Non-beam Hazards and Laser Safety (part 2)

___ 4 CHEMICAL HAZARDS

___ 4.1 LASER DYES

Laser dyes and associated solvents are often composed of toxic and carcinogenic chemicals dissolved in flammable solvents. This creates the potential for fires, chemical spills, and personnel exposures above permissible limits. Frequently, the most hazardous aspect of a laser operation is the mixing of chemicals that make up the laser dye. Dye lasers normally use a lasing medium composed of a complex fluorescent organic dye dissolved in an organic solvent. The use of dimethylsulfoxide (DMSO) as a solvent for cyanide dyes in dye lasers is of particular concern and should be discouraged. DMSO aids in the transport of dyes into the skin.

___ 4.1.1 Recommended Guidelines for Laser Dye Use

1. All dyes must be treated as hazardous chemicals. Most solvents suitable for dye solutions are flammable and toxic by inhalation or skin absorption.

2. Obtain material safety data sheets (MSDSs) for all dyes and solvents.

3. Use and store all dyes and solvents in accordance with your institution's chemical hygiene plan if one exists.

4. Prepare and handle dye solutions inside a chemical fume hood.

5. Wear a lab coat, eye protection, and gloves. Contact an industrial hygienist for assistance in glove selection.

6. Pressure-test all dye laser components before using dye solutions. Pay particular attention to tubing connections.

7. Install spill pans (secondary containment) under pumps and reservoirs.

While plastic trays maybe economical and available, they are flammable. A superior choice for secondary containment is metal trays.

8. Be alert to contaminated parts.

9. Keep dye mixing areas clean.

10. Tie down tubes so that if they disconnect, one does not have an angry snake on one's hands.

___ 4.2 COMPRESSED AND TOXIC GASES

Hazardous gases (e.g., fluorine, hydrogen chloride) may be used in such laser applications as excimer lasers. If hazardous gases are used, standard operating procedures should contain references for the safe handling of compressed gases, such as seismic restraints, use of gas cabinets, proper tubing and fittings, and so on. The use of mixtures with inert gases, rather than the pure gases, is generally preferred. Hazardous gases should be stored in appropriately exhausted enclosures, with the gases permanently piped to the laser using the recommended metal tubing and fittings. An inert gas purge system and distinctive coloring of the pipes and fittings is also prudent. Compressed gas cylinders should be secured from tipping. Standard compressed gas safety rules should be followed. If questions arise, consult your compressed gas safety specialists.

___ 4.3 FUMES, VAPORS, AND LASER-GENERATED AIR CONTAMINANTS FROM BEAM-TARGET INTERACTION

Air contaminants may be generated when certain class 3B and class 4 laser beams interact with matter. When the target irradiance reaches a given threshold (approximately 107 W/cm^2), target materials, including plastics, composites, metals, and tissues, may vaporize, creating hazardous fumes or vapors that may need to be captured or exhausted. The plume is the cloud of contaminants created when there is an interaction between the beam and the target matter. The plume may contain metallic aerosols dust, chemical fumes, and aerosols containing biological contaminants. Some specific examples follow:

1. Ozone produced around flash lamps and concentrations of ozone that build up with high-repetition rate lasers

2. Asbestos fibers released from the firebricks used as backstops for carbon dioxide lasers

3. Polycyclic aromatic hydrocarbons from mode burns on poly(methyl methacrylate) type polymers

4. Hydrogen cyanide and benzene from cutting aromatic polyamide fibers

5. Fused silica from cutting quartz

6. Heavy metals from etching

7. Benzene from cutting polyvinyl chloride

8. Cyanide, formaldehyde, and synthetic and natural fibers associated with other processes

Special optical materials used for far infrared windows and lenses have been the source of potentially hazardous levels of airborne contaminants. For example, calcium telluride and zinc telluride burn in the presence of oxygen when beam irradiance limits are exceeded. Exposure to cadmium oxide, tellurium, and tellurium hexafluoride should also be controlled.

Exposure to these contaminants must be controlled to reduce exposure below acceptable OSHA or national permissible exposure limits. The MSDS may be consulted to determine exposure information and permissible exposure limits. In general, three major control measures are available: exhaust ventilation, respiratory protection, and isolation of the process. Whenever possible, reticulation of the plume should be avoided. Exhaust ventilation, including use of fume hoods, should be used to control airborne contaminants.

___ 5 FIRE HAZARDS

Class 4 laser systems represent a fire hazard along with some focused class 3B lasers. Enclosure of class 3 laser beams can result in potential fire hazards if enclosure materials are likely to be exposed to irradiances exceeding 10 W/cm^2.

The use of flame retardant materials is encouraged. Opaque laser barriers (e.g., curtains) can be used to block the laser beam from exiting the work area during certain operations. While these barriers can be designed to offer a range of protection, they normally cannot withstand high irradiance levels for more than a few seconds without some damage, including the production of smoke, open fire, or penetration. Users of commercially available laser barriers should obtain appropriate fire prevention information from the manufacturer. Operators of class 4 lasers should be aware of the ability of unprotected wire insulation and plastic tubing to ignite from intense reflected or scattered beams, particularly from lasers operating at invisible wavelengths. Class 4 lasers can also ignite gas atmospheres and cases exist of class 3B lasers igniting a gas environment, when particulates in the gas environment absorb the laser radiation and get hot enough to ignite the gas atmosphere.

___ 6 ERGONOMICS AND HUMAN FACTORS

Ergonomic problems can arise from a laser operation that causes awkward arm and wrist positions. If these positions occur for prolonged periods of time, medical problems such as repetitive strain injuries may arise. Back injuries can occur from stretching or strains caused by poorly designed enclosures that are repeatedly taken on and off.

___ 7 SEISMIC SAFETY

In some parts of the world, natural disaster conditions such as earthquake need to be considered in the design and use of equipment. Examples would be fastening electronic racks to the floor or walls, and having at least two locking wheels on rolling racks and tying down computer monitors. When possible, bolt down heavy laser equipment. One should be aware of tall objects (bookcases, optical storage racks) that if tipped over would block access into or out of a work space.

___ 8 AREA ILLUMINATION

Adequate lighting is a standard recommendation. If low light levels are required, have luminescent strips or arrows to show the way to exits and emergency equipment.

___ 9 MECHANICAL HAZARDS

___ 9.1 ROBOTICS

With the increasing use of robotics in a variety of settings, robotic safety cannot be overlooked. A common error is to assume the robot is in the off position when it’s in a pause mode. Robotic hazards can occur from a number of sources:

overloading on the robot, effects from RF sources, software errors, corrosive atmosphere effects on material, and metal stress. The majority of robot laser tools or applications present little hazard from the laser beam. The wide variety of robotic uses presents a broad spectrum of hazard situations. Here is some food for thought when selecting or thinking about robot safeguarding systems. An effective robotic safeguarding system should be based upon a hazard analysis of the robot system's use, programming, and maintenance operations. Among the factors to be considered are the tasks a robot will be programmed to perform, programming procedures, environmental conditions, location and installation requirements, possible human errors, scheduled and unscheduled maintenance, possible robot and system malfunctions, normal mode of operation, and all personnel functions and duties.

An effective safeguarding system protects not only operators but also any others who might work on the robot or integrated equipment and those in the surrounding area. A combination of safeguarding methods may be used. Redundancy and backup systems are especially recommended, particularly if a robot or robot system operates in hazardous conditions or handles hazardous materials.

The safeguarding devices employed should not themselves constitute or act as a hazard or curtail necessary vision or viewing by human operators.

___ 9.1.1 Robot Accidents

Robotic incidents can be grouped into four categories:

Impact or Collision Accidents: Unpredicted movements, component malfunctions, or unpredicted program changes related to the robot's arm or peripheral equipment can result in contact accidents.

Crushing and Trapping Accidents: A worker's limb or other body part can be trapped between a robot's arm and other peripheral equipment, or the individual may be physically driven into and crushed by other peripheral equipment.

Mechanical Part Accidents: The breakdown of the robot's drive components, tooling or end-effector, peripheral equipment, or its power source is a mechanical accident. The release of parts, failure of the gripper mechanism, or failure of end-effector power tools (e.g., grinding wheels, buffing wheels, deburring tools, power screwdrivers, and nut runners) are a few types of mechanical failures.

Other Accidents: Other accidents can result from working with robots.

Equipment that supplies robot power and control represents potential electrical and pressurized fluid hazards. Ruptured hydraulic lines could create dangerous high-pressure cutting streams or whipping hose hazards. Environmental accidents from arc flash, metal spatter, dust or electromagnetic or RF interference can also occur. In addition, equipment and power cables on the floor present tripping hazards.

___ 10 SUPPLEMENTAL ELECTRICAL SAFETY SECTION

___ 10.1 LIFE-THREATENING EFFECTS

Charles F. Dalziel, Ralph Lee, and others have established the following criteria for the lethal effects of electric shock:

1. Currents in excess of a human's "let-go" current (>16 mA at 60 Hz) passing through the chest can produce collapse, unconsciousness, asphyxia, and even death.

2. Currents (>30 mA at 60 Hz) flowing through the nerve centers that control breathing can produce respiratory inhibition, which could last long after interruption of the current.

3. Cardiac arrest can be caused by a current greater than or equal to 1 A at 60 Hz flowing in the region of the heart.

4. Relatively high currents (0.25 to 1 A) can produce fatal damage to the central nervous system. Currents greater than 5 A can produce deep body and organ burns, substantially raise body temperature, and cause immediate death.

5. Electricity flowing through the human body can shock, cause involuntary muscle reaction, paralyze muscles, burn tissues and organs, and kill. The typical effects of various electric currents flowing through the body on the average 150-lb male and 115-lb female body are given in Table.1.

___ 10.2 BURNS

Although a current may not pass through vital organs or nerve centers, internal electrical burns can still occur. These burns, which are a result of heat generated by current flowing in tissues, can be either at the skin surface or in deeper layers (muscles, bones, etc.) or both. Typically, tissues damaged from this type of electrical burn heal slowly. Burns caused by electric arcs are similar to burns from high-temperature sources. The temperature of an electric arc, which is in the range of 4,000 to 35,000°F, can melt all known materials, vaporize metal in close proximity and burn flesh and ignite clothing at distances up to 10 ft from the arc.

___ 10.3 DELAYED EFFECTS

Damage to internal tissues may not be apparent immediately after contact with the current. Internal tissue swelling and edema are also possible.

___ 10.4 CRITICAL PATH

The critical path of electricity through the body is through the chest cavity.

Current flowing from one hand to the other, from a hand to the opposite foot, or from the head to either foot will pass through the chest cavity, paralyzing the respiratory or heart muscles, initiating ventricular fibrillation, and/or burning vital organs.

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TABLE.1 Effects of Electric Current on the Human Body

Effect/Feeling

Direct Current (mA) Alternating Current (mA) Incident Severity 60 Hz 10,000 Hz 150 lb 115 lb 150 lb 115 lb 150 lb 115 lb Slight sensation 1 0.6 0.4 0.3 7 5 None Perception threshold 5.2 3.5 1.1 0.7 12 8 None Shock, not painful 9 6 1.8 1.2 17 11 None Shock, painful 62 41 9 6 55 37 Spasm, indirect injury Muscle clamps source 76 51 16 10.5 75 50 Possibly fatal Respiratory arrest 170 109 30 19 180 95 Frequently fatals >0.03-sec ventricular fibrillation 1300 870 1000 670 1100 740 Probably fatal >3-sec ventricular fibrillation 500 370 100 67 500 340

Probably fatal >5-sec ventricular fibrillation 375 250 75 50 375 250

Probably fatal Cardiac arrest - - 4000; 4000 - - Possibly fatal Organs burn - - 5000; 5000 - - Fatal if it’s a vital organ

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___ 10.5 INFLUENTIAL VARIABLES

The effects of electric current on the human body can vary depending on the following:

1. Source characteristics (current, frequency, and voltage of all electric energy sources)

2. Body impedance and the current's pathway through the body

3. How environmental conditions affect the body's contact resistance

4. Duration of the contact

___ 10.5.1 Source Characteristics

An alternating current (ac) with a voltage potential greater than 550 V can puncture the skin and result in immediate contact with the inner body resistance.

A 110-V shock may or may not result in a dangerous current, depending on the circuit path, which may include the skin resistance. A shock greater than 600 V will always result in very dangerous current levels. The most severe result of an electrical shock is death.

The worst possible frequency for humans is 60 Hz, which is commonly used in utility power systems. Humans are about five times more sensitive to 60-Hz alternating current than to direct current. At 60 Hz, humans are more than six times as sensitive to alternating current than at 5000 Hz, and the sensitivity appears to decrease still further as the frequency increases. Above 100 to 200 kHz, sensations change from tingling to warmth, although serious burns can occur from higher RF energy.

At much higher frequencies (e.g., above 1 MHz), the body again becomes sensitive to the effects of an alternating electric current, and contact with a conductor is no longer necessary; energy is transferred to the body by means of electromagnetic radiation.

___ 10.5.2 Body Impedance

Three components constitute body impedance: internal body resistance and the two skin resistances at the contact points with two surfaces of different voltage potential. One-hand (or single-point) body contact with electrical circuits or equipment prevents a person from completing a circuit between two surfaces of different voltage potential. Table 2 lists skin-contact resistances encountered under various conditions. It also shows the work-area surfaces and wearing apparel effects on the total resistance from the electrical power source to ground.

This table can be used to determine how electrical hazards could affect a worker in varying situations.

Delayed reactions and even death can be caused by serious burns or other complications. The most dangerous current flow via the chest cavity is through the heart when the shock occurs in the time relative to the normal heart rhythm. This current may cause ventricular fibrillation, which is defined as repeated, rapid, uncoordinated contractions of the heart ventricles. Ventricular fibrillation that alters the heart's normal rhythmic pumping action can be initiated by a current flow of 75 mA or greater for 5 sec or more through the chest cavity.

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TABLE 2 Human Resistance (Q) for Various Skin-Contact Conditions Body Contact Condition Dry (ohms) Wet (ohms) Finger touch 40,000-1,000,000 4,000-15,000 Hand holding wire 15,000-50,000 3,000-5,000 Finger-thumb grasp 10,000-30,000 2,000-5,000 Hand holding pliers 5,000-10,000 1,000-3,000 Palm touch 3,000-8000 1,000-2,000 Hand around 1.5-inch pipe or drill handle 1,000-3,000 500-1,500 Two hands around 1.5-inch pipe 500-1,500 250-750 Hand immersed - 200-500 Foot immersed - 100-300

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Above: Fig. 1 Endotrachel tube on fire.