Radiation and its Relevance to Flying
European law requires that as airline employees flying in jet aircraft, we be informed about the additional radiation we are exposed to, over and above the natural background at ground level. The amount of this radiation may be greater than that permitted for the unacquainted general public and produces a very low additional risk of inducing cancer.
Radiation is a natural part of our environment, and can be divided into high energy and penetrative "ionising" radiation, and "non-ionising", such as light, heat or radio wave. Ionising radiation can come from natural sources such as rocks and soil, medical such as x-rays and cosmic from the sun and outer space. The biological effect of radiation on the human body depends not only on its energy but also its composition and the tissue exposed, so a weighting factor is applied to produce a meaningful unit of "harmful effect", called the sievert (Sv). This can be subdivided into a thousand milli sieverts (mSv). The general public in this country are on average exposed to 2.2 mSv from natural sources.
A jet aircraft cruising at altitude is subject to increased cosmic radiation the higher it flies into the atmosphere, which normally gives us some protection at ground level. The aircraft itself provides no shielding from these penetrative rays. The magnetic field of the earth provides some protection from cosmic radiation. Protection is greatest over the equator and least over the North and South Polar Regions. At cruising altitudes, radiation levels over the magnetic poles are approximately double those over the equator. Cosmic radiation is affected by the 11-year solar cycle. During the active solar phase, the increased magnetic field linked to increased levels of solar wind helps shield the earth, meaning that cosmic radiation levels in the atmosphere are at a minimum and increase as the solar activity decreases. Very rare solar storms can give rise to high doses; particularly at high latitudes and at supersonic altitudes and for this reason Concorde was designed with a radiation detector.
Cosmic Radiation Measurement Methods
Direct measurement by active dosimeters can be difficult to perform and generally require a lot of interpretation. Measurement by passive radiation badges as worn by radiographers and nuclear workers can also be difficult in the mixed radiation of cosmic rays. Since everyone on any particular flight receives the same dose, it is convenient to assess the dose by calculation. Calculations are based on mathematical methods validated by in flight and ground measurements. They use time-averaged solar conditions but at present do not account for solar flares. Calculation of route doses based on typical flight profiles use computer codes such as CARI, EPCARD or PC-AIRE.
Airlines have been involved with direct measurements for more than 20 years on long haul flights either independently or as part of EU sponsored projects. There is consistency of measurement data and comparability between that and calculation results, which indicate that aircrew receive an average annual occupational exposure in the range of 2 to 4 mSv.
Current occupational dose limits in national and European legislation are based on the International Commission for Radiological Protection (ICRP) publication 60. The dose limits apply to the dose received in addition to background radiation and radiation from medical examinations and treatments.
-Members of the public: 1 mSv per year.
-Occupationally exposed workers: 20 mSv per year averaged over 5 years.
-Pregnant occupationally exposed workers: The exposure of the foetus must be as low as reasonably achievable and unlikely to exceed the public dose of 1 mSv from declaration of pregnancy.
In Guidance, the EU has proposed a 6 mSv per year action level. This level is arbitrary and has no radiobiological significance. An employer must take action to reduce the exposure of any worker whose dose exceeds 6 mSv over a rolling twelve-month period.
At jet cruising levels the cosmic radiation we receive from space over the working year gives us an added risk of developing a fatal cancer later in life. By flying, perhaps 500 hours a year at 35,000 feet, we double the added risk of cancer from natural background radiation that a ground based worker has. The risk is not of definitely being damaged, the dose is far too low for this, and rather our risk of cancer is statistically slightly increased. About 25% of us will die of cancer from all causes and a whole flying career of forty years in a jet aircraft might increase this to perhaps 25.5%. The risk of a pilot being killed by an accident in a commercial airliner over a lifetime's flying is half that of the small added risk of fatal cancer from cosmic radiation, so radiation cannot be ignored.
There have been several studies concerning patterns of mortality and the incidence of cancers in those who fly. Early work on mortality data for flight deck crew using the Proportional Mortality Ratio (PMR) approach generated hypotheses related to excesses for melanoma, colon cancer and brain cancer. Many early studies lack statistical significance because of small sample sizes, but the common finding from more recent cancer incidence data is that of an excess for melanoma. However, even in these larger studies, exposure to radiation is not separated from other possible causes. Aircrew and passengers are exposed to other potential carcinogens, such as, ozone, fuels, other radiations, and also to circadian rhythm disruption and different lifestyle factors. The statistical relationship between the incidence of these malignancies and radiation exposures is at best an association and no causal link has been established.
One of the three main principles of radiation protection is known as ALARA, which is an acronym for keeping exposure As Low As Reasonably Achievable, taking into account social and economic factors. Measuring the radiation does not of course reduce the risk of such exposure. It is neither practicable nor economic for an aircraft to carry additional shielding to reduce exposure, nor is it practical to change latitude since that would increase flight time and thus exposure time.
Altering altitude will have some benefit in reducing exposure. At any particular weight, an aircraft has a particular "Optimum" altitude for a given cruise speed. To fly grossly below optimum is uneconomic and this is unacceptable on the ALARA principle in relation to the small reduction in risk that would be achieved. For any given weight there will be, by symmetry, two altitudes of equal cost. One of these is above the optimum level and one below. The principles of radiation protection suggest that the lower of these two levels be used more often than the upper. The captain has many considerations when choosing a flight level and this should be one of them.
With respect to pregnant crewmembers, some employers insist on no further exposure following pregnancy declaration in order to comply with the ALARA principle. In addition, there will be physiological factors to be considered in determining whether or not pregnant crewmembers continue to fly.
Civilian aircraft operating above 15,000 metres (approximately FL490) such as Concorde are required to perform quarterly measurements or to carry radiation warning monitors, which give an alarm in the case of intense solar particle events. If there is an alarm, a decision is made as to whether to descend to a lower level
More detailed Department for Transport guidance
is available at:
BALPA - May 2003
Airport Medical Services Limited