|
Cosmic radiation
There are many different types of radiation which form a natural part of the environment in which we live and which are present in the earth, buildings and food we eat. Natural radiation in the form of light and heat are essential to life. Other forms of radiation generated by man, such as microwaves for cooking, radar for Air Traffic Control and x-rays for medical examinations add value to our daily activities.
Types of radiation The different types of radiation are most easily classified according to the effects they have on matter. There are two broad categories:
The amount of cosmic radiation which reaches the earth from the sun and outer space varies and depends on the latitude and height above sea level. Man, animals and plants, have all evolved in an environment with a background of natural radiation and with few exceptions, it is not a significant risk to health.
What is cosmic radiation? Cosmic radiation comes from two sources.
The lower energy particles of solar radiation do not contribute significantly to levels of cosmic radiation except at times of increased activity from the sun and solar flares. The amount of cosmic radiation entering the atmosphere follows an 11 year cycle with the intensity of radiation being lowest when solar activity is at its highest. The next solar maximum is due in ? at which time cosmic radiation levels will be at a minimum. Cosmic radiation is effectively absorbed by the atmosphere and is also affected by the earth's magnetic field. The effect on the body will depend on the latitude and altitude at which the individual is flying and also the length of time in the air.
How is it measured? Radiation may be measured either directly using sophisticated equipment as in Concorde, or estimated using a computer software program. This program looks at the route, time at each altitude, and the phase of the solar cycle and calculates the radiation dose received by crew or passengers for a particular flight. British Airways and other airlines have compared actual measurements taken on board an aircraft with computer estimations and the two are very similar. Radiation is measured in units called Sieverts. The effects of ionising radiation depend not only on the dose absorbed, but also on the type and energy of the radiation and the tissues involved. These factors are taken into account in arriving at the Dose Equivalent measured in Sieverts (Sv). However, doses of cosmic radiation are so low that figures are usually quoted in microsieverts (µSv), that is millionths of a Sievert or, millisieverts (mSv), that is thousandths of a Sievert. Residents of the United Kingdom are exposed to a total overall background ionising radiation level of approximately 2.6 mSv per annum.
How much radiation might I be exposed to? In flight exposure will depend on the route, altitude and aircraft type. However, on average, dose rates received will be in the order of:
For passengers, the International Commission on Radiological Protection (ICRP) recommends a limit of 1 mSv per year. This equates to about 200 hours flying per year on subsonic trans-equatorial routes.
The risks When ionising radiation passes through the body, energy is transmitted to the tissues which affects the atoms within the individual cells. This may result in: (i) Development of cancer. A cell may be altered as a result of being irradiated and subsequently become cancerous. The likelihood of this happening will depend on the dose received. For an accumulated dose of 5 mSv per year over a career span of 20 years (more than the anticipated annual exposure for long haul crew) the likelihood of developing cancer due to the radiation will be 0.4%. This though needs to be put in perspective as we know from national mortality data that approximately 23% of the population will die from some type of cancer and so the additional exposure will increase the risk from 23% to 23.4%. Compared with all other risks encountered during the working life, this is very low. (ii) Genetic risk. A child conceived after exposure of the mother or father to ionising radiation is at risk of inheriting radiation induced genetic defects. These may take the form of anatomical or functional abnormalities apparent at birth or later in life. The risk following an accumulated dose of 5 mSv per year over a career span of 20 years will be 1 in 1,000. Again this needs to be considered against a background incidence in the general population of approximately 1 in 50 for genetic abnormalities. (iii) Risk to the health of the foetus. With regard to pregnancy, although the risks to the unborn child from cosmic radiation are minimal when compared with other risks during pregnancy, radiation exposure should be kept to a level 'as low as reasonably achievable'. Individual passengers will therefore need to make their own assessment of risk taking into account the likely exposure.
Effects on crew British Airways works closely with the UK Government, Civil Aviation Authority and the National Radiological Protection Board and has monitored cosmic radiation on board its aircraft for more than 20 years. Monitoring instruments were permanently installed on Concorde and regular measurements are also made on long range Boeing 747-400 aircraft. In addition, British Airways has undertaken epidemiological studies examining the incidence of disease and life expectancy of flight crew over the last 40 years. It has been found that pilots and flight engineers have an increased life expectancy of between 3 and 5 years compared to the general population. Death rates from heart disease and all cancers combined are considerably less in flight crew then for the population of England and Wales as a whole and, although rare, death from melanoma (which is associated with exposure to sunlight) was the only cause of cancer in excess. Cancer such as leukaemia, which may be linked to radiation exposure, was lower than for the general population. Further larger studies are continuing to which British Airways is contributing and as a result, more information will be available in due course.
Summary As required by UK legislation, British Airways estimates the dose received by flight and cabin crew for each sector using a sophisticated computer model. This takes into account all the factors previously discussed including the aircraft climb and descent profiles, latitude of the flight, altitude, time of year and point in the solar cycle. The computer estimates are periodically validated by measurements performed in flight.
Cosmic radiation is both a complex and emotive subject. It cannot be seen, touched, smelt or tasted and yet it is present all around us. Whilst we know that there is no level of radiation exposure below which effects do not occur, we can estimate the probability of any harm occurring based on the exposure received. This, coupled with the knowledge of doses received by British Airways crew and the available epidemiological studies, reassures us that there is an extremely low probability of individuals suffering an abnormality or disease as a result of exposure to cosmic radiation.
|
||||||||||||||||||||||||||||||||||||||||||||||||||
