Cosmic Radiation and Aircrew Exposure: Implementation of European Requirements in Civil Aviation, Dublin, 1-3 July 1998

Abstract

The European Union's Basic Safety Standards Directive (96/29/Euratom) lays down safety standards for the protection of workers and the general public against the effects of ionising radiations. Article 42 of the Directive deals with the protection of aircrew. It states that for crew of jet aircraft who are likely to be subject to exposure to more than 1 mSv y-1 appropriate measures must be taken, in particular: to assess the exposure of the crew concerned, to take into account the assessed exposure when organising working schedules with a view to reducing the doses of highly exposed aircrew, to inform concerned workers of the health risks involved in their work, to apply Article 10 to female aircrew. (The unborn child shall be treated like a member of the public.) This Directive must be transformed into national law of the 15 member states of the European Union by 13 May 2000. The European Commission and the Radiological Protection Institute of Ireland sponsored this International Conference. The objective of this conference was to assist both the airline industry and the national regulatory organisations in identifying the means available to comply with the requirements of the Directive. Over 200 delegates attended the conference from more than 25 countries. The welcoming addresses were made by Mary Upton (Director of the Radiological Protection Institute of Ireland), Joe Jacob (Minister for State responsible for Nuclear Safety) and James Currie (Director-General for the Environment, Nuclear Safety and Civil Protection). Mr Currie stated that there was a need for political decisions to be based on good science, and that technological trends will lead to higher and longer flights, and therefore higher radiation doses. The first day concentrated on the scientific basis of measurement, calculation and monitoring of cosmic radiation. The first speaker, Dr Heinrich from the University of Siegen, Germany, talked about the physics of cosmic radiation fields. He pointed out that of all the particles that come from outside our solar system 85% are hydrogen, 12.5% are helium and 1.5% are heavier particles. The flux of these particles changes over the 11-year solar cycle: if the solar activity is high then the cosmic radiation flux is low. The Earth's geomagnetic field affects whether or not a particle will reach the Earth. The nearer the equator, the higher the cut-off rigidity and hence the greater the shielding. At the poles the cut-off rigidity is at its lowest, hence the greater the number of particles which reach the Earth. The speaker summarised by saying that in order to make an accurate assessment of the radiation dose due to cosmic radiation one must know which primary cosmic rays are involved, the solar modulation effects, the geomagnetic shielding and particle interactions in the atmosphere. Alternatively dosimetric measurements can be made at different altitudes, latitudinal and longitudinal positions for the most relevant radiation components. The second speaker of the morning, Dr Hilton Smith, the Ex-Scientific Secretary of the ICRP, gave a talk entitled `Quantifying Radiation Risk'. The talk started by explaining that high LET radiations have a greater probability of causing DNA damage than low LET radiations and that DNA can be damaged in a number of ways: the single-strand break, two single-strand breaks, the double-strand break (the hallmark of ionising radiation) and base damage. The possible effects of these interactions are the death of the cell, inhibition of cellular division and change in cell structure. The physical and chemical effects occur over very short periods, but the biological effects may not be noticed for many years. The speaker described risk estimation based on A-bomb survivors, medical therapy, medical diagnosis of patients, occupational studies of uranium miners and radium workers. The human fatal cancer risk has been calculated by the ICRP to be 5% Sv-1 for the public. The maximum likelihood of cancer occurring is at age 70. At the end of the presentation there was a discussion on whether or not protons of a certain energy should have a quality factor of 5. It was suggested that the factor should be equal to one. Dr Bartlett of the NRPB gave the next talk on Radiation Protection Concepts and Quantities for Occupational Exposure to Cosmic Radiation. Dr Bartlett explained that there are significant differences between the exposure condition of aircrew and occupational exposure generally. There are a greater range of radiation types and energies. Half of aircrews' doses are due to neutrons. UK Classified radiation workers receive 2% of their dose from high LET radiations and aircrew receive 50%. Dose distributions and characteristics of the working populations are different, with 53% of aircrew being female, as opposed to 7% of Classified UK radiation workers. The field intensity on aircraft is predictable, and, with the exception of rare solar flare events, there is no risk of accidental exposures. The speaker highlighted the variation in cosmic radiation dose as a function of altitude illustrated by the radiation doses at 15, 10 and 6.7 km being 10, 5 and 1 µSv h-1. It was interesting to note the comparison made between the average radiation dose of 1 mSv y-1 in the nuclear industry and 2 mSv y-1 for aircrew. The speaker said that it is necessary to appreciate that people living in high radon areas in the UK receive approximately 8 mSv per year. Dr Bartlett highlighted how the requirements for the protection of aircrew from the Basic Safety Standard Directive (BSS96) differed from those for occupational exposures in general, namely that there are not explicit dose limits, other than that to be applied to the exposure of the foetus. There are no requirements for the designation of areas or classification of workers and there is no reference to the principle of ALARA, but there is a requirement to take account of the assessed exposure when arranging work schedules with a view to reducing higher doses. Dr Bartlett summed up by saying that dose assessment will probably be done by folding roster information with estimates of route doses. The last speaker of the morning session was Dr Maria Blettner, from the International Agency for Research on Cancer, Lyon, France. She talked about epidemiological studies for individuals occupationally exposed to radiation. The speaker emphasised that the results of early studies regarding cancer mortality are equivocal; elevated cancer risks have been observed in some studies, but not in others. The low cumulative dose up to 100 mSv is associated with poor statistics. Therefore it is difficult to calculate the relative risk of exposure with a high degree of confidence. The speaker also highlighted the difficulty in obtaining a comparison population since aircrews have characteristics and lifestyles that differ from the general population. The speaker stressed the need for large studies in this field of epidemiology. Dr Blettner summarised her speech by saying that the results of a cohort of some 22 000 pilots and 47 000 crewmembers can be formed from the workers in nine different countries and that pooled analyses are expected in 2001. The next speaker was Dennis O'Sullivan, from the Dublin Institute of Advanced Studies, who gave a talk entitled `Overview and Present Status of EC Research Programme'. The objectives of the EC programme were highlighted as follows: to develop and calibrate instrumentation for use at altitude, to measure flux and energy spectra of neutrons and charged particles, to measure LET spectra and ambient dose equivalent, to estimate dose contribution by solar particle events and finally to compare results with calculations. The airlines involved in these studies were Aer Lingus, Alitalia, BA, Lufthansa and Scandinavia Airlines. Tests were carried out on several routes, on both subsonic and supersonic aircraft. A detailed set of measurements were obtained over a five-year period. Professor O'Sullivan said that the NRPB used TLDs for low and high LET radiations and PADC for neutrons. The investigation of dosemeter response was carried out using Monte Carlo codes. The active instruments used for measurements were the tissue equivalent proportional counter (TEPC) and a Bonnersphere spectrometer using eight spheres. The instrumentation used was calibrated in the CERN-CEC reference field. In summary, it was found that the shape of the neutron spectrum does not change with altitudes and that the maximum dose rate was found to be under the seats of the aircraft. Dr Lindbourg of the Swedish Radiation Protection Institute gave a short talk on the importance of using the TEPC for cosmic ray measurements, as it is the only means of reading directly absorbed dose to tissue and the radiation quality (in terms of lineal energy). Dr Schewe from PTB, Germany, gave the next talk on reference fields and calibration procedures. The speaker highlighted the difficulties in measuring radiation fields onboard aircraft, as the calibration fields used are often vastly different to the radiation field the instrumentation is being exposed to. The speaker said that this could lead to errors in the measurements in excess of 50%. One way around this is to use realistic reference fields, which produce similar particle compositions and particle fluences as those present in the cosmic radiation at aircraft altitudes. For this work the reference field facility in one of the secondary beams lines of the CERN Super Proton Synchrotron was used. In summary it was shown that the TEPC could be used as a reference instrument for evaluating ambient dose equivalent in aircraft. The next speaker was Dr Tommasino of the ANPA, Rome, who talked about in-flight measurement of radiation fields and doses. He stated that the problem of radiation dose assessment has been developed within the multinational research programmes of the Commission of the European Communities. The speaker talked about the different dosimetric systems formed by the TEPC, ANPA-stack, DIAS-stack and Extended Rem-counter. The ANPA-stack and DIAS-stack detectors have been developed under the CEC research programme specifically for the measurement of cosmic radiation on aircraft. The experiments were carried out between 1994 and 1997 at the period of the solar minimum, and therefore represent an upper limit on the dose due to galactic cosmic rays. The speaker gave an example of a flight from Tokyo to Milan, where the ambient dose equivalent was 4.83 µSv h-1 and the annual dose, assuming a 700 hour year, was 3.38 mSv y-1. In conclusion the speaker said that the measurements from all four dosimetric systems were consistent. Dr Schraube from the National Research Centre for Environment and Health, Germany, gave the last presentation of the day, on the experimental verification and calculation of route doses. The verification was restricted to neutrons. The speaker showed that theoretical calculations could be matched to experimental data. Therefore the fluences at all positions of interest in the Earth's atmosphere could be calculated. It was then possible to calculate the doses on aviation routes using the computer package European Program for the Calibration of Aviation Route Dose (EPCARD). The second day of the conference concentrated on the airline industry perspective of the cosmic radiation problem. The first speaker was B Lecouturier, of the Federation des Syndicats de Transport, Brussels. She gave an introductory presentation on the view of cabin crew. The speaker highlighted the inconsistency of some EU states: for example, some states insist that pregnant workers stop flying, while others do not. In conclusion the speaker said that cabin crew wish for a correct assessment of their cosmic radiation dose, medical surveillance and further epidemiological studies. The second speaker of the day was Dr Balouet, also from the Federation des Syndicats de Transport, Brussels. His talk was entitled, `Ionising Radiations and Cabin Crew Concerns'. The main concerns of cabin crews were as follows: uncertainty in the quality factor for neutrons, heavy ions are not taken into account when calculating radiation doses, 25-60% of some routes if flown for a standard working year could exceed the 6 mSv level, European crew flying on non-European airlines, solar flares, which give relatively high radiation exposures. The next speaker was Wallace Friedberg of the Civil Aeromedical Institute, USA, who gave a talk on the guidelines provided by the FAA to promote radiation safety for Air Carrier Crewmembers. Wallace recalled the information the FAA has provided, including: (a) guidelines for air carrier training programmes on in-flight radiation exposure, including recommended radiation exposure limits; (b) estimates of the galactic radiation dose received on a wide variety of air carrier flights; (c) tables for estimating healthy risks from galactic radiation exposure; and (d) support for research on the effects of irradiation during pregnancy, including possible galactic radiation effects on the reproductive health of female flight attendants. The speaker highlighted the availability of a computer program (CARI) available to the general public, which can be used for estimating the galactic radiation dose received from a non-stop flight between any two locations in the world. Sandy Mitchell of the European Cockpit Association was the next speaker. He gave a talk on the concept of `As Low as Reasonably Achievable' in relation to cosmic radiation. The speaker began by saying that aviation activity was increasing by 5% per annum and flights below 25 000 ft pose no radiation exposure problem. The speaker then drew attention to the strategies that could be employed to reduce radiation exposure, which include restriction of altitude to 31 000 ft, restriction of annual flying hours to 500 hours, increased aircraft shielding, fitting all aircraft with active monitoring and the introduction of annual medical examinations for aircrew. It was also suggested by the speaker that pregnant flying aircrew could undertake ground duties or be given unpaid leave. Alternatively, they could be transferred to regional routes where doses are very low. The speaker concluded by performing a cost-benefit analysis of reducing the cruising altitude of aircraft. This lead to increased fuel consumption but reduced collective dose. It was shown that the collective dose reduction would not be great enough to justify the costs incurred. The session after lunch concentrated on the airlines and the air industry. Dr Oksanen of Finnair gave the fist lecture of this session in a talk entitled `The Operator: Experiences and Views'. The speaker began by summarising the Association of European Airlines (AEA) involvement in cosmic radiation, which has included dose measurement and estimation, production of educational material and epidemiological studies. The speaker talked about the differing methods of route dose estimation using active monitoring, passive monitoring and computer modelling. The AEA airlines believe that the EURATOM Directive may best be implemented by route dose estimation using a common mathematical model. This would have the advantage that modelling is accepted worldwide as a credible and practical method of dose estimates and overcomes the logistic problems, the likelihood of equipment failure and error inherent in direct measurement. In addition, it allows for consistency of route dose estimates among various airlines and finally offers the opportunity for independent scrutiny and audit required. Michael Bagshaw, the Head of Medical Services for British Airways, gave a very interesting talk on in-flight measurements. Mr Bagshaw began by talking about the cosmic ray detection methods used on the Concorde, consisting of GM tubes and boron trifluoride detectors. The information from these detectors is then fed directly to the pilot. The system alarms at 500 µSv h-1. However, since Concorde entered service in 1976 no Concorde has had to reduce altitude due to cosmic radiation. Mr BAGSHAW said the effective dose when averaged over 113 flights was 13 µSv h-1, with the London to New York route dose being 43 µSv h-1. On average flight crew get 3-4 mSv y-1 and cabin crews get 2-3 mSv y-1. Interestingly, it is BA policy to ground crew on the declaration of pregnancy. A crewmember who does not declare she is pregnant may still fly, even if she knows she is pregnant. A pregnant crewmember therefore has a choice. Christopher Hume from British Aerospace gave a short presentation regarding the manufacturer's perspective. A number of issues that are going to increase cosmic radiation dose were highlighted. Future aircraft are going to fly at much higher altitudes, the BIZ JET is going to fly at 60 000 ft and the SCT is going to fly at 70 000 ft. Some new routes may go directly over the North Pole. The speaker mentioned that shielding of the aircraft in order to reduce doses was impractical. Clive Dyer from the DERA Space Department in Farnborough made an unscheduled and very interesting presentation concerning the cosmic radiation effects on avionics. He stated that there are common links between the interaction of radiation within electronics and that within tissue at the DNA level. His talk described the different ways in which ionising radiation can interact with electronics and cause a number of different effects, including bit-flips, destructive burn-out, gate rupture and dielectric failure. Professor Dyer mentioned that the reduction in component size means larger upsets in the electronics. The speaker then concentrated on single-event effects on equipment in space, where the problems were first predicted in 1962 and observed in 1975. PCs on the Space Shuttle and Mir require frequent reboot, typically every nine hours. The speaker concluded by saying that single-event effects can now be seen at ground level because of the design of modern computer chips. The final day concentrated on regulatory aspects. L Bergau, from the Medical Department of Lufthansa German Airlines, gave the first talk on medical aspects. His study involved the comparison of female aircrew and female ground crew. The study excluded anyone who had undergone medical treatment with ionising radiation and heavy smokers. Two thousand cells from each individual were scored. The results showed that the numbers of dicentrics were the same in the cabin crew, aircrew and ground crew, demonstrating that the low cosmic radiation exposures seem not to increase cancer risk. Mr Ulback from the National Institute of Radiation Hygiene, Denmark, gave the next speech. His talk was simply entitled `Radiation Protection'. The speaker outlined how the legislation had been derived from the ICRP, through the EU commission and EU council, and finally adopted in the legislation of the member states. The final speaker, Mr Courades from the European Commission, spoke about EU legislation. Mr Courades said that the EU Directive would affect more states than are currently members because other countries wish to become part of the EU. It was highlighted that Article 42 applied only to aircrew (civilian and military) and not passengers. It was also pointed out that classification of the workplace is not required onboard aircraft. Mr Courades said that it was difficult to have a generic dose for a specific flight because of `free flight' where an aircraft changes altitude frequently. The speaker summed up by stressing the need for a common implementation of the directive.