Introduction

• As pilots fly ever higher in the atmosphere, and overcrowding leads to increased use of currently in frequently used routes, such as trans-bowler, the issue of radiation and aviation becomes more important. Due to long hours of flight the exposure time has also increased.

The Nature of Cosmic Radiation

• Radiation is the transfer of energy from a source. It may be in the form of electromagnetic radiation such as x-rays and gamma rays. It may also be in the form of particles such as neutrons and protons.

• Cosmic radiation is the collective term for the radiation which comes from the Sun (the solar component) and from the galaxies of the Universe (the galactic component). Cosmic radiation is ionising, i.e. it can displace charged particles from atoms.

• This can lead to the disruption of molecules in living cells. Processes in the cell repair most of this damage.

• Cosmic radiation consists of a complex mixture of radiations and their interactions in the atmosphere are similarly complex. Nevertheless, the Earth's atmosphere substantially shields the Earth from cosmic radiation, and doses of cosmic radiation are greater with increasing altitude.

• Cosmic radiation particles may be electrically charged and so may be deflected by the Earth's magnetic field, it is for this reason that doses of cosmic radiation are greater at higher latitudes towards the Earth's magnetic poles.

• The deflection of cosmic radiation particles is least for higher energy particles and for particles of all energies travelling parallel to the magnetic field lines.

• The deflection is greatest for lower energy particles such that apart from exceptional solar events, the solar component of cosmic radiation is of no direct concern. The output of radiation from the Sun varies in an approximate 11 year cycle. At times of maximum solar output, associated with increasing numbers of sunspots, the magnetic field embedded within the Sun's radiation serves to deflect more of the galactic cosmic radiation component away from Earth.

• For this reason doses are about 20% lower than the mean value during maximum solar activity and about 20 % higher during solar minimum. Mainly, but not exclusively, during solar maximum there is a small probability of a solar flare giving rise to exceptionally high numbers of energetic particles such that there are increased levels of cosmic radiation at aircraft altitudes.

Doses from Cosmic Radiation

• Ionising radiation is measured in terms of 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 tissues of the body differ.

• To account for this, tissue absorbed doses are multiplied by radiation weighting factors to give equivalent doses, and by tissue weighting factors to give the effective dose in sieverts (Sv) to the whole body.

• Cosmic radiation is made up of many different types of radiation of a wide range of energies and consequently is difficult to measure to a high degree of accuracy.

• Cosmic radiation effective dose rates increase with altitude up to a maximum at about 20 km (66,000ft), and with increasing latitude reaching a constant level at about 50°.

• The effective dose rate at an altitude of 8 km (26,000ft) in temperate latitudes is typically up to about 3 microSv per hour (1000 microSv = 1 mSv) but near the equator only about 1 to 1.5 microSv per hour. At 12 km (39,000ft), the values are greater by about a factor of two.

Cosmic Radiation and Flying

• Operators should make arrangements to give information and provide education regarding the risks of occupational exposure to radiation to their air crew, air crew being defined as flight crew, cabin crew and any person employed by the aircraft operator to perform a function on board the aircraft while it is in flight.

• Female air crew should be made aware of the need to control doses during pregnancy and to notify their employer if they become pregnant so that any necessary dose control measures can be introduced.

• Aircraft capable of operating at altitudes greater than 15 km (49,000ft) should carry an active radiation monitor, which monitors current levels of radiation, to detect any significant short-term variation in radiation levels during flight. (This is to comply with world standards).

• Such variations may arise as a result of solar flares, or other solar events, which can cause a sharp increase in the solar component of cosmic radiation, especially at altitudes above 15km.

• Potential exposure resulting from such an event can be greatly reduced by a controlled descent if active monitoring is used.

• The galactic component of cosmic radiation, which is more important at lower altitudes, is not typically subject to large sudden changes.

• Air crew operating such high flying aircraft should be subject to the same general monitoring regime as for those operating between 8 and 15 km but account should be taken of the greater potential variability of dose.

• Active monitoring may be used to assess the doses to which air crew are exposed to provide a warning of high dose rates.

• In principle, the need to detect high dose rates could be achieved by some means other than an on-board monitor e.g. satellite or ground based solar monitoring systems. However, at present, such techniques provide no more than retrospective estimates of dose.

What Needs to Be Done

• Operators should ensure that studies are carried out so as to assess the likely magnitude of the exposure to cosmic radiation of air crew for whom they are responsible. These studies should cover all rostering arrangements.

• For operators of turbo-prop aircraft, predictions of dose, the maintenance of records of route dose rates, and individual air crew records of predicted dose may not be required because, owing to the normal flight characteristics of these aircraft, the annual doses of crew are likely to be low (less than 1mSv per year).

• No controls are necessary for an individual member of air crew whose annual dose can be shown to be less than 1 mSv.

• For flights at temperate latitudes at a typical altitude of 10.6 km (35,000 ft), and for average solar activity, the effective dose rate is about 5 to 6 microSv h-1, and therefore a total time at altitude of about 200 hours is needed to accumulate 1 mSv.

• Near the equator at this altitude, the time needed is about 400 hours. At an altitude of 11.8 km (39,000 ft) these times are about 150 and 300 hours respectively, and at an altitude of 10 km (33,000 ft), about 250 and 500 hours respectively.

• Airlines generally work in terms of 'block hours'. These start from when the aircraft is pushed back from its stand and finish when its engines are switched off after landing. Block hours may therefore be considerably greater than flying hours, and this must be recognised when estimating doses.

• Operators whose air crew may receive an effective dose greater than 1 mSv, generally those operators whose aircraft operate above 8km (26 000ft), should carry out a detailed  assessments of exposure, expressed in millisieverts per year . If the assessed annual dose is less than 6mSv per year, then no further action to be taken. However, it should be noted that the European Commission guidance on this issue recommends individual monitoring. This is also common practice for ground-based workers with annual doses between 1 and 6 mSv.

• Individual monitoring is to be regarded as best practise but it is recognised that this can impose unjustifiable cost for some operators. In these circumstances an acceptable course of action would be to rely on an assessment of maximum doses where this shows that air crew will not be approaching annual doses of 6 mSv. A suitable cut off point would be where the assessment indicates a maximum annual dose of 4 mSv. The purpose of such monitoring would be to ensure that annual doses did not exceed 6 mSv.

• Where an assessment of maximum doses indicates that air crew are liable to excede 6 mSv per annum, individual monitoring must be carried out. In addition, operators should adjust an air crew member's roster to reduce exposure with the aim of preventing, where possible, doses in excess of 6 mSv per annum.

• Where the assessment of individual doses is necessary, this may be done by combining roster information with route doses. For example, a flight from northern Europe to the eastern seaboard of the USA will result in a value of effective dose of about 30 to 40 microSv. For a longer flight from northern Europe to Japan, the total effective dose is about 50 to 70 microSv. Transatlantic flights at the altitudes used by supersonic aircraft may give similar total effective doses as in subsonic aircraft, the higher dose rates being offset by the shorter flight times.

• Unusually high levels of cosmic radiation at altitudes relevant to civil aviation can result from solar particle events (SPEs). These are produced by sudden, sporadic releases of energy in the solar atmosphere (solar flares), and by coronal mass ejections (CMEs). Only a small fraction of SPEs , a few per year, cause an observable increased intensity of cosmic radiation fields at aviation altitudes. These can be detected by ground monitors and are referred to as ground level events (GLEs).

• The largest events often take place on either side of the period of maximum solar activity as measured by sunspot number. Any rise in dose rates associated with an event is quite rapid, usually taking place in minutes. The duration may be hours to several days. The prediction of which events will give rise to significant increases in dose rates at aircraft altitudes is not currently possible.

What Operator Should Do

• The operator should have a system of record keeping which should be detailed in the Operations Manual and which should be available for inspection by the Aviation Authorities.

• To facilitate using flight time and duty rosters for the maintenance of radiation records, and to give a more accurate record of dose received, the ANO requirements relate to a rolling year. This defines the year of record as being the period of 12 months expiring at the end of the previous month and accords with one of the definitions in the ANO with regard to flight time limitation records.

• Similarly, the radiation records should be kept for a 12 month period after the last complete 12 month period recorded, i.e. for two years. This accords with the requirement to keep flight time limitation records for one year after the flight referred to. However, individuals exposed to more than 6mSv in any 12 month period must have their records kept for 30 years (even if the person is deceased), or until the individual reaches 75 years, whichever is the longer period of time.

• Individuals are entitled to have access to their records and to be able to obtain a copy to offer their new employer, should they change employment