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Cosmic Radiation Exposure when Flying
People travelling in aircraft may be exposed to more ionising radiation than they would be exposed to on the ground.
People travelling in aircraft may be exposed to more ionising radiation than they would be exposed to on the ground. The increased exposure occurs because the Earth’s atmosphere provides less protection from cosmic radiation at the typical cruising altitudes of commercial aircraft, which are usually between 7000 and 12000 metres.
Aircrew and frequent flyers get the most additional exposure because of the extra time they spend at cruising altitudes. Large studies of pilots and aircrew have generally shown no significant association with an increased health risk.
The first publication in ARPANSA’s Radiation Protection Series provides recommended ionising radiation dose limits for the public and for people exposed to ionising radiation as a result of their occupations. Read on for more detail.
All living organisms are exposed to ionising radiation on a continuous and daily basis. This type of exposure is referred to as background radiation. The international unit of radiation dose is called the sievert (Sv). A sievert is a large unit of dose and the unit most in use is the millisievert (1000 mSv = 1 Sv) The sources of background radiation include radioactive materials and their decay products in the natural environment (referred to as terrestrial), in building materials and from outer space (referred to as cosmic radiation). There is considerable variation in the background radiation levels throughout the world. The world average is 2.4 mSv/year and the average Australian background radiation dose is around 2 mSv each year.
ARPANSA Radiation Protection Series No. 1 (Republished 2002) describes Australia’s system for radiation protection and includes an occupational exposure standard. In general, an occupational dose limit of 20 mSv per year applies, and for members of the public a 1 mSv per year dose limit applies. When a pregnancy is declared by an employee, the unborn child must be protected to the same level as the general public. The latter is particularly important, as the most significant radiation risk at low doses is likely to involve the unborn child.
Sources and exposures
Cosmic radiation is mainly in the form of particles from outer space. Some contribution also occurs from the sun, together with solar particle events. Solar particle events are rare occurrences that can result in higher exposures for short periods of time. The Earth’s atmosphere offers considerable protection from cosmic radiation, such that at ground level only small exposures occur. Because Australia has few high mountain ranges, background cosmic radiation at ground level is low.
Airflight involves a change in the exposure to ionising radiation. As altitude increases during flight, there is an initial lowering of the exposure due to the reduction of the terrestrial (Earth-based) component of background radiation. As altitude increases further, the cosmic radiation component increases and can exceed the initial radiation exposure at ground level. The important part of a flight from an overall cosmic radiation exposure perspective is the cruising phase of jet airflight. This typically involves altitudes between 7000 and 12000 metres.
In addition to altitude, latitude – the distance from the equator – also has an influence on the exposure level. Exposures increase the further that the flight path is away from the equator.
The groups with the most significant occupational exposure to cosmic radiation are cabin crew, pilots and flight engineers. Measurements and modelling of Australian aircrew exposures have indicated an additional dose from commercial airflight of around 1.8 mSv per year for those involved in domestic routes, and around 4 mSv per year for those involved in international flight routes. These figures are similar to those experienced by aircrew in other countries. It is also possible for people who fly very frequently, for example 10-20 hours per week on long haul flights, to approach and exceed a 1 mSv per year dose. The table below provides some indication of doses for a number of routes and also indicates flying hours and number of flights taken to achieve a 1 mSv dose.
|Hours Exposure for Effective Dose of 1 millisievert|
|Altitude (ft)||Altitude (m)||Hours at latitude 30° S (1)||Hours at equator (2)|
Source(s): (1) CARI-6 model - Averaged over a complete Solar Cycle
(2) Effects of Cosmic Radiation on Aircrew – Herbert R Meyer
|Route Estimates||Dose/Flight (µSv)||Flights for 1 mSv|
Data provided by Capt Ian Getley and adapted for presentation
1 mSv = 1000 µSv
Ionising radiation causes biological effects by directly damaging cells, tissues, components of cells and enzymes. Damage to important parts of the cell such as the DNA – the genetic material – can occur by ionising radiation directly breaking chemical bonds or by interacting with cellular chemicals that create agents that will break chemical bonds, or by mechanisms that change how cell divide, communicate or die. Damage to the DNA is felt to be an important step with regards to the risk of cancer and the risk of heritable defects, although human cells do have an enormous capacity to repair such damage.
Australian and international radiation protection guidance assumes that for exposure to ionising radiation there is no threshold dose below which there are no effects, i.e. that small doses confer a small risk, and that risk can be judged from the known effects at high exposures. Exposures of aircrew to cosmic radiation are typically less than a quarter of the occupational dose limit of 20 mSv per year. Large studies involving the health of pilots and aircrew have generally shown no significant association with an increased risk of cancer and, in particular, with the types of cancer that might be expected to arise from radiation exposure. This observation fits with the likely risks of low level exposure, and the scientific basis upon which the exposure standards are underpinned.
For pregnant aircrew, the Australian and international guidance is that the unborn child should be treated in broad terms as a member of the public. The public dose limit is 1 mSv. Radiation exposure to the unborn child of less than 1 mSv will not lead to significant radiation related health effects.
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