Currently the dose limit for workers exposed to radiation is 20 mSv in a year to
the whole body, but this is rarely ever approached by biologists because the levels
of radiation used are so low. Limits are set for individual organs. The most important
of these to know are for hands (500 mSv per year) and for lens of the eye (150 mSv
per year).
Dose limits are constantly under review and, although limits are set, it is against
internationally agreed guidelines to work up to such a limit, that is we are not allowed
to assume that all is satisfactory if the limit is not exceeded. Instead, the ALARA principle
is applied, to work always to adose limit that is As Low AsReasonably Achievable. Work
that may cause a worker to exceed three-tenths or one-tenth of the dose limit must be
carried out in a controlled area or a supervised area, respectively. In practice, work in the
biosciences rarely involves a worker receiving a measurable dose. Supervised areas are
common but not always required (e.g. for^3 Hor^14 C experiments). Controlled areas are
required only in certain circumstances,for example for isotope stores or radioiodination
work. A potential problem, however, in biosciences is the internal radiation hazard. This is
caused by radiation entering the body, for example by inhalation, ingestion, absorption or
puncture. This is a likely source of hazard where work involves open sources (i.e. liquidsSource
(measured in
Bq or Ci) Dose equivalent (measured in rem or sievert,
expresses damage to human)
Absorbed dose
(measured in rad or gray,
expresses energy absorbed
by any material)Flux (no. of particles m–2s–1)
(proportional to )distance^12Fig. 14.11The relationship between radioactivity of source and absorbed dose.Example 7CALCULATION OF DOSE
Question A 1 mCi source of^125 I gives a dose of 10 mSv h^1 at 1 cm. What will be the dose rate
at 5 cm?
Answer Using the formula above 10 12 ¼new dose 52.
Therefore the new dose is 10/25¼0.4 mSv h^1.578 Radioisotope techniques