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Ionizing radiation exposure to the lens of the eye is a known cause of cataractogenesis. Historically, it was believed that the acute threshold dose for cataract formation was 5 Sv, and annual dose limits to the lens were set at 150 mSv. Recently, however, the International Commission on Radiological Protection has reduced their threshold dose estimate for deterministic effects to 0.5 Gy and is now recommending an occupational limit of 20 mSv per year on average. A number of organizations have questioned whether this new threshold and dose limit are justified based on the limited reliable data concerning radiation-induced cataracts. This review summarizes all of the published human epidemiological data on ionizing radiation exposure to the lens of the eye in order to evaluate the proposed threshold. Data from a variety of exposure cohorts are reviewed, including atomic bomb survivors, Chernobyl liquidators, medical workers, and radiotherapy patients. Overall, there is not conclusive evidence that the threshold dose for cataract formation should be reduced to 0.5 Gy. Many of the studies reviewed here are challenging to incorporate into an overall risk model due to inconsistencies with dosimetry, sample size, and scoring metrics. Additionally, risk levels in the studied cohorts may not relate to occupational scenarios due to differences in dose rate, radiation quality, age at exposure and latency period. New studies should be designed specifically focused on occupational exposures, with reliable dosimetry and grading methods for lens opacities, to determine an appropriate level for dose threshold and exposure limit.

Radiation dose may be measured by means of the increase which it produces in the thermoluminescence sensitivity of quartz. The measurement is possible even if the quartz was heated after the dose was received (for example for several hours at temperatures up to 300°C). This makes the quartz suitable as a heat-resistant dosimeter. THERMOLUMINESCENCE (TL) phosphors and film badges are the principal existing dosimeters. However the records they store are partly erased by heating for more than a few hours at temperatures above about 50°C [see, for example, Ref. 1 for the behaviour of LiF (TLD-loo)]. Observations upon the radiationsensitive T L peak in quartz at about 100°C (for a heating rate of 20"C/sec) show that this material may be used as a heat-resistant dosimeter. It is possible to use such a low temperature T L peak by measuring the increase in T L sensitivity which results from thermal annealing of the exposure dose.@) The dosimetry does not, then, involve the measurement of the stored T L due to the exposure dose, but requires the measurement of the change in sensitivity produced by that dose.The quartz used is prepared by firing geological a-quartz at 1000°C for 18 hr. T o determine the T L sensitivity, So, of the material so produced (called hereafter the virgin material), a 1 rad test dose is given to a sample, and the resulting TL measured.Throughout this work, all doses are of beta radiation from ' OSr plaque sources. The T L is measured by heating the irradiated material at 20"C/sec, and the light detection system is an EM1 9 6 3 5 9 photomultiplier plus Corning (7-59) and Chance Pilkington (HA3) filters (the sensitisation being greatest in the region of violet emission).At the end of the period of dosimetry, during which the quartz received the exposure dose, the quartz is thermally annealed by heating at PO"C/sec to 550°C. The final sensitivity, S, is obtained by giving this sample the same test dose as used on the virgin material, and measuring the resulting TL. The sensitivity ratio 8 567 S/So is a function of exposure dose, as shown in Fig. 1, curve a. The same growth of the sensitivity ratio is obtained if the 100°C traps filled by the exposure dose are still filled when the anneal is made, or have been emptied by drainage at room temperature before the anneal. Thus it appears that the peak retains a memory of its radiation history independent of the time elapsed between the exposure dose and the anneal.The temperature of 550°C is decided upon from evaluation of the sensitivity ratio as a function of maximum temperature to which the sample is annealed. The greatest ratio occurs at 550°C. For example, the data for an exposure dose of 1 krad is shown in Fig. 2, curve a. The behaviour of S/S, shown in Figs.1 and 2 is very sensitive to the firing temperature, and must be established for each batch of quartz unless the firing temperature is reproduced within about 5°C. For different quartzes fired under identical conditions, these sensitivity characteristics may differ considerably. As yet, no anneal...

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