Cutaneous sensation threshold for 3.8 μm radiation from a short duration pulsed laser on the calves of human subjects: A pilot study

The objective of this study was to determine the threshold for sensing a 3.8 μm laser on the skin. The identification of the skin sensation threshold for invisible lasers is useful in providing scientific guidance to the safety community regarding accidental skin exposures. Knowledge of the level of sensation of lasers might be used to show that sensation may not coincide with the exposure limits, and could in fact be orders of magnitude greater than limits. This type of information can be used in conjunction with studies performed on animals to help better define the minimum radiant exposure required to elicit minor skin changes. Future standards for skin exposure could potentially be based on sensation thresholds, if the difference between gross morphologic skin changes and sensation provide a sufficient safety margin. This study involved exposing ten volunteers on their calf to incrementally increasing energy from single 3.8 μm, 4 μs pulses with a spot size of approximately 4 cm². The laser pulse was sensed by the subjects between an average of 2.3 and 3.1 J/cm², a level significantly above the maximum permissible exposure, but below which significant gross morphologic changes occur to the skin. Exposures greater than 2.4 J/cm² resulted in desiccation with concomitant separation of the superficial layer of epidermis in all subjects. Mild erythema presented within five minutes at exposures greater than 6.4 J/cm², resolving within 72 h.