Читать книгу Patty's Industrial Hygiene, Physical and Biological Agents - Группа авторов - Страница 71

Lasers have found widespread use in industry, science, and medicine. In consumer products, they are engineered to be completely safe and are placed into a “no‐risk,” “eye‐safe” hazard category referred to as Class 1. The general public also employs Class 2 lasers, which are generally safe when properly used as alignment devices and pointers. However, in the workplace, higher‐risk groups are encountered. Class 3 laser products pose an eye hazard and Class 4 lasers may pose additional hazards, such as a fire hazard or a significant hazard to the skin. In addition to these broad hazard categories, three sub‐classes also exist to better cite the very limited hazards of Class 3 lasers on the border of posing a hazard (Class 3R) or to address the increased risk from optically aided viewing: Classes 1M and 2M. The user must employ laser hazard controls for Class 3 and 4 laser products, and safety requirements generally become more stringent for the higher class. Special controls are required in several application areas, such as laser material processing and surgery. Eye protection is required when laser beams are accessible in the open. Class 4 lasers employed in material processing, such as welding and cutting, or in surgery may produce airborne contaminants that will generally require local exhaust ventilation.

A laser produces coherent optical radiation. To date, lasers have been developed which emit monochromatic radiant energy having wavelengths ranging from the extreme ultraviolet (UV) to the far infrared (IR) (sub‐millimeter) part of the electromagnetic spectrum. This nonionizing range of the electromagnetic radiation is frequently termed the “optical spectrum” and ranges from approximately 100 nm (at the edge of the soft X‐ray, or “grenz‐ray” region) in the vacuum UV, to 1 mm (1000 μm) at the edge of the microwave band of the radiofrequency spectrum (corresponding to a frequency of 300 GHz). Wavelengths are generally expressed in terms of nanometers (nm) in the UV and visible spectral regions and in terms of micrometers (μm) in the IR region. The optical spectrum is frequently divided into seven photobiological spectral bands as shown in Figure 1.

The term “laser” is an acronym for Light Amplification by Stimulated Emission of Radiation. Despite the fact that stimulated emission is a process that was theoretically predicted by Albert Einstein in 1916, the first successful laser was not demonstrated until 1960 (1). Lasers have many applications in the research laboratory, in industry, medicine, and surgery, and even in office settings, construction sites and even households. For many applications, such as videodisk players, laser printers, computers, and optical fiber communication systems, the laser's radiant energy output is enclosed; the user faces no safety hazard or health risk. In such applications, the presence of a potentially higher‐class laser embedded in the product may not be obvious to the user. However, in some medical, industrial, or research applications, the laser's emitted radiant energy is accessible and may pose a potential hazard to the eye and skin.

Because the laser process (sometimes referred to as “lasing”) can produce a highly collimated beam of optical radiation (i.e. UV, visible, or IR radiant energy), a laser can pose a hazard at a considerable distance – quite unlike most hazards encountered in the workplace. Perhaps it is this characteristic more than any other that has led to special concerns expressed by workers and by radiological and occupational health and safety experts. Nevertheless, lasers can be used safely when appropriate hazard controls are applied. Guidelines for safe exposure and safety standards (5, 6) for lasers exist worldwide and are currently quite similar or “harmonized.” All of the safety standards make use of a hazard classification system, which groups laser products into one of four broad hazard classes according to the laser's output power or energy and its ability to ca use harm. Safety measures are then applied commensurate to the hazard classification (5, 7–10).

FIGURE 1 The optical spectrum. The photobiological spectral bands of the International Commission on Illumination (the CIE) are shown.

The motivation for laser hazard classification was to minimize the need for measurements by the practicing industrial hygienist or safety specialist. It was recognized in the late 1960s that it was very impractical to expect many in the laser user community to conduct highly specialized (and potentially costly) measurements. Instruments (laser radiometers) are costly and a wide variety would be required to assess the risks from every possible laser wavelength and pulse duration. It was also recognized at that time that virtually no laser could be constructed that was truly “eye‐safe” unless the beam were enclosed or at least altered in some way. That is, the physical threshold for laser action was normally above a permissible exposure limit (EL) for direct exposure of the eye. Therefore, laser safety standards and regulations evolved that placed the responsibility upon the manufacturer to perform the first step of hazard evaluation – the determination of the potential hazard of the laser output, and the type of risk – as defined by a hazard level, the laser class (Table 1). The hazard class then would be employed by the laser user for a risk assessment. The user or safety community at the user's site would then employ the laser classification to complete the risk assessment and determine appropriate control measures. This task became that of the laser safety officer (LSO) in the ANSI Z136‐series of laser safety standards for the safe use of lasers. The Federal Laser Product Performance Standard, 21CFR1040, and the international laser product performance standard, IEC 60825‐1, are harmonized (i.e. consistent) and both require the manufacturer to perform measurements to determine the laser hazard class and to label the product with the hazard class (as well as to incorporate certain system safety features based upon the hazard class). The system‐safety features such as interlocks on access panels are particularly important for many Class 1 lasers that incorporate a very hazardous, higher‐class, embedded laser within a safety enclosure, known as the protective housing.

TABLE 1 Simplified summary of laser hazard classification.^a

Hazard class	Hazard	Control measures	Examples
1	Not hazardous (“eye‐safe”)	No user control measures; all safety requirements are on the manufacturer	Some small infrared diode lasers; totally enclosed laser scribing systems
1M	Not hazardous unless viewed with a collecting telescope	Warnings to the user not to look into the beam with telescope	Free‐space laser telecommunication system
2	Not a realistic hazard because of the eye's aversion response to bright light (visible lasers only)	Warning label not to stare into the beam; afterimages will result	Low‐power 1‐mW laser pointers; some hand‐held bar‐code scanners
2M	Not a realistic hazard because of the eye's aversion response unless viewed with a telescope	Warning not to view with telescope and not to stare into the beam	Some expanded‐beam laser alignment tools
3R	Marginally hazardous – a transitional hazard classification. Hazardous only under worst‐case viewing conditions, but MPE can be exceeded, but a low risk of actual eye injury exists	Warning not to look into the direct beam. The MPE will be exceeded at close distances. Limit use to mature users	1–5‐mW laser pointers; alignment lasers used in industry and at construction sites for positioning, leveling, etc.
3B	Risk of eye injury upon direct intra‐beam viewing and from some specular reflections; generally not a realistic risk of significant skin injury	Stronger warnings, emission indicators and other features and controlled use and eye protection	Low‐power medical treatment lasers; military hand‐held Nd rangefinders
4	Higher risk of eye injury from the direct beam of specular reflections, and potentially even from diffuse reflections from some pulsed lasers; significant risk of skin injury	Stringent user controls with controlled areas, use of eye protection; laser barriers; beam enclosures	Surgical lasers above 0.5‐W, multi‐kilowatt industrial material‐processing lasers, research laser systems

^aSee the detailed discussion on hazard classes in the section on hazard evaluation measurements and classification.

Lasers operate at discrete wavelengths within the optical spectrum, and although most lasers are monochromatic (emitting one wavelength, or single color), it is not uncommon for one laser to emit several discrete wavelengths. For example, the argon‐ion laser emits several different lines within the near UV (e.g. near 350 nm) and several lines in the visible spectrum, but this laser is frequently designed to emit only one green “line” (wavelength) at 514.5 nm and/or a blue line at 488 nm. Some small laser pointers designed to emit a visible (e.g. green) wavelength may also emit an incompletely blocked, near‐IR pump beam (11) that can pose an unexpected hazard.

Although several thousand different laser lines (i.e. discrete laser wavelengths characteristic of different active media) have been demonstrated in the physics laboratory, perhaps only 20–30 have been developed commercially to the point where they are routinely applied in everyday technology (9, 12–16). Guidelines for human exposure have been developed and published which basically cover all wavelengths of the optical spectrum in order to allow for currently known laser lines and future lasers (2, 3, 5).