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August 31, 2010

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Fire, earth, water – Rays and means

Evolving life on Earth, from the time of primordial single-cell organisms to humankind, has been immersed in a sea of radiation. In certain work environments it is a source of incidental pollution, so Dr Chris Ide explains the risks and suggests what can be done to address them.

There are two main types of radiation – ionising, and non-ionising – both of which are part of the electromagnetic spectrum. Ionising radiation consists of high-energy electromagnetic waves, such as x- or gamma rays, which have sufficient energy to pass through the body; or charged subatomic particles – alpha and beta – which, when they collide with an atom or molecule, have sufficient energy to knock off electrons and leave an ion, carrying an electrical charge. These particles can generally be stopped by cardboard, or intact skin.

Although radiation exposure can vary, average background exposure in the UK is about 2.4mSv per year, ranging from 1 – 10mSv,1 But there are parts of the world where background levels are much higher, such as Ramsar in Iran, where levels of up to 260mSv have been recorded.

About half of our exposure to ionising radiation is due to radon,2 chemical symbol Rn, a colourless, odourless, tasteless gas, which is derived from the radioactive breakdown of uranium. Since small amounts of uranium are ubiquitously scattered throughout the earth’s crust, and concentrated in some building materials, radon is being constantly generated.

There are several different types – isotopes – of radon, each one arising from a different point in the sequence of radioactive disintegration. Two of them –  220Rn and 219Rn – have half-lives of less than one minute, so hardly have time to leave the ground. By contrast, the main Rn isotope of importance – 222Rn – has a half-life of about 3.8 days.

Radon gas is significant because it is responsible for about 5 per cent of cases of lung cancer. Compared to the well over 80 per cent of lung cancers that can be attributed to tobacco smoking, this may seem trivial but lung cancer is the most common fatal cancer worldwide, so, in these circumstances, a relatively small proportion can still equate, in absolute terms, to a considerable number of cases. Furthermore, this may well rise if the proportion of lung cancers due to smoking widely falls, as it is already beginning to do in British men.3

Generally speaking, radon gas is very rapidly diluted to harmless levels. However, in soils where the proportion of uranium is higher, a greater proportion of radon is found. The UK has been quite intensively surveyed, and maps are available through the Health Protection Agency (HPA), showing the percentage of homes in various 5km2 locations where the current action limit of 200 becquerels per m3 is likely to be exceeded,4 giving a lifetime exposure risk of lung cancer of about 3 per cent.5 Interestingly, in Scotland, this includes Her Majesty’s highland residence in Balmoral!

The HPA can provide sampling devices for householders (or business owners) to enable them to measure the radon level in their own homes, or premises. While a screening service is available, dependable results really require the sampling to take place over a period of three months, in order to iron out short-term fluctuations. These can occur, for example, during periods of very cold (or extremely warm) weather, when ventilation may be more restricted (or liberal) than usual. Once again, this is not particularly a problem, unless the radon-containing air is allowed to accumulate in confined spaces, such as cellars, basements and attics.

Should the radon levels require remedial action, well-established techniques exist, such as constructing a radon sump to expel the gas to the outside air, or laying an impermeable membrane to prevent the gas entering through the floor. The estimated cost of a radon sump is about £1000, together with £50 a year for the power costs, while membranes cost around £100.

Medical and diagnostic exposures account for about 14 per cent of ionising radiation doses and are generally confined to health-service environments, where exposure is carefully calculated to minimise it for both subjects and employees, although errors occasionally occur.

The dose administered varies very widely, from about 0.01 – 0.015mSv for a chest x-ray to 10mSv for an adult abdominal CT scan, which is about 40 times the dose for a plain film of the abdomen.6 Radiotherapy used for curative treatment of tumours can result in exposures of up to 60 gray, usually as a result of multiple exposures over several weeks.

Another source is cosmic rays – sub-atomic particles that arrive at the Earth’s atmosphere from the sun and elsewhere in the universe – together with x and gamma radiation, which may also be generated by particles colliding with atoms in the atmosphere. The amount of exposure depends on a combination of position relative to the earth’s magnetic field, and altitude.

Cosmic rays do present a hazard to astronauts, and are likely to present a considerable obstacle to prolonged interplanetary exploration. While not many readers will advise space travellers, more will be employed by organisations whose employees fly frequently, either as passengers or aircrew. For the purpose of engine efficiency, jet aircraft have been flying at high altitudes for many years. While this increases exposure to cosmic radiation, generally speaking, this does not seem to result in a significant increase in serious disease.

Health effects

The health hazards associated with radioactivity and ionising radiation became apparent very quickly, and many of the pioneer doctors, scientists and technicians who worked in this field in the late 19th and early 20th centuries paid a high price, as did some of their patients. Roentgen announced the discovery of x-rays in 1895, but within two years, 69 cases of skin damage had been reported,7 and within 15 years, every health hazard associated with exposure to ionising radiation had been identified.

Since then, drawing on studies of the mortality experience of many groups, such as those professionally exposed,8 patients who were treated with ionising radiation for a wide variety of conditions,9 Japanese atomic bomb survivors,10 and workers involved in the production of nuclear materials and power generation,11,12 the safe dose levels of radiation have been constantly refined and reduced.

There are two main types of health hazard associated with ionising radiation – stochastic and non-stochastic, or deterministic. The main stochastic hazard associated with ionising radiation is cancer, such as leukaemia, thyroid, or lung. Cases of these cancers occur on a random basis and there is no variation in their severity (you can’t have a ‘little’ leukaemia – you either have it, or you don’t). However, the number of cases in a population is proportionate to the dose received.

By comparison, the principal non-stochastic, or deterministic effects include radiation burns, the neurological, blood and gastro-intestinal manifestations of radiation sickness, cataracts, and damage to unborn children. For these phenomena, there is a threshold (a factor of time and dose) below which they do not occur. Nonetheless, it is interesting to note that surveys of residents living in areas such as the aforementioned Ramsar, and other locations with high levels of naturally-occurring background radiation, do not appear to have worse health than those dwelling in areas where the radiation dosage approximates to more normal levels.13

Non-ionising radiation

Ultraviolet radiation is another form of ubiquitous radiation, particularly for outdoor workers, such as those involved in agriculture, forestry and the construction industry. Its principal source is sunlight. UV radiation has beneficial effects, insofar as it synthesises vitamin D, which helps strengthen bones. In moderation, it also helps us feel good, although excessive acute exposure results in painful sunburn, and premature aging of the skin.

In the longer term, skin cancers develop, in particular so-called ‘rodent ulcers’, which usually affect older people. Generally, the prognosis is good, since they are easily treated, but they may require extensive plastic surgery in serious cases.

The other, more serious skin cancer associated with UV exposure is malignant melanoma, which tends to spread both locally and to more distant parts of the body. Protecting the skin by – where appropriate – loose-fitting clothing, use of sun creams, wide-brimmed hats and, where feasible, avoiding exposure will reduce this risk.

Finally, those who work outdoors in winter may suffer from ‘snow-blindness’ – a type of conjunctivitis caused by UV rays reflected from snow fields. Appropriate eye protection, in addition to some of the precautions previously mentioned, will help eliminate this problem.

A perhaps unexpected problem is occult exposure to UV light. Many years ago, when I worked for the HSE, I took a phone call from a food factory in which some workers were complaining of sunburn (in January!) I visited the premises, spoke separately and privately to the employees and manager involved – yes, they were sunburnt. Walking through the workplace, I noted several insectocuters (pest-control devices that lure flying bugs to their doom using UV light, and electrocuting them against a charged grid, their bodies dropping into a tray). “Aha!”, I thought, “this is a simple photodermatitis” (a skin eruption caused by the interaction of light and a chemical). However, all the substances had been in use for several years, and in other parts of the factory with identical insectocuters and chemical usage, there were no skin problems whatsoever.

I sought advice from a consultant dermatologist with a long-standing interest in work-related health problems who was equally baffled, so we returned to the premises with a health physicist, who measured UV radiations in 10 representative locations. In seven, the employees were exceeding their recommended daily exposure to UV; in three of these, this was occurring within 20 minutes of starting work! We shut off the insectocuters, and looked at the emitting tubes – they were of the type used to emit UV rays for sterilising surgical instruments.14 

The wavelengths associated with visible light are not without their health and comfort problems. Inadequate levels of illumination can result in hazards not being fully visible, possibly leading to slips, trips and falls. It is also important to ensure that adequate illumination is not hampered by shadows cast by obstructions. Poor lighting also causes loss of efficiency among employees because of headaches and eyestrain.

To some extent, we are dealing with personal comfort here – what will suit one person might not necessarily suit another, so it is important that workers are able to exert a degree of control over their local lighting. Useful advice on this complex subject is available from several sources, particularly the HSE.15

Infrared radiation has a wide variety of applications, such as night-vision imaging, remote temperature sensing, as a source of heat, and for short-range communications. If over-exposure occurs, there is the possibility of damage to the eyes, by the gradual formation of cataracts.

Microwave radiation was originally a feature of radar emissions, used to detect, track and identify distant objects. This is still a vital feature of systems whose purpose varies from air-traffic control to speed traps. Most of us initially became familiar with microwaves from their use in cooking, which, according to popular legend, was first noticed by a Raytheon employee, who was making cavity magnetrons in World War 2. He was surprised that the foil-wrapped chocolate in his pocket always melted when in the vicinity of these devices –  and the rest, as they say, is history!
Microwaves are also an important part of telecommunications networks; mobile phones would not exist without them. Nonetheless, concern has been expressed over the long-term health implications of their use, especially from the point of view of the relationship between mobile-phone use and tumours of the brain and salivary glands. Much of the research tends to show what I call ‘The pantomime phenomenon’ (“Oh yes it does!. . .Oh no, it doesn’t!”)

As this controversy had been ongoing for almost a decade, great things were expected from the ‘Interphone’ study – the largest survey to date of the biggest number of users with more than 10 years’ use.16 At first, the results were reassuring, showing no rise in tumour incidence, apart from among the highest users, but uncertainties rapidly developed in the interpretation of the data, with some of the principal authors seeming to draw widely differing conclusions, the typical disclaimer being “more research is needed”.

However, it is important to remember that, although no currently available mobile phone exceeds any exposure thresholds, the current limits are set in relation to local-tissue heating effects, rather than the longer timescales usually required for tumour induction. This is why there are concerns over their use by children and young people, who have a longer time in which to accumulate damage. A NIOSH blog17 contains some provoking thoughts, but points out that the greatest hazard would seem to be the short-term one of distraction while driving, or carrying out other safety-critical tasks.

As usual, safety advisors should be able to obtain sound advice from publications from reputable organisations, their peers, and, with the judicious application of common sense, should be able to pilot their clients through electromagnetic uncertainties. 

References
www.unscear.org/docs/reports/gareport.pdf (accessed 22 July 2010)
2    National Physical Laboratory –
www.npl.co.uk/science-+-technology/ ionising-radiation/radiation-sources
3    info.cancerresearchuk.org/cancerstats/ types/lung/incidence/  (accessed 22 July 2010)
4    
www.hpa.org.uk/Topics/Radiation/ UnderstandingRadiation/Understanding RadiationTopics/Radon/radon_Map/    (accessed 17 July 2010)
5    
www.hpa.org.uk/web/HPAweb&HPAweb Standard/HPAweb_C/1197636998945 (accessed 23 July 2010)
6    Bremner DJ, Hall JH (2007): ‘Computed tomography – an increasing source of radiation exposure’, in N Eng Med J 2007;357:2277-84
7    
www.ccnr.org/ceac_B.html (Section B1)
8    Berrington A, Derby SC, Weiss HA, Doll R (2001): ‘100 years of observation on British radiologists: mortality from cancer and other causes 1897 – 1997’, in Br J Radiol 2001,74;882:507 – 19
9    L Ron E (2003): ‘Cancer risks from medical radiation’, in Health Phys 2003,85;1:47 – 59
10    Little, MP (2009): ‘Cancer and non-cancer effects in Japanese atomic bomb survivors’, in J Radiol Prot 2009;29 (2A):A43 – 59
11    Muirhead CR, Goodill AA, Haylock RG, et al (1999): ‘Occupational radiation exposure and mortality: second analysis of the National Registry for Radiation Workers’, in J Radiol Prot 1999’19;1:3 – 26
12    Loomis DP, Wolf SH (1997): ‘Mortality of workers at a nuclear materials production plant at Oak Ridge, Tenessee 1947 – 1990’, in Am J Ind Med 1997,31;1:121
13    Ahmed JU (1991): ‘High levels of natural radiation: report of an international conference in Ramsar’, in IAEA Bulletin 1991;2:36 – 38
14    Forsyth A, Ide CW, Moseley H (1991): ‘Acute sunburn due to accidental irradiation with UVC’, in Contact Dermatitis 1991;21;141-142.
15    HSE (1997): Lighting at work, HSG38, Health and Safety Executive, ISBN 97 807 176 123 21
16    The INTERPHONE Study Group (2010): ‘Brain tumour risk in relation to mobile telephone use: results of the INTERPHONE international case-control study’, in Int J Epidemiol 2010,39;3:675 – 94
17    
www.cdc.gov/niosh/blog/ (accessed 27 July 2010)

1    Report of the United Nations Scientific Committee on the Effects of Atomic radiation to the General Assembly – 

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