Pigmentation-dependent skin injury thresholds with nanosecond single-pulse laser exposure

Maximum permissible exposure limits defined in ANSI Z136.1 are broadly protective, but they do not explicitly account for pigmentation differences in skin. As a result, the lack of experimental data quantifying the effect of pigmentation on injury thresholds constrains accurate interpretation when melanin is a primary optical absorber, which has direct implications for the performance of optics-based imaging methods.

We experimentally determined the pigmentation-dependent laser exposure dose that produces a visible injury in 50% of exposed sites (i.e., ${\mathrm{ED}}_{50}$ injury thresholds) with nanosecond, 750-nm-wavelength pulses, and we provide the first known quantification of relationships between melanin index (MI) and individual typology angle (ITA) or erythema index (EI) through colorimetric measurements.

Fifty total abdominal skin sites per swine were identified on three swine with different pigmentation phenotypes (median ITA values per swine ranged $-18°$ to 87°). Single-laser pulses with 5-ns pulse duration, 750-nm-wavelength, and energies of 35, 40, 50, 60, and 65 mJ per pulse were delivered to 10 sites per energy level per swine. Laser-exposed sites were visually assessed at early post-exposure intervals, followed by colorimeter measurements of MI, ITA, and EI. Probit regression analysis was used to compute the ${\mathrm{ED}}_{50}$ thresholds, followed by a polynomial regression analysis conducted to relate ITA to MI and EI to MI.

Dark-skinned swine ( $\text{median}\mathrm{ITA}=-18°$ ) exhibited a 27.7% lower ${\mathrm{ED}}_{50}$ ( $323.94\mathrm{mJ}/{\mathrm{cm}}^2$ ) than light-skinned swine ( $\text{median}\mathrm{ITA}=69.5°$ , ${\mathrm{ED}}_{50}=448.36\mathrm{mJ}/{\mathrm{cm}}^2$ ), which is consistent with increased melanin-mediated absorption in the darker-skinned swine. The lightest-skin swine ( $\text{median}\mathrm{ITA}=87°$ ) did not develop visible erythema within the tested exposure range, preventing ${\mathrm{ED}}_{50}$ estimation. Mathematical MI-ITA and MI-EI relationships revealed pigmentation-dependent optical and inflammatory responses across the three swine.

The measured differences in ${\mathrm{ED}}_{50}$ values quantify the impact of skin pigmentation on laser-induced injury risk and demonstrate that darker skin has greater susceptibility to damage with the investigated laser parameters, relative to lighter skin. This foundational work addresses a previously uncharacterized source of biological variability that is relevant for the interpretation of laser safety margins and the design and evaluation of optics-based systems.