pable of yielding useful quantitay uniform radiation environment, be used as a basis for evaluating ‘ossarily more accurate—mothods Jation exposure-levels. yhich is suggested by the New he basic limitation of the pocket mique in terms of measuring ‘oO environmental radiation is now ining mean leakage rates under while being worn and handled. » fundamental reason why this st partially overcome by suitably on, and thus the pocket chamber red as a potentially practical ono rent. It should be remarked that y fulfilled their basic purpose in n, namely, the determination of exposure-levels between areas. umber of other possible methods iopulation exposure to environr example, photographic film ve been applied to this general ucecess. O’Brien et al.!° describod m iodide) system which Roser 1 in the measurement of popula- mn @ limited scale. The approxi- hancement of the film response vtor is almost too great for the f Brazil: such a method would sible in areas of more normal tain kinds of studies. Tho basic st of the dosimeters, which preise. Tho problem of reciprocity be taken into account in the ters. simeter has been described by shic film and a plastic scintillator ensitive than the sodium iodide 1 energy dependence and good §.D. for two weeks’ exposure at eciprocity failure was observed ive. The main problem seems to on temperature in its response, arate. The error present in any ‘known, so that the use of this commended. cent progress in increasing tho ographic film by means of postt and improved development er such film useful for environithout the necessity for external se. McLaughlin® has reported 10 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 very promising approach to the problem of determin- ing human exposure to low-level ionizing radiation has evolved out of the recent development of thermoluminescent materials 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, respectively, with approximately +20 per cent 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 -dose of several hundred milliroentgens, 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 future 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 exposure to environmental radiation, and field tests along the lines 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 y-dose levels, render the detailed examination of human exposure to environmental radiation on a routine basis increasingly feasible, even in the extremely low-level radiation fields that are characteristic of the normal environment. 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 results seem to indicate the adequacy of in site measurements in establishing a radiation profile over extensive areas, a profile that when sufficiently uncomplicated may be properly interpreted in terms of population exposure to environmental radiation. These results also emphasize some of the problems associated with adequately calibrating the response of personnel dosimeters underfield conditions. lt appears that the use of highly accurate ionization-chamber and spectrometric techniques for 7m situ moasurements will be required in the near future for all populasion studies of the type described here, if only to provide a standard by which the adequacy of the new techniques for direct human exposure measuretl