Laser Safety

OSHA Technical Manual: Laser Hazards

OSHA Fact Sheet

OSHA Laser Safety Standards


Introduction

Laser – The term LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. Light can be produced by atomic processes which generate laser light. A laser consists of an optical cavity, a pumping system, and an appropriate lasing medium

Uses

Lasers are used for a variety of applications. For example:

Research

  • study of mechanisms at interfaces
  • detection of single molecules

Medical

  • eye surgery
  • therapy for Carpel Tunnel Syndrome
  • photocoagulation

Commercial

  • supermarket checkout or stock inventory scanners
  • determining site boundaries for construction

Industrial

  • cutting
  • welding

Basic Structure and Function

The light emitted from a laser is monochromatic, that is, it is of one color/wavelength. In contrast, ordinary white light is a combination of many colors (or wavelengths) of light.

Lasers emit light that is highly directional, that is, laser light is emitted as a relatively narrow beam in a specific direction. Ordinary light, such as from a light bulb, is emitted in many directions away from the source.

The light from a laser is said to be coherent, which means that the wavelengths of the laser light are in phase in space and time. Ordinary light can be a mixture of many wavelengths.

These three properties of laser light are what can make it more of a hazard than ordinary light. Laser light can deposit a lot of energy within a small area.

The lasing medium of a laser is a substance that emits light in all directions and can be a gas, liquid, solid, or semi-conducting material.

The excitation mechanism of a laser is the source of energy used to excite the lasing medium. Excitation mechanisms typically used are electricity from a power supply, a flashtube, lamp, or the energy from another laser.

A laser's feedback mechanism is used to reflect light from the lasing medium back into itself and typically consists of two mirrors at each end of the lasing medium. As the light is bounced between the mirrors, it increases in strength, resulting in amplification of the energy from the excitation mechanism in the form of light.

The output coupler of a laser is usually a partially transparent mirror on one end of the lasing medium that allows some of the light to leave the lasing medium in order that the light be used for the production of the laser beam. The output coupler is usually part of the feedback mechanism.

Laser Types

There are many types of lasers available for research, medical, industrial, and commercial uses. Lasers are often described by the kind of lasing medium they use: gas, liquid, solid, semiconductor, or dye.

Lasers are also often characterized by duration of laser light emission.

A continuous wave (CW) laser is a laser, which emits a steady beam of light, whereas a pulsed laser emits laser light in an off-and-on or pulsed manner.

A Q-switched laser is a pulsed laser, which contains a shutter-like device that does not allow emission of laser light until opened. Energy is built up in a Q-switched laser and released by opening the shutter-like device to produce a single very intense laser pulse.

Hazard Potential

Damage to the eyes and skin is of most concern in laser accidents.

Thermal effects are the major cause of tissue damage by lasers. Energy from the laser is absorbed by the tissue in the form of heat, which can cause localized, intense heating of sensitive tissues. The amount of thermal damage that can be caused to tissue varies depending on the thermal sensitivity of the type of tissue. Thermal effects can range from erythema (reddening of the skin) to burning of the tissue.

Factors that affect thermal damage to tissue are:

  • amount of tissue affected
  • wavelength of light
  • energy of the beam
  • length of time that the tissue is irradiated

Beam Hazards

Laser beams are capable of causing a localized vaporization of tissue, which in turn can create a mechanical shockwave to be propagated through the tissue (acoustic effects). Shockwaves can cause tearing of tissue.

Laser light can also cause changes to the chemistry of cells, which can result in changes to tissue (photochemical effects).

The three parts of the eye of concern in laser injuries are the cornea, lens and retina.

The cornea is the transparent layer of tissue covering the surface of the eye. The cells on the surface of the cornea have a lifetime of only about 48 hours, therefore cell turnover is quite fast. Injury to cells on the surface of the cornea is generally repaired quickly, but injury to deeper layers of the cornea can result in permanent change to the cornea.

The lens of the eye focuses light to form images in the eye. Damage to the lens can cause the destructive interference of light within the lens, resulting in a "milky" area or cataract.

The retina is made up of layers of nerve cells and is used for reception of the light in the eye. Damage to cells in the retina can result in loss of vision.

Visible and IR-A wavelengths of light are transmitted through the cornea and lens of the eye, and are absorbed mostly by the retina. The visible and IR-A portions of the spectrum (400-1200 nm) are often referred to as the "Retinal Hazard Region" because these wavelengths of light can damage the retina.

The amount of hazard to the retina from viewing of a laser beam in the Retinal Hazard Region increases with increased pupil size and increased duration of the laser beam.

UV-A wavelengths of light are mostly absorbed in the lens of the eye and can cause photochemical damage to the lens.

UV-B, UV-C, IR-B and IR-C are absorbed by the cornea of the eye. Exposure to these wavelengths can result in conjunctivitis, "milky" cornea, and inflammation.

The layers of the skin which are of concern in a discussion of laser hazards to the skin are the epidermis and the dermis.

The epidermis layer lies beneath the stratum corneum and is the outermost living layer of the skin.

UV-B and UV-C, often collectively referred to as "actinic UV," can cause erythema and blistering as they are absorbed in the epidermis. UV-B is a component of sunlight that is thought to have carcinogenic effects on the skin.

The dermis mostly consists of connective tissue and lies beneath the epidermis.

IR-A wavelengths of light are absorbed by the dermis and can cause deep heating of skin tissue.

Hazard Determination

Maximum Permissible Exposure (MPE), is the maximum level of laser radiation to which a human can be exposed without adverse biological effects to the eye or skin.

There are three factors involved in the determination of the MPE:

  • the wavelength of the laser light
  • the energy involved in the exposure
  • the duration of the exposure

MPE values for eyes and skin are listed for various combinations of wavelength and exposure duration in tables 5 and 7 of ANSI Standard Z136.1-1993, which is available from the Laser Institute of America. The EHS department has a copy of ANSI Z136.1-1993.

NHZ stands for Nominal Hazard Zone, and is the zone inside which laser radiation that is direct, reflected, or scattered exceeds the MPE for the laser.

Control measures are not needed outside the NHZ.

NOHD is an acronym for Nominal Ocular Hazard Distance. The NOHD is the distance along the axis of the direct laser beam to the human eye beyond which the MPE of the laser is not exceeded.

Non-beam hazards

Electrical Hazards

The most widely encountered non-beam hazard from lasers is electric shock, or even death, from sources of electricity. Sources of electrical hazard from lasers come primarily from the power supply of CW lasers and the capacitor banks of pulsed lasers.

The following precautions should be followed to help prevent electrical injury when working around laser equipment:

  • Use one hand when working around power supplies, capacitors or other electrical equipment
  • Avoid wearing metallic items
  • Never handle electrical equipment when hands are wet or when standing on wet ground

Personnel knowledge in CPR is highly recommended in case an electrical accident occurs.

Chemical Hazards

One of the major sources of chemical hazards from lasers is from the organic dyes used in dye lasers. Most dyes used in dye lasers are fluorescent organic compounds. Some dyes (the rhodamines) are considered to be mutagenic or carcinogenic, while other dyes (polymethine compounds) are toxic.

Additionally, some of the solvents used during dye preparation can be irritants, highly toxic, and/or highly reactive.

Other hazards include gases from gas lasers, as well as gases that are formed by the interaction of the laser with target materials, and coolants such as liquid nitrogen.

To prevent chemical accidents, the following are suggested:

  • Preparation of dye solutions should be carried out in fume hoods and/or glove boxes
  • Personal protective equipment such as lab coats, gloves and goggles, should be worn during dye preparation
  • Dye solutions and reagents should be stored properly
  • Adequate ventilation shall be provided if gases are produced

There are hazards from light generated by lasers, which do not originate from the beam itself. These optical hazards include:

  • UV light as a product of laser welding
  • UV light from discharge tubes and pumping lamps
  • visible and IR-A light from pumping systems

Prevention 

  • Hazardous levels of non-beam optical emissions shall be shielded.

Explosion Hazard

The following are potential sources for explosion hazards in lasers:

  • lamps (arc lamps, filament lamps)
  • capacitor banks
  • static electricity buildup in circulating dye solutions containing non-polar solvents such as dioxane

To prevent explosion hazards

  • Components that are capable of exploding should remain enclosed in housings that can withstand a potential explosion.
  • Use static dissipaters (ground wire) in tubing for circulating dye solution

Fire Hazard

Potential sources of fire hazards include:

  • electrical circuits
  • improper beam enclosures
  • ignition of gases or fumes from the laser
  • flammable laser dyes

In order to prevent fire hazard,

  • Beam enclosures should be constructed of flame resistant materials
  • Electrical circuitry shall be evaluated for the potential to cause fire

Laser Hazard Classes

Lasers are classified in four major hazard categories known hazard classes. The classes are based upon a scheme of graded risk. They are based upon the ability of a beam to cause biological damage to the eye or skin. In the FLPPS, the classes are established relative to the Accessible Emission Limits (AEL) provided in tables in the standard. In the ANSI Z 136.1 standard, the AEL is defined as the product of the Maximum Permissible Exposure (MPE) level and the area of the limiting aperture. For visible and near infrared lasers, the limiting aperture is based upon the "worst-case" pupil opening and is a 7 mm circular opening.
Lasers and laser systems are assigned one of four broad Classes (I to IV) depending on the potential for causing biological damage. The biological basis of the hazard classes are summarized in Table below.
Class I: cannot emit laser radiation at known hazard levels (typically continuous wave: cw 0.4 mW at visible wavelengths). Users of Class I laser products are generally exempt from radiation hazard controls during operation and maintenance (but not necessarily during service).
Since lasers are not classified on beam access during service, most Class I industrial lasers will consist of a higher class (high power) laser enclosed in a properly interlocked and labeled protective enclosure. In some cases, the enclosure may be a room (walk-in protective housing) which requires a means to prevent operation when operators are inside the room.
Class I.A.: a special designation that is based upon a 1000-second exposure and applies only to lasers that are "not intended for viewing" such as a supermarket laser scanner. The upper power limit of Class I.A. is 4.0 mW. The emission from a Class I.A. laser is defined such that the emission does not exceed the Class I limit for an emission duration of 1000 seconds.
Class II: low-power visible lasers that emit above Class I levels but at a radiant power not above 1 mW. The concept is that the human aversion reaction to bright light will protect a person. Only limited controls are specified.
Class IIIA: intermediate power lasers (cw: 1-5 mW). Only hazardous for intrabeam viewing. Some limited controls are usually recommended.
NOTE:There are different logotype labeling requirements for Class IIIA lasers with a beam irradiance that does not exceed 2.5 mW/cm2 (Caution logotype) and those where the beam irradiance does exceed 2.5 mW/cm2 (Danger logotype).
Class IIIB: moderate power lasers (cw: 5-500 mW, pulsed: 10 J/cm2 or the diffuse reflection limit, whichever is lower). In general Class IIIB lasers will not be a fire hazard, nor are they generally capable of producing a hazardous diffuse reflection. Specific controls are recommended.
Class IV: High power lasers (cw: 500 mW, pulsed: 10 J/cm2 or the diffuse reflection limit) are hazardous to view under any condition (directly or diffusely scattered) and are a potential fire hazard and a skin hazard. Significant controls are required of Class IV laser facilities.

LASER CLASSIFICATIONS--SUMMARY OF HAZARDS
Applies to
--- wavelength ranges ---
  ------------ Hazards ------------
Class
UV
VIS
NIR
 IR 
  Direct ocular
Diffuse ocular
Fire
I X X X X   No No No
IA -- X* -- --   Only after
1000 sec
No No
II -- X -- --   Only after
0.25 sec
No No
IIIA X X** X X   Yes No No
IIIB X X X X   Yes Only when laser
output is near
Class IIIB limit
of 0.5 Watt
No
IV X X X X   Yes Yes Yes

Key: X
*
**
=
=
=
Indicates class applies in wavelength range.
Class IA applicable to lasers "not intended for viewing" ONLY.
CDRH Standard assigns Class IIIA to visible wavelengths ONLY. ANSI Z 136.1 assigns Class IIIA to all wavelength ranges.

How to Determine the Class of Lasers During Inspection

The classification of a laser or laser product is, in some instances, a rather detailed process. It can involve determination of the AEL, measurement of the laser emission, measurement/determination of the emission pulse characteristics (if applicable), evaluation of various performance requirements (protective housing, interlocks, etc.) as specified by the FLPPS and/or ANSI standards.

It should be stressed that classification is a required specification provided by the laser manufacturer and the label that specifies the class is found in only one location on the laser product. The class of the laser will be specified only on the lower left-hand corner (position three) of the warning logotype label.

The logotype is the rectangular label that has the laser "sunburst" symbol and the warning statement of CAUTION (Class II and some Class IIIA) or DANGER (some Class IIIA, all Class IIIB and Class IV). This label will also have the type of laser designated (HeNe, Argon, CO2, etc.) and the power or energy output specified (1 mW CW/MAX, 100 mJ pulsed, etc.).

Class I lasers have no required labeling indicating the Class I status. Although the FLPPS requires no classification labeling of Class I lasers it does require detailed compliance with numerous other performance requirements (i.e., protective housing, identification and compliance labeling, interlocking, etc.)

Control Measures

Engineering controls are design features or devices that are applied to a laser or its environment for the purpose of reducing laser hazards. Engineering controls are considered to be the most effective types of control.

Examples of engineering controls are

  • beam housings,
  • beam shutters,
  • attenuators, and
  • remote firing controls.

A list of required and recommended engineering control measures for each laser class is available in the Table below:

Administrative controls consist of procedures and information provided to personnel for the purpose of reducing laser hazards.

Examples of administrative controls include :

  • warning signs and labels,
  • standard operating procedures, and
  • safety training.

A list of required and recommended administrative control measures for each laser class is available in

CONTROL MEASURES FOR THE FOUR LASER CLASSES [ANSI Z 136.1 (1993)]
  ------------------ Class -----------------
Engineering Control measures I IA II IIIA IIIB IV


Protective housing X X X X X X
Without protective housing --Supervisor shall establish alternate controls --
Interlocks on protective housing a a a X X X
Service access panel b b b b b X
Key switch master _ _ _ _ · X
Viewing portals _ _ Shall if MPE is exceeded Shall if MPE is exceeded Shall if MPE is exceeded Shall if MPE is exceeded
Collecting optics _ _ Shall if MPE is exceeded Shall if MPE is exceeded Shall if MPE is exceeded Shall if MPE is exceeded
Totally open beam path _ _ _ _ X X
Limited open beam path _ _ _ _ X X
Remote interlock connector _ _ _ _ · X
Beam stop or attenuator _ _ _ · · X
Activation warning system _ _ _ _ · X
Emission delay _ _ _ _ _ ·
Class IIIB laser controlled area _ _ _ _ X _
Class IV laser controlled area _ _ _ _ _ X
Laser outdoor controls _ _ _ _ X X
Temporary laser controlled area b b b b _ _
Remote firing & monitoring _ _ _ _ _ ·
Labels _ X X X X X
Area posting _ _ · · X X
Administrative & procedural controls _ X X X X X
Standard operating procedures _ _ _ _ · X
Output emission limitations _ _ _ --Supervisor determines--
Education and training _ _ _ X X X
Authorized personnel _ _ _ _ X X
Alignment procedures _ _ X X X X
Eye protection _ _ _ _ · X
Spectator control _ _ _ _ · X
Service personnel b b b b X X
Laser demonstration _ _ X X X X
Laser fiber optics _ _ X X X X
Key: 
 
 
 
 
 
X
a.
b.
_
 ·
Shall if MPE is exceeded
 = 
 = 
 = 
 = 
 = 
 = 
 
Shall.
Shall if embedded Class IIIA, Class IIIB, Class IV.
Shall if embedded Class IIIB or Class IV.
No requirement.
Should.
Shall if MPE is exceeded.

Personal Protective Equipment (PPE): are devices worn by individual to protect against hazardous environment.

Of particular importance in prevention of laser hazards is eyewear. Laser protective eyewear is usually made of filters, which absorb and/or reflect specific wavelengths of laser light.

An important factor to look for, when choosing laser protective eyewear is the optical density (OD) of the lenses. The OD of the eyewear is a measure of its capacity to filter light; OD is the opposite of transmission. The higher the OD, the less light is transmitted to the eye. The OD must be chosen so as not to impair vision significantly, yet at the same time, must be chosen so as to be capable of reducing the laser light to the MPE.

When choosing laser protective eyewear it is also important to select eyewear that is designed to protect against the wavelengths of the laser light that will be used. Eyewear designed to filter shorter wavelengths of light are not appropriate for use with lasers that emit longer wavelengths of light.

Finally, it is also important that some attention be paid to the style of the laser protective eyewear so that comfort to the wearer is maximized. Laser protective eyewear cannot protect personnel if not worn.

Always check with your supervisor or somebody more familiar with particular laser equipment.