Answer to Question #349 Submitted to "Ask the Experts"

First of all, the units, mR/hr apply only to gamma or x radiation. However, the same question could be posed in terms of dose equivalent rate, mrem/hr, which is applicable to all types of ionizing radiation. As a matter of reference, an exposure of 1 mR gamma or x radiation results in a dose equivalent of approximately 1 mrem (with a small variation depending upon the energy and irradiation conditions). The current regulatory occupational exposure limit in the United States is 5000 mrem per year. If continuous exposure is interpreted as 2000 working hours per year, this translates into 2.5 mrem/hr for radiation workers. However, occupational exposures should be kept as low as reasonably achievable with 5000 mrem/yr as a regulatory upper limit. This does not necessarily mean that radiation does in excess of 5000 mrm/yr are unsafe for radiation workers, but this is the limit applied for regulatory purposes. Because of factors such as the general population being more heterogeneous and containing individuals of all ages and conditions of health, the regulatory limit is lower than for occupational exposure. The limit for radiation delivered in addition to the natural background radiation to individuals of the general public is 500 mrem in any one year and 100 mrem/yr on a prolonged basis. If we interpret continuous exposure as 8766 hours per year (24 hrs/day x 365.25 days/yr), 100 mrem/yr translates to 0.011 mrem/hr or 11 microrem/hr. This would be in addition to the natural background radiation which is quite variable but which averages about 300 mrem/yr or 34 microrem/hr. This does not necessarily mean that radiation doses in excess of 500 mrem in a single year of 100 mrem on a prolonged basis are unsafe when added to the pre-existing natural background but rather these are the limits accepted for regulatory purposes. Charles E. Roessler, CHP, PhD

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From a wide range of radiobiological studies (primarily conducted at the cellular level), dose rate effects are observable in the range of 1-100 cGy/min (1-100 rad/min). In this interval, the biological effect of a given TOTAL dose is decreased as the dose rate is decreased (presumably due to biological repair of radiation damage). Dose rates below 1 cGy/min do not alter the biological response significantly from 1 cGy/minute. Similarly, above 100 cGy/minute, biological responses are not altered significantly from 100 cGy/minute. If further information is needed you may wish to referr to Radiobiology for the Radiologist by Eric J. Hall, J. B. Lippincott Company, for a technical discussion of dose rate effects. Kenneth L. Mossman, Ph.D.