Protection from radiation
Because exposure to high levels of ionising radiation carries a risk, should we attempt to avoid it entirely?
Even if we wanted to, this would be impossible. Radiation has always been present in the environment and in our bodies. However, we can and should minimise unnecessary exposure to significant levels of man-made radiation.
Radiation is very easily detected. There is a range of simple, sensitive instruments capable of detecting minute amounts of radiation from natural and man-made sources. There are four ways in which people are protected from identified radiation sources:
- Limiting time: For people who are exposed to radiation in addition to natural background radiation through their work, the dose is reduced and the risk of illness essentially eliminated by limiting exposure time.
- Distance: In the same way that heat from a fire is less the further away you are, the intensity of radiation decreases with distance from its source.
- Shielding: Barriers of lead, concrete or water give good protection from penetrating radiation such as gamma rays. Radioactive materials are therefore often stored or handled under water, or by remote control in rooms constructed of thick concrete or lined with lead.
- Containment: Radioactive materials are confined and kept out of the environment. Radioactive isotopes for medical use, for example, are dispensed in closed handling facilities, while nuclear reactors operate within closed systems with multiple barriers which keep the radioactive materials contained. Rooms have a reduced air pressure so that any leaks occur into the room and not out from the room.
Standards and regulation
Radiation protection standards are based on the conservative assumption that the risk is directly proportional to the dose, even at the lowest levels, though there is no evidence of risk at low levels. This assumption, called the 'linear no-threshold (LNT) hypothesis', is recommended for radiation protection purposes only such as setting allowable levels of radiation exposure of individuals. It cannot properly be used for predicting the consequences of an actual exposure to low levels of radiation. For example, it suggests that, if the dose is halved from a high level where effects have been observed, there will be half the effect, and so on. This could be very misleading if applied to a large group of people exposed to trivial levels of radiation and could lead to inappropriate actions to avert the doses.
Much of the evidence which has led to today's standards derives from the atomic bomb survivors in 1945, who were exposed to high doses incurred in a very short time. In setting occupational risk estimates, some allowance has been made for the body's ability to repair damage from small exposures, but for low-level radiation exposure the degree of protection may be unduly conservative.
Most countries have their own systems of radiological protection which are often based on the recommendations of the International Commission on Radiological Protection (ICRP). The 'authority' of the ICRP comes from the scientific standing of its members and the merit of its recommendations.
The three key points of the ICRP's recommendations are:
- Justification. No practice should be adopted unless its introduction produces a positive net benefit.
- Optimisation. All exposures should be kept as low as reasonably achievable, economic and social factors being taken into account.
- Limitation. The exposure of individuals should not exceed the limits recommended for the appropriate circumstances.
National radiation protection standards are based on ICRP recommendations for both Occupational and Public exposure categories.
The ICRP recommends that the maximum permissible dose for occupational exposure should be 20 millisievert per year averaged over five years (ie 100 millisievert in 5 years) with a maximum of 50 millisievert in any one year. For public exposure, 1 millisievert per year averaged over five years is the limit. In both categories, the figures are over and above background levels, and exclude medical exposure.
In Australia, radiation protection regulations are set by States and Territories, as well as by the Environment Protection (Nuclear Codes) Act 1978. Two Codes of Practice have been developed to cover:
- Radiation Protection and Radioactive Waste Management in Mining and Mineral Processing, 2002.
- Safe Transport of Radioactive Material, 2001.