slusion whi . Ith ig g : ig . lgpest barat the basicinlimitation by¢ the Now technique t¢ on of <posure to envi a six-fold increase in the response of commercial radiographic film by use of these techniques that permits a determination of a 3-mr. y-ray exposure with a precision of +0-2 mr, A vory promising approach to the problom ofdetermin- ing human exposure to low-level ionizing radiationhas evolved out of the recent development of thermoluminescent matorials for personnel dosimetry. Commercially available dosimeter systems using lithium fluoride** and calcium fluoride** are claimed to provide measurable responses at the 10-mr. and 5-mr. level of y-ray exposure, respoctively, with approximately +20 per cont accuracy (S.D.). These limits may eventually be somewhat lowered and the precision improved with refinements in readout techniques. Cullen” has recently utilized 156 lithium fluoride dosimeters for a population exposure investigation in a high background area in Brazil, with 50 mg of the material placed in religious medals to be worn for a threemonth period. This exposure time provided a total y-dose of several hundred milliroentgons, well above the minimum now routinely detectable. In general, thermoluminescent dosimeters have the significant advantages of small size and relatively low unit cost, and may prove to be a useful tool for futuro population investigations. The Health and Safety Laboratory is at present engaged in evaluating the available thermoluminescent dosimeter systems for their applicability to the routine measurement of human ex- 48feasible Of Brazil. in ape? . a method would . M reas of certain kinds Of st _More normal ' Cost of ad use, udies. the dosi The basic : The problen which Pre- 0 be taken into woe reciprocity imeters, account in tho raphie tr , has be en descri > fi bed b a Plastic Scintillat oy posure to environmental radiation, and field tests along the linea of the New England survey are planned when sufficiently promising dosimeter systems are developed. These recent advances in direct personnel dosimetry, particularly in the extension of the sensitivity limits to ever lower +-dose levels, render the detailed examination of human oxposure to environmental radiation on a routine basis increasingly feasible, even in the extremely low-level radiation fields that are characteristic of the normal onvironment. But the reliability and reproducibility of the readings of the various types of dosimeter in terms of absolute dose under the stresses of actual field use remain to be thoroughly explored. The New England survey rosults seem to indicate the adequacy of in situ measurements in establishing a radiation profile over extensive arcas, a profile that when sufficiently un- Scent Progross . . . graphic film be increasing the and Improve complicated may be properly interpreted in terms of population exposure to onvironmental radiation. These results also emphasize some of the problems associated with adequately calibrating the response of personnel dosimeters underfield conditions. It appears that the use of highly accurate ionization-chamber and spectrometric techniques for in situ measurements will be required in the near future for all populaéion studies of the type described hore, if only to provide a standard by which the adequacy of the new techniques for direct human exposure measure11