Class 4 lasers must be equipped with a key switch and a safety interlock.
Chemical hazards may include materials intrinsic to the laser, such as beryllium oxide in argon ion laser tubes, halogens in excimer lasers, organic dyes dissolved in toxic or flammable solvents in dye lasers, and heavy metal vapors and asbestos insulation in helium cadmium lasers. In this case, there is no eye aversion response. Many lasers are considered class 1 only because the light is contained within an interlocked enclosure, like DVD drives or portable CD players. Some lasers are so powerful that even the Infrared lasers are particularly hazardous, since the body's protective glare aversion response, also referred to as the "Lasers can cause damage in biological tissues, both to the eye and to the skin, due to several mechanisms.The eye focuses visible and near-infrared light onto the retina. Optical connector inspection microscopes are available which incorporate blocking filters, thus greatly improving eye safety. Many lasers are high voltage devices, typically 400 V upward for a small 5 mJ pulsed laser, and exceeding many kilovolts in higher powered lasers. Technicians may also use red lasers for fault finding at around 628–670 nm.

A special case is systems operating at 670–1000 nm, where the beam may appear to be a dull red, even if the light beam is actually very intense. High power optical amplifiers are used in long distance systems. A laser can be classified as Class 1M if the power that can pass through the pupil of the naked eye is less than the AEL for Class 1, but the power that can be collected into the eye by typical magnifying optics (as defined in the standard) is higher than the AEL for Class 1 and lower than the AEL for Class 3B. Ned Tijdschr Geneeskd. Arch Ophthalmol, 122, 1210–1217 Further experienced discomforts than these may be psychological rather than physical. not expanded beam) cannot typically exceed about 100 mW, above which power level single mode connectors become unreliable, so if there is a single mode connector in the system, the design power level will always be below this level, even if no other details are knownOptical microscopes and magnifying devices also present unique safety challenges. Beam power density may not exceed 2.5 mW/cmLasers in this class may cause damage if the beam enters the eye directly. Definition: A cranial orthosis is a device that is intended for medical purposes to apply pressure to prominent regions of an infant's cranium in order to improve cranial symmetry and/or shape. Even moderately powered lasers can cause injury to the eye. Most single mode / multi mode fiber systems actually use infra-red light, invisible to the human eye. Furthermore, some lasers emit more than one wavelength of light, and this may be a particular problem with some less expensive frequency-doubled lasers, such as 532 nm "green laser pointers" which are commonly pumped by 808 nm infrared laser diodes, and also generate the fundamental 1064 nm laser beam which is used to produce the final 532 nm output. This generally applies to lasers powered from 5–500 mW. Lasers in this class are mostly dangerous in combination with optical instruments which change the beam diameter or power density, though even without optical instrument enhancement direct contact with the eye for over two minutes may cause serious damage to the retina. If the IR radiation is allowed into the beam, which happens in some green laser pointers, it will in general not be blocked by regular red or orange colored protective eyewear designed for pure green or already IR-filtered beam. Consensus standards, such as American National Standards Institute (ANSI) Z136, provide users with control measures for laser hazards, as well as various tables helpful in calculating maximum permissible exposure (MPE) limits and accessible exposures limits (AELs). 50 is an FDA guidance document that determines what components of IEC 60825 and IEC 60601-2-22 can be used to meet requirements found in 21CFR1040 for products employing lasers intended for sale in the United States. A Class 3B laser is hazardous if the eye is exposed directly, but diffuse reflections such as those from paper or other Class 4 is the highest and most dangerous class of laser, including all lasers that exceed the Class 3B AEL. Any system employing typical optical connectors (i.e. A laser beam can be focused to an intensity on the retina which may be up to 200,000 times higher than at the point where the laser beam enters the eye. Classification is also dependent on the wavelength and on whether the laser is pulsed or continuous. orthosis, cranial, laser scan: Regulation Description: Cranial orthosis. A comprehensive resource for safe and responsible laser use Laser Classes chart Lasers are classified based on their potential for causing injury — especially eye damage, since the eye is most susceptible to excess laser light. The World Association for Laser Therapy (WALT) and the North American Association for Laser Therapy (NAALT) have determined that the proper terminology to use for LLLT is photobiomodulation (PBM).Laser manufacturers have also begun using terms such as “high power,” “deep penetrating,” and “deep tissue” to describe their Class 4 offerings.

There are four main classes for visible-beam lasers: Class 2, Class 3R, Class 3B and Class 4.

These regulations impose upon manufacturers required safety measures, such as labeling lasers with specific warnings, and wearing laser safety goggles when operating lasers.