Appendix C- Exposure Limits and Laser Classification

The principle mechanism for laser injury is thermal. As a result, there is a threshold for the amount of thermal energy transferred to exposed tissue below which there will be no injury. In order to prevent laser injury, a system of exposure limits has been established. Limits were promulgated as a consensus standard by the American National Standards Institute (ANSI Z136.1-1993, "American National Standard for Safe Use of Lasers"). These limits were incorporated into regulations by the State of Arizona (Arizona Administrative code Title 12 Chapter 1 Article 14, "Rules for the Control of Non-Ionizing Radiation"). Separate exposure limits exist for direct viewing of the laser beam, viewing of a diffuse reflection, and for skin exposure.

A system of hazard classification has also been developed and is part of the ANSI Standard and State Regulations. It is usually more convenient to establish safety controls based on the laser class than use of the exposure limits. All lasers fall into one of 6 hazard classes.

Class I (Depends on wavelength)

Any laser or laser system that cannot emit laser radiation in excess of the maximum permissible exposure levels (MPE) discussed above is a Class I laser. There is no hazard.

HOWEVER, it is important to note that Class I laser systems often imbed more hazardous lasers in the device. Removal of a protective housing may result in access to radiation in excess of MPE's and implementation of the Laser Safety Program for ARRA regulated lasers (IIIb and IV).

Class II (less than or equal to 1 mW average power and if applicable a pulse duration of less than 0.25 seconds)

These are low power lasers emitting in the visible wavelengths (0.4 to 0.7 mm). These lasers could result in exposure exceeding MPE's if intentionally viewed for more than 0.25 seconds. Although, typically this does not happen because of the normal "aversion response" people are not allowed to stare at any laser beam. When people view laser light, the natural response is to blink and move the head and eye away from the bright uncomfortable laser beam. An example of a Class II laser is a HeNe pointer laser of 1 mW or less.

Class IIa

These laser are lasers emitting light in the visible wavelengths (0.4 to 0.7 mm) which are not intended for prolonged viewing and will not produce a hazard if viewed directly for periods not exceeding 1000 seconds. The exposure will not exceed MPE's if viewed for less than this amount of time.

Class IIIa (1 to 5 mW)

There are medium power lasers that represent a potential hazard to the eye. Examples would be visible lasers from 1 to 5 mW.

Note: Class IIIa includes lasers with an accessible output between 1 and 5 times the Class I AEL for wavelengths shorter than 0.4 µm or longer than 0.7µm or less than 5 times the AEL for wavelength between 0.4 and 0.7 µm.

Class IIIb (access of IIIa power levels but typically less than 0.5 W average power)

The hazard for IIIb lasers is potentially greater than that for IIIa. The hazard is still limited to direct viewing of the laser beam, however. These are lasers with output less than 0.5 Watt. These lasers do not produce hazardous diffuse reflections or represent a skin exposure hazard.

Class IV (0.5 W power or higher)

These are high powered lasers that represent hazards (eye damage, skin injury, and or potential flammable material ignition source) for direct viewing, viewing of diffuse reflections, and skin exposure.

The power levels for Class IV lasers are at 0.5 W and higher.