Question 172: (a) If the annual limit to the head is five
rem deep dose equivalent, what is the purpose of the 15 rem
eye dose equivalent? (b) How can a person receive 15 rem
eye dose equivalent without exceeding the annual TEDE limit?
Answer: (a) The purpose of the 15 rem non-stochastic
limit to the lens of the eye is to prevent lens opacities
(cataracts). The dose limit to the head a stochastic
limit) and the dose limit to the eye (non-stochastic limit)
are measured at different depths in tissue, 1 cm tissue
equivalent depth for deep dose and 0.3 cm for eye dose; and
for low penetrating radiation (such as beta or low-energy
x-rays), doses at different tissue depths can be
significantly different.
(b) The 15 rem eye dose equivalent applies to the exposure
to the lens of the eye and is measured at a tissue depth of
0.3 cm. The 5 rem TEDE limit is the sum of the deep dose
equivalent at a tissue depth of 1 cm and the committed
effective dose equivalent. In general, a person can
receive 15 rem to the eye (measured at 0.3 cm) without
exceeding the 5 rem limit on deep dose equivalent when the
head is exposed to beta or low-energy photon radiation,
although it would be rare. for an individual to receive 15
rem eye dose equivalent without exceeding a deep dose
equivalent of 5 rem. (Reference: 10 CFR 20.1201)