Do we over-state the risk of multiple pulsed exposures?

Laser safety standards committees have struggled for years to adequately formulate a sound method for treating repetitive-pulse laser exposures. Safety standards for lamps and LEDs have ignored this issue because averaged irradiance appeared to adequately treat the issue for large retinal image sizes and skin exposures. Several authors in recent meetings have questioned the current approach of three conditions as still not sufficient to treat pulses of unequal energies or certain pulse groupings. Schulmeister et al (2007) employed thermal modeling to show that the total-on-time pulse rule was conservative. However, Lund (2007) has further developed the approach of probability summation put forth by Menendez et al. (1991), whereby the additivity is really the result of an increasing probability of detecting injury for multiple pulses, and related this to the slope of the probit curve for the threshold study. Since the uncertainty in the minimal, ophthalmoscopically visible retinal lesion (MVL) threshold is great, ACGIH, ANSI and ICNIRP committees traditionally applied large risk reduction factors ("'safety factors") of one order of magnitude when deriving intrabeam, "point-source" exposure limits. This reduction factor took into account the probability of visually detecting the low-contrast lesion, the probability of striking an area of high pigmentation, etc. Thus the N^-1/4 reduction factor may just be an artifact of detecting the lesion. The reduction factor exponent is much smaller for large spot sizes where these probabilities are quite reduced. Two recent studies, by D.J. Lund, on repetitive pulse studies in animal models, and by Schulmeister et al., on multiple pulse studies in retinal expiants, support this interpretation of the available data.