Physics and Engineering of Radiation Detection

11.6. Biological Effects of Radiation 661

and when should one start worrying about the exposure. Unfortunately there is no
simple answer to this since most of the effects of radiation are statistical in nature.
One could, of course, argue that for extremely high dose the damaging effects are
almost certain. This is definitely true but the effect is still statistical since in such
a case due to very high intensity of radiation the probability of cells being damaged
becomes very high. Having said that, there have been several extensive studies to
determine the safe levels for radiation workers and general public. Based on those
studies the limits for radiation workers and general public have been suggested by
international commission on radiological protection. These limits, as given in Table
11.6.1, are followed by most organizations.

Table 11.6.1: Exposure limits of different body parts for radiation workers and
general public (26).

Exposure Type Maximum Permissible Equivalent Dose (mSv)

Occupational General Public

Skin 500/year 50/year

Eye lens 150/year 15/year

Hands 500/year

The effective whole body dose limit set by ICRP is 20mSvper year for radiation
workers and 1mSvper year for general public (26). This limit and the ones given in
Table 11.6.1 are taken as the absolute maximum and therefore no radiation worker
should ever be allowed to keep working in radiation environments after receiving
doses higher than these limits. For additional safety, another principle that has now
been almost universally adopted, is the so called ALARA principle. ALARA stands
for As Low As Reasonably Achievable. What this means is that one should strive to
aggressively limit the exposure to individuals within the limitations of the working
environment and the cost.

Table 11.6.2: A typical 3-level ALARA system for radiation workers of age 19 and
older.

Maximum Permissible ALARA (mSv)

Whole Body Dose Level-1 Level-2 Level-3

20/year 0.2/month 0.3/2 months 0.4/3 months

The ALARA principle does not relax the maximum permissible doses as those
are still the ones given in Table 11.6.1. On the contrary, it actually provides a