Radiation is "the transfer of energy from a source" which may be in the form of electromagnetic radiation such as gamma rays and x-rays or in the form of mobile and highly accelerated sub atomic particles. Ionising radiation is radiation which can displace charged particles, which means that molecules within the cell walls of living organisms can be disrupted. "Cosmic Radiation" is the collective term for the ionising radiation present in the earth’s atmosphere which has originated either from the sun or from outside our solar system (galactic radiation) as Primary Particles or has been created as a result of the interaction of these primary particles with the earth’s atmosphere to create Secondary Particles.
Galactic cosmic radiation in the local environment of the solar system consists of about 86% positively charged hydrogen nuclei (protons), 11% alpha particles (helium nuclei), 2% electrons (or beta particles) with heavy metal ions and antimatter particles called positrons making up the rest. Within the solar system, lower energy particles are deflected by the magnetic field of the solar wind - the stream of plasma emitted by the Sun. This follows an 11 year activity cycle which at its maximum output, coincides with increased numbers of sunspots and serves to deflect relatively more of the galactic cosmic radiation component away from the earth with the converse also applying. Whilst its routine effect on cosmic radiation levels is beneficial, the solar maximum is also the most likely time for the generation of Solar Particle Events (SPEs) in which solar flares or coronal mass ejections release large amounts of energy which cannot be forecast although they are short lived - hours to days in duration. Other types of short term and generally unpredictable solar activity can also vary the magnetic fields of both the sun and the earth and can produce major but very temporary increases in cosmic radiation at aircraft altitudes but they are infrequent and make little difference to the rates of dose accumulation. The term "Space Weather" is used to describe all these solar-caused or solar-moderated phenomena.
Nearer the Earth, the Primary Particles of cosmic radiation are nearly all electrically charged and many are deflected by the earth’s magnetic field. This effect varies according to the energy state of the impacting particle and its trajectory in relation to the magnetic field. Least deflection occurs for high energy particles and those travelling parallel to the lines of force of the magnetic field. The latter is the case at high latitudes whereas at the equator, the magnetic field is perpendicular to the direction of particle motion and a much greater proportion of particles are deflected.
The result of the interaction of Primary Particles with the atmosphere is a complex radiation field which has a composition very different to that further out in space. The balance depends on altitude, latitude and the position in the solar cycle. For a 35000 ft altitude in temperate latitudes, typical shares of ambient radiation will be neutrons (55%), electrons and positrons (20%), protons (15%), photons (5%), and muons (5%). By comparison, at sea level at the same latitudes, the particles present are nearly all muons. This difference from the "outer space" balance of particles is a result of both the proton deflection and the fact that the majority of Secondary Particles released when Primary Particles collide with atoms of hydrogen and helium in the upper atmosphere are neutrons.
How Does Total Cosmic Radiation Vary at Aircraft Altitudes?
From the explanation above; the influences on radiation levels at typical aircraft altitudes of around 35000 ft can be summarised. In order of their relative importance, the four main influences are:
- Altitude - atmospheric shielding means that exposure increases as altitude increases. The exposure at aircraft altitudes is about 100 times greater than on the ground, but the increase is not linear and below about 25000 ft altitude, the increase over surface levels is modest. The maximum exposure is reached at 60,000 ft altitude.
- Latitude - the difference in shielding between high and low latitudes has a much more marked effect on exposure at aircraft altitudes than at the surface. At 50° exposure is up to 4 times greater than at the equator.
- Normal Solar Activity - the 11 year solar maximum reduces exposure by about 25% and the corresponding solar minimum increases it by about 25%
- Random Solar Activity - SPE produce large but short-lived increases in exposure which double exposure during their occurrence.
What effect does it have and how is it measured?
Any ionising radiation can displace charged particles, which means that molecules within the cell walls of living organisms can be disrupted. Whilst processes within cells can repair most of this damage, there is, despite a lot of research in the area, considerable uncertainty as to the potential effects to aircrew of exposure to cosmic radiation and it is presently believed that only long term epidemiological studies of their health will better inform our knowledge as to both the degree of risk and the nature of it. Whilst ionising radiation can be objectively measured by absorbed dose - the energy deposited per unit mass - equal absorbed doses of different types of radiation cause biological effects of different magnitudes and the sensitivity of different body tissues to these different types varies. The risk from human exposure to environmental ionising radiation of any type is therefore estimated by calculating an "effective radiation dose" measured in sieverts (Sv). At the quantities involved these are measured in the smaller units of micro sieverts (μSv) and milli sieverts (mSv) with 1000μSv equal to 1 mSv. Effective dose measurement uses weighting factors to takes into account the variation in tissue sensitivity and known biological effects. Cosmic Radiation is quite different in its composition compared to other forms of ionising radiation to which humans are exposed. Neutrons typically contribute up to 50% of the effective radiation dose that aircrew receive as a result of flying whereas occupational exposures of affected ground level workers are more likely to be dominated by gamma radiation and X-rays. The World Health Organisation notes that the biological effects of heightened exposure to ionising radiation attributable to neutrons are not fully understood. Present thinking is based upon the premise that as well as high levels of radiation being harmful, lower levels carry a risk which is proportional to dose.
Aircrew Exposure Monitoring
In 1990, the International Committee on Radiological Protection (ICRP) recommended that jet aircrew should be considered occupationally exposed to ionising radiation. This has led to the introduction of corresponding exposure monitoring schemes in many jurisdictions. In Europe, aircrew liable to receive more than 1 mSv per year must be assessed and informed of the potential risks with tactical adjustment of working duties expected as a response to exposures significantly greater than the average. The purpose of this is to ensure that no aircrew will approach the internationally accepted limiting effective dose for all ionising radiation of 20 mSv per year averaged over 5 consecutive years or 50mSv in any one year. Whilst it is considered that it is most unlikely that aircrew would reach these limits under any current circumstance, the cautious approach to the risk being taken by some jurisdictions reflects the scientific uncertainty surrounding the possible cumulative effects of long term aircrew exposure to specifically cosmic radiation. The one particular area where aircrew may be affected by the general limits is the specific exception they make because of the potential for a heightened risk to the foetus in pregnant females by imposing an equivalent dose limit for such workers of 1 mSv during the declared term of the pregnancy. As a result of this, many airlines now have a policy of transferring pregnant aircrew to ground duties as soon as pregnancy is confirmed. To put this in perspective, it is estimated than a typical return transatlantic flight leads to an effective cosmic radiation dose of about 60μSv.