonds, including those of biological molecules in the human body. At high exposure, this can damage enough tissue to cause death. At low exposure, this tissue damage can contribute to the creation of various cancers, and to a lesser extent, [can contribute to] damage to the cells in reproductive organs, creating a risk of genetic effects for the offspring of those of childbearing age.”
Less radiation will be received on a lower-altitude flight because of the greater amount of radiation shielding provided by the Earth’s magnetic field. This shielding is greatest near the equator and gradually decreases to zero as one goes north or south. Galactic cosmic radiation levels over the polar regions are about twice those over the geomagnetic equator at the same altitudes.1
“A small portion of background radiation at sea level also comes from the cosmic and solar radiation that significantly impacts aviation,” Shonka says. “Background [radiation] varies considerably over varying locations, altitudes and geological material. On average, humans (on the ground) get about 1 millisievert (mSv) of ionizing radiation from external radiation sources of background. (Sieverts and millisieverts are measurements of radiation exposure.)”
The exact risk per unit exposure is not precisely known. “It should be noted that there is a vigorous debate in the radiation protection community that surrounds the issue of how to extrapolate the cancers observed in more highly exposed populations down to exposures that occupational workers or members of the public are routinely exposed to,” Shonka says.
The main consequences of constant, long-term exposure to ionizing radiation are so-called chromosome aberrations (that is, damage to the DNA), which are the first indicators of a possibly cancerous disease, according to Eberbach. “While gammas, protons and neutrons interact differently with the body’s tissues, the end result is comparable,” she says.
Shonka said that “current risk estimates (for gamma or neutron radiation) are that if 100,000 people were exposed to 100 mSv, one would expect roughly 1,000 excess cancers. This very roughly corresponds to the results seen in the atomic bomb survivor populations in Japan. Below 100 mSv, exposures are considered to be low exposure. In a population that is not exposed to added radiation, one would expect 40 percent to get cancer at some point in their lives, and 25 percent to die from cancer. Below 100 mSv, if one assumes a linear no–threshold model, then 10 mSv (10 times less dose than stated above) to 1 million people (10 times more people) would also produce the same total population (called collective) dose and produce the same 1,000 excess cancers. These cancers would be in addition to the 400,000 individuals who get cancers that are normally expected in that many people.”
Occupational limits are 20 mSv per year, with an additional requirement that radiation be controlled to as low as reasonably achievable (ALARA). “Because of this ALARA practice, typical radiation workers average less than 10 percent of that limit. The few workers who reach the limit in any one year … typically do not continue to receive that high exposure for their entire career. According to the 2008 United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) compilation, aircrews were the highest exposed of all radiation workers and averaged 3 mSv per year and have three- to 10-times higher radiation exposure than radiation workers in other industries,” says Shonka. “This discrepancy occurs because other jobs involving exposure to radiation (such as nuclear power) have benefitted from decades of effort to ensure exposures were ALARA, and the lack of a tool for aircrews to control the radiation encountered in flight. Until 2000, aircrews were essentially unregulated radiation workers, a fact that continues to exist in the U.S. today.”
In AC 120-61B, FAA said that UNSCEAR’s 2000 report ranked aircrews as the fourth most exposed group of employees, with an average annual effective dose of 3 mSv.
The exposure to ionizing radiation needs to be managed with extra care in case of childbearing crewmembers, who should be reminded that radiation exposure to the fetus should not generally exceed the general population limit of 1mSv. According to an ECA position paper, “This legally requires pregnant crewmembers to inform their operator as soon as they receive confirmation of their pregnancy. Noting that a flight crewmember may have exceeded that limit before confirmation of pregnancy, operators should have effective provisions in place to ensure that the crewmember does not exceed a dose of 1 mSv after declaration of pregnancy. In addition to the limit for the fetus, the operator will have to provide for measures to further reduce the exposition of the fetus according to ALARA.”
Monitoring and Measurement
The exposure of aviation employees is typically not measured directly with dosimeters. In countries in the European Union (EU), government agencies provide air carriers with calculations of the dose from cosmic radiation, and companies with employees who are frequent flyers receive the information on a flight-by-flight basis. “The calculated doses are then used by the carriers to control the aircrew doses to regulatory limits. Because cosmic radiation has been studied for over 100 years, the models used to perform these calculations are well accepted,” explains Shonka. “The calculations have not included radiation from solar storms that produce radiation similar to galactic cosmic radiation; however, France has recently begun to incorporate solar events into their model. This is particularly important for trans-polar flights because higher latitudes will have higher doses from solar events.”
Estimates of the radiation dose are made complex by the fact that the cosmic radiation field consists of multiple different components. “A simple recording of total dose, such as may be given by a Geiger counter, will therefore give little indication of the effective dose to biological tissues. Nevertheless, radiation can be measured directly using sophisticated equipment as was carried on board the supersonic Concorde of British Airways and Air France, or indirectly using computer software programs,” according to the International Air Transport Association’s Medical Manual. “The latter, when supplied with such details as the route, altitudes flown, time at each altitude, and the phase of the solar cycle, are able to calculate an estimate of the radiation dose received by crew for a particular flight. Many studies have been undertaken comparing actual measurements with computer estimation with the two showing good agreement.”
FAA, which began to investigate the health effects of ionizing radiation in the early 1960s, through its Office of Aerospace Medicine and its Civil Aerospace Medical Institute (CAMI), developed a dose calculation computer program referred to as CARI. (CAMI originally was called the Civil Aeromedical Research Institute (CARI).) The current version of the program is called CARI-7 and is the seventh major release. According to FAA, the program calculates the effective dose of galactic cosmic radiation received by an individual on an aircraft. The program accommodates both waypoint defined and shortest route flight paths.
Eberbach points out that the fact that radiation exposure is usually not monitored with a dosimeter device but rather is calculated is actually in contrast to a basic principle in radiation protection to prefer measurements over calculation, whenever possible. “Long-haul crews are exposed roughly two to three times as much as short-haul crews. Few airlines carry dosimeter equipment on board, and if they do, they are mostly only a few devices that are carried for scientific purposes. Comprehensive fitting with dosimeters would be a best practice but rarely ever takes place. In the calculations department, some are using better computation models than others. Some let their crews see their record only once a year; others have a constant access to their data,” she says.
The fact that dosimeters are rarely deployed is due, in part, to the technical difficulties in measuring the complex radiation environment using dosimeters, Shonka says.
The European Example
An important control function regarding ionizing radiation is performed by regulations that are devised to keep the risk of cancer and genetic effects at levels comparable to those of other risks accepted in industry. “In 1990, the International Commission on Radiological Protection (ICPR) declared that aircrews were radiation workers. Following that, the EU passed some regulations applying the same limitations that all workers exposed to radiation have,” says Shonka. “In 1996, the EU Directive 42 was issued. It mandated that member countries were required to establish programs for aircrews and frequent flyers likely to exceed 1 mSv per year, with five years allowed for implementation. Specifically, employers are required to: assess exposure, make schedule changes for high dose crew, inform aircrew of health risks and limit pregnant aircrew to 1 mSv per year.”
If the exposure exceeds 6 mSv per year, annual medical follow-up is mandated in Europe. “Exposures should be reduced to ALARA levels through crew rostering, etc. The aircrews who are assigned to commuter-type flights typically fly at lower altitudes and do not exceed 1 mSv per year, and thus do not require monitoring. Aircrew who might be pregnant are provided additional considerations to prevent dose to the fetus exceeding 0.5 mSv per month,” says Shonka. “Iceland … also follows similar regulations to the EU and instructs its carriers … to use an EU member state with an accepted model to provide the radiation dose calculations for them.”
In addition to its dosage calculator, FAA has published information about radiation exposure, primarily in the form of advisory circulars. In AC 120-61B, FAA said it accepts the most recent recommendations of the American Conference of Government Industrial Hygienists and that for non-pregnant crewmembers, the FAA-recommended limits for exposure to ionizing radiation are the same as recommended by ICRP. The recommended occupational exposure limit for ionizing radiation is a five-year average effective dose of 20 mSv per year, with no more than 50 mSv in a single year. FAA also says that exposure management should be based on the ALARA principle.
Eberbach believes that the EU basic safety standards are a good example of a reasonable regulatory framework. “Classifying aircrews as occupationally exposed and mandating to treat their exposure as a planned exposure situation [very specific terminology and consequences] is exemplary. Unfortunately, there is no such thing as a worldwide legally binding framework,” she says.
According to ECA, an important aspect is the education of aircrews. Air carriers should inform potential employees of the possible consequences and risks of radiation exposure. “Crewmembers should receive education on applying ALARA principles to minimize their radiation exposure where they can influence their flight duty assignments. Reducing exposure times by flying fewer hours may coincide positively with efforts to reduce yearly limits of flight hours in the interest of flight safety. Similarly, flight crewmembers may influence their lifelong radiation exposures by making use of their options regarding selection of aircraft type(s) flown, the types of operation (short haul/long haul), and their retirement age. Operators are encouraged to provide crews with the possibility of such career choices. Crewmembers should also be aware that lightning strikes expose crews to additional ionizing radiation doses,” an ECA position paper says.
Shonka says that the implementation of the EU regulations is a straightforward exercise. “When a flight operated by a European airline lands, data on the flight (e.g., altitudes, speeds, waypoints, times) [are] downloaded and sent to the government agency that performs the calculation and provides the cosmic radiation dose estimate for that specific flight. Carriers and employers of frequent flyers are required to incorporate those results into the work history of their employees for that flight to ensure regulatory requirements are met.”
In the past 30 years, the number of trans-polar operated has grown significantly, saving airlines time and money. But “trans-polar flights carry a significantly enhanced risk of high radiation levels because of solar storms. Even normal cosmic radiation levels are higher at the poles,” says Shonka. “Efforts are under way to build a model for this region; however, radio communications would likely be disrupted during a solar event, underscoring the need for on-board radiation detectors to permit the aircrew to decide whether to divert or not during a solar event.”
The International Civil Aviation Organisation (ICAO) addresses the risk of ionizing radiation exposure in its Manual of Civil Aviation Medicine (Doc 8984). In the manual, ICAO says: “In view of the fact that ionizing radiation is now assumed to play a role in mutagenic or carcinogenic activity, any procedure involving radiation exposure is considered to entail some degree of risk. At the same time, however, the radiation-induced risks associated with flying are very small in comparison with other risks encountered in daily life. Nevertheless, such risks are not necessarily acceptable if they can be easily avoided.
“Theoretically, the radiation exposure in aircrew can be reduced by optimizing flight routes and crew scheduling, and by installation of radiation warning devices. Such devices are particularly effective in detecting high momentary radiation during solar flares and can thus be used in determining a need for a lower cruising level. Female crewmembers should be aware of the possible risk to the fetus and should be scheduled in such a way as to minimize the exposure during pregnancy.”2
ICRP recommends that airlines should educate and inform employees about the doses and effects of cosmic radiation. “Airlines should also assess and log these doses, making each employee’s personal dose information available to them. Provisions should be in place to adjust duties for aircrew who have declared a pregnancy, if needed. National authorities and airline companies should raise awareness about cosmic radiation and offer their support to make informed decisions about cosmic radiation exposure in relation to flying.”
Mario Pierobon is a safety management consultant and content producer.
Notes
- FAA. AC 120-61B, In-Flight Radiation Exposure.
- ICAO. Manual of Civil Aviation Medicine (Doc 8984), Third Edition — 2012.
Featured image: © ChrisD600 | iStockphoto
Types of radiation: © generalfmv | iStockphoto
Online exposure calculator: U.S. Federal Aviation Administration
Transpolar routes: route maps, Rolypolyman | Wikimedia PD; base map, Chen-Pan Liao | Wikimedia CC-BY-SA 4.0