Lasers are classified by output power and are a hazard to the eye, with the potential for thermal injury being the primary mechanism. The Maximal Permissible Exposure, or MPE, is the level of laser radiation to which a person may be exposed without hazardous effects to the eye or skin. A system of hazard classification has been developed and is part of the ANSI Standard and State Regulations, however it is usually more convenient to establish safety controls based on the laser class than use of the exposure limits. In general, Class IIIa lasers have a power output of 1 to 5 milliwatts (1-5mW), Class IIIB lasers output power up to 500 mW, or 0.5 watts, and Class IV lasers include all of those with power output higher than 0.5 watts (500 mW).
The classification scheme makes no distinction between the Class IV therapy lasers, cosmetic and hair removal lasers, surgical lasers, and a military laser capable of shooting down a satellite. All of these are greater than 500 mW, and therefore all of them are Class IV. Lumping together every laser with power output greater than 500 mW is somewhat unfortunate, and has led to misunderstandings and discussions on several state physician licensing boards. One state chiropractic board first balked at the notion of its members using Class IV lasers, assuming that the intended usage was hair removal or a cosmetic procedure. However, after proper education and demonstration with a Class IV Therapy Laser, the board unanimously approved the use of such devices when used in a manner consistent with the scope of practice.