Sunday, December 27, 2009

Radiation protection : Optimization of protection

2- Optimization of protection

In relation to any particular source of radiation within a practice, the dose to any individual from that source should be below an appropriate dose constraint, and all reasonable steps should be taken to adjust the protection so that exposures are "as low as reasonably achievable" ALARA, economic and social factors being taken into account.

Constraints are imposed on a practice involving exposure to radiation at the planning stage. For workers, the value of the dose constraint should be chosen so as to reflect the annual value of dose that can reasonably be reached in a particular industry or procedure; it may well be a small fraction of the dose limit. For members of the public, a typical constraint, 0.3 mSv in a year, can be used as a planning value for a new source of radiation exposure, such as a factory that intends to discharge radioactive material to the environment.
In most countries the annual doses to individual members of the public from practices that cause exposure have been brought-below 0.3 mSv in a year - the primary dose constraint recommended by ICRP for the public. Even the groups of people who are most exposed to radioactive discharges from nuclear facilities. because they live nearby or have particular eating habits, typically receive annual doses that are a fraction of this constraint.

Dose constraints or guidance levels are also appropriate for medical exposures of patients, the objective being to minimize doses in a sensible way. Some routine medi­cal procedures can give significant doses (i.e. several mSv) and, importantly, can vary greatly from hospital to hospital. The use of guidance levels can provide a practical means of reducing doses to patients without a reduction in the diagnostic information available to physicians.

Since we assume that no radiation dose is entirely free from risk, it is important to pay attention to all doses and to reduce them whenever it is reasonably achievable. Even­tually the point must come when further reductions in dose become unreasonable, because social and economic costs would outweigh the value of the reductions. On the other hand, the benefits and risks associated with a particular practice are often not distributed evenly in society, and so this second requirement - the optimization of protection recommended by ICRP - also includes a constraint on the procedure, in the form of restrictions on doses or risks to people so as to prevent inequitable exposures from radiation.

Optimization of protection has been increasingly influential during the past two decades throughout the world and, in most countries, the average annual dose to radiation workers is well below (i.e. by a factor of ten or more) the 20 mSv per year that ICRP has recommended. Some groups of workers receive doses a few times the average, and some workers receive more than 20 mSv/a, but the number doing so is a very small percentage of the total. Analysis by UNSCEAR shows that the average annual dose to workers from man-made sources is 0.6 mSv, whereas the average annual dose to workers from enhanced natural sources (e.g. in mining) is higher at 1.8 mSv

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