and solvents or flammable solvents, ultraviolet or visible radiation from the pump lamps, and electric shock from power supplies for lamps.

Lasers are classified according to their relative hazards: Class I lasers, including laser printers, compact disc players, and unfocused laser diodes, are either completely enclosed or have such a low output of power that even a direct beam in the eye could not cause damage. Class II lasers, including supermarket scanners and visible laser bar code scanners, are visible light lasers with power of less than 1 milliwatt (mW). These can be a hazard if a person stares into the beam and resists the natural reaction to blink or turn away. Class IIIA lasers have powers between 1 and 5 mW and can present an eye hazard if a person stares into the beam and resists the natural reaction to blink or turn away, or views the beam with focusing optical instruments. Class IIIB lasers are visible, ultraviolet, and infrared lasers with powers in the 5 to 500 mW range and produce eye injuries instantly from both direct and specularly reflected beams. Class IV lasers are visible, ultraviolet, and infrared lasers with continuous powers in excess of 500 mW or pulse energies in excess of a threshold that depends on wavelength and pulse duration. Class IV lasers present all of the hazards of Class III lasers and may also produce eye or skin damage from diffuse scattered light. Anyone who is not the authorized operator of a laser system should never enter a posted laser-controlled laboratory if the laser is in use.

3.E.6 Radiofrequency and Microwave Hazards

Radiofrequency (RF) and microwaves occur within the range 10 kilohertz (kHz) to 300,000 megahertz (MHz) and are used in RF ovens and furnaces, induction heaters, and microwave ovens. Extreme overexposure to microwaves can result in the development of cataracts and/or sterility. Microwave ovens are increasingly being used in laboratories for organic synthesis and digestion of analytical samples. Use of metal in microwave ovens can result in arcing and, if a flammable solvent is present, in fire or explosion. Superheating of liquids can occur. Capping of vials and other containers used in the oven can result in explosion from pressure buildup within the vial. Inappropriately selected plastic containers may melt.

3.E.7 Electrical Hazards

The electrocution hazards of electrically powered instruments, tools, and other equipment can almost be eliminated by taking reasonable precautions, and the presence of electrically powered equipment in the laboratory need not pose a significant risk. Many electrically powered devices are used in homes and workplaces in the United States, often with little awareness of the safety features incorporated in their design and construction. But, in the laboratory, as well as elsewhere, it is critical that these features not be defeated by thoughtless or ignorant modification. The possibility of serious injury or death by electrocution is a very real one if careful attention is not paid to engineering, maintenance, and personal work practices. Equipment malfunctions can lead to electrical fires. Every worker should know the location of electrical shutoff switches and/or circuit breaker switches and should know how to turn off power to burning equipment by using these switches.

Some special concerns arise in laboratory settings. The insulation on wires can be eroded by corrosive chemicals, organic solvent vapors, or ozone (from ultraviolet lights, copying machines, and so forth). Eroded insulation on electrical equipment in wet locations such as cold rooms or cooling baths must be repaired immediately. In addition, sparks from electrical equipment can serve as an ignition source in the presence of flammable vapor. Operation of certain equipment (e.g., lasers, electrophoresis equipment) may involve high voltages and stored electrical energy. The large capacitors used in many flash lamps and other systems are capable of storing lethal amounts of electrical energy and should be regarded as "live" even if the power source has been disconnected.

Loss of electrical power can produce extremely hazardous situations. Flammable or toxic vapors may be released from freezers and refrigerators as chemicals stored there warm up; certain reactive materials may decompose energetically upon warming. Hoods may cease to function and to protect workers. Stirring (motor or magnetic) required for safe reagent mixing may cease. Return of power to an area containing flammable vapors may ignite them.

3.E.8 Magnetic Fields

Increasingly, instruments that generate large static magnetic fields (e.g., frequently, NMR spectrometers) are present in research laboratories. Such magnets typically have fields of 25,000 to 160,000 gauss (2.5 to 16 teslas), far above Earth's magnetic field, which is about 0.5 G. The magnitude of these large static magnetic fields falls off rapidly with distance, which is fortunate, because effects on magnetic media such as credit cards and computer disks are thus limited (see Chapter 6, Table 6.1). Strong attraction occurs when the magnetic field is above 50 to 100 G and increases by the seventh

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