What are the laser MPEs?
At the center of all laser safety guidelines and programs is the concept of Maximum Permissible Exposure (MPE). This is defined as "the level of laser radiation to which an unprotected person may be exposed without adverse biological changes in the eye or skin." This definition traces back to the first ANSI Z136.1 Safe Use of Lasers standard in 1973 as well as IEC standards.
There is no single "Maximum Permissible Exposure" value. Instead, there are a range of MPE values, depending on the laser wavelength, the exposure time, and the biological target (eye or skin). For example, the MPE value for all-day (8 hour) eye exposure to laser light is different from the MPE values for an accidental eye exposure (less than 1/4 second for visible light; less than 10 seconds for infrared and ultraviolet light).
The values for eye MPEs are generally much lower than for skin MPEs. This is because the eye is much more sensitive to excess light.
The MPE pages at this website list some of these values, and has links to more detailed information.
More information on how to use MPE values can be found in the ANSI Z136 series of standards, available from the Laser Institute of America, as well as other documents, books and online resources.
How to reduce hazardous laser exposure
The goal of all laser safety programs is to reduce laser light exposure so it is at or below the MPE level. This could be done through engineering controls (barriers, interlocks, enclosures), procedural controls (following specified procedures and practices), and/or use of personal protective equipment (laser protective eyewear, non-flammable skin covering).
Usually, engineering controls are preferred. They do not require a person to remember to follow procedures, or to always be wearing the correct laser safety glasses or goggles for the laser's wavelength and power.