Dose limit for the eyes reduced

The prevailing belief in radiation protection fraternity has been that human radiation-related cataract occurs only after relatively high doses and the ICRP guidelines on minimal doses for cataract induction in humans are given in the table for single exposure and protracted exposure scenarios.

Table: ICRP Guidelines on Minimal Lens Doses for Cataract Induction

End point	Brief exposure (Sv)	Protracted exposure (Sv)	Annual dose (Sv)
Detectable opacities	0.5 to 2	5	>0.1
Visual impairment	5	>8	>0.15

Epidemiological studies among Chernobyl clean-up workers, Atom - bomb survivors in Japan, astronauts, residents of contaminated buildings, radiological technicians and recent surveys of staff in interventional rooms indicate that there is an increased incidence of lens opacities at doses below 1 Gy.

The IAEA studies on radiation induced cataract among cardiologists and support staff in cardiac catheterization laboratories, published in “Radiation Research” received wider attention since it pointed towards possibility of opacities in the lens of the eyes below the currently specified threshold by International Commission of Radiological Protection (ICRP). However, there are issues such as difficulty in accurate dose estimation in eyes of medical staff as hardly any data is available that can be used to correlate with lens opacities. Only rough estimations based on work load and typical factors used in the procedures performed by staff could be made. In contrast, there is much better dosimetry in A-Bomb survivors and much longer follow up period.

Based on the overwhelming data, the ICRP released a statement in 2011 recommending a change in the threshold dose for the eye lens and dose limits for eye for occupationally exposed persons.

According to this statement, the threshold in absorbed dose for the lens of the eye is now considered to be 0.5 Gy. Further, for occupational exposure in planned exposure situations the Commission now recommends an equivalent dose limit for the lens of the eye of 20 mSv in a year, averaged over defined periods of 5 years, with no single year exceeding 50 mSv.

The Commission continues to recommend that optimisation of protection be applied in all exposure situations and for all categories of exposure. With the recent evidence, the Commission further emphasises that protection should be optimised not only for whole body exposures, but also for exposures to specific tissues, particularly the lens of the eye, and to the heart and the cerebrovascular system.

The implementation of the limit amongst the occupational workers in nuclear and radiological facilities is operationally difficult in view of the fact that there is hardly any reliable and recorded dosimetric data available. To begin with, however, some rough assessment of the exposure to eyes can still be made using whole-body exposure data.