Maximum Permissible Exposure ("MPE") can be be thought of as your laser safety speed limit. MPE is the maximum level of laser radiation the human eye or skin can be exposed to without hazardous effects or biological changes. MPE is dependent on the wavelength of the laser, the time duration, and the energy involved (cw vs pulsed, and the energy/power of the laser). This is an integral concept in laser safety. Fundamentally, MPE borrows the concept of energy density, or how much energy over a unit of area is permissible at a particular wavelength without biological harm given a specific time duration.
It should be noted, different types of viewing (direct/intrabeam vs diffused) have different assumed time durations for the MPE calculation. Furthermore, this article is merely designed to give a high-level conceptual understanding of the MPE calculation; the computation(s) should be done by a Laser Safety Officer ("LSO"). Also, it is important to note many professional labs have a higher threshold (or require a lower MPE than calculated).