Nonbeam Hazards and Laser Safety

INTRODUCTION

While beam hazards (exposure to the laser beam) are the most prominent laser hazards, other hazards pose an equal or possibly greater risk of injury or death.

To date, we have not melted anyone with a laser beam, but we have had laser users electrocuted and patients die from operating room fires with the laser as the heat or ignition source. As laser applications expand further into our society, a greater number of associated, or nonbeam, hazards will need to be considered to maintain a safe workplace. Here are two examples: at a construction site, which is the greater hazard to the worker - the alignment laser used in a trench or a lack of adequate bracing of the trench, which could cave in and cause a worker's death? When laying an optical fiber for communications, should one be more concerned about confined work in a manhole or the laser fiber? Clearly, one must always be aware of hazards associated with an activity. The number-one associated hazard with laser use is the possible electrical hazard.

The long list of associated hazards can be broken down into hazard categories:

physical, chemical, biological, mechanical, and ergonomic human factors. The laser safety officer (LSO) need not be an expert in these areas, but needs to be aware that such hazards exist and be alert for them. Once determined, the appropriate safety specialists should perform a proper evaluation. In some cases users have grown so accustomed to the hazards around them that such hazards may not even occur to them if they are asked to list workplace hazards. An example might be small quantities of optical cleaning solvents.

___ 1.1 A LISTING OF NONBEAM HAZARDS

A. Physical

1. Noise

2. Vibration

3. Incoherent radiation

4. X-rays

5. High temperature

6. Low temperature

7. Electricity

B. Chemical

1. Toxic substances

2. Carcinogenic substances

3. Irritant substances

4. Dust and particulates

5. Fire

C. Biological

1. Microbiological organisms

2. Viruses

D. Mechanical

1. Trailing cables and pipes

2. Sharp edges

3. Moving parts

4. High-pressure water

E. Ergonomic

1. Workstation layout

2. Manual handling

3. Person-machine interface

4. Ease of operation

5. Shift patterns

___ 2 ELECTRICAL HAZARDS

Most lasers contain high-voltage power supplies and often-large capacitors or capacitor banks that store lethal amounts of electrical energy. In general, systems that permit access to components at such lethal levels must be interlocked.

However, during maintenance, service, and alignment procedures, such components often become exposed or accessible. This has caused numerous serious and fatal shocks.

___ 2.1 GOOD PRACTICE GUIDELINES FOR ELECTRICAL HAZARDS

1. All equipment should be installed in accordance with recognized national electrical codes.

2. All electrical equipment should be treated as if it were "live."

3. Working with or near live circuits should be avoided. Whenever possible, unplug the equipment before working on it.

4. A buddy system should be used when work on live electrical equipment is necessary, particularly after normal work hours or in isolated areas. Ideally, the person should be knowledgeable of first aid and CPR.

5. Rings and metallic watchbands should not be worn, nor should metallic pens, pencils, or rulers be used while one is working with electrical equipment.

6. Live circuits should be worked on using one hand when it’s possible to do so.

7. When one is working with electrical equipment, only tools with insulated handles should be used.

8. Electrical equipment that gives the slightest perception of current when touched should be removed from service, tagged, and repaired prior to further use.

9. When working with high voltages, consider the floor conductive and grounded unless standing on suitably insulated dry matting normally used for electrical work.

10. Live electrical equipment should not be worked on when one is standing on a wet floor or when the hands, feet, or body are wet or perspiring.

11. Don’t undertake hazardous activities when fatigued, emotionally stressed, or under the influence of medication that dulls or slows the mental and reflex processes.

12. No one should work on lasers or power supplies unless qualified and approved to perform the specific tasks.

13. Before working with electrical equipment, de-energize the power source. Lock and tag out the disconnect switch.

14. Check that each capacitor is discharged, shorted, and grounded before working near capacitors.

15. When possible, use shock-preventing shields, power supply enclosures, and shielded leads in all experimental or temporary high-voltage circuits.

16. Follow lockout/tagout procedures when working with hard-wired equipment.

___ 2.2 POTENTIAL ELECTRIC HAZARD PROBLEMS

1. Uncovered electrical terminals

2. Improperly insulated electrical terminals

3. Hidden power off/on warning lights

4. Lack of personnel training

5. Buddy system not being practiced during maintenance and alignment work

6. Non-earth-grounded or improperly grounded laser equipment

7. Excessive wires and cables on the floor that create fall and trip hazards What follows is additional information on some of the more common non beam hazards you might encounter.

___ 3 PHYSICAL HAZARDS

___ 3.1 CRYOGENIC FLUIDS

Cryogenic liquids (especially liquid nitrogen) may be used to cool the laser crystal and associated receiving and transmitting equipment. These liquefied gases are capable of producing skin burns, and as they evaporate, they replace the oxygen in the area; this is a particular danger in small unventilated rooms. Adequate ventilation must be ensured. The storage and handling of cryogenic liquids must be performed in a safe manner. Insulated handling gloves that can be quickly removed should be worn. Clothing should have no pockets or cuffs to catch spilled cryogenics. Suitable eye protection must be worn. If a spill occurs on the skin, flood the skin contact area with large quantities of water. Cryogenic fluids are potentially explosive when ice collects in valves or connectors that are not specifically designed for use with cryogenic fluids. Condensation of oxygen in liquid nitrogen presents a serious explosion hazard if the liquid oxygen comes in contact with any organic material. All cryogenic liquids should be used with caution because of the potential for skin or eye damage from the low temperature and the hazards associated with pressure buildups in enclosed piping or containers. Portable containers should only be used where there is sufficient ventilation.

Don’t place containers in a closet or other enclosed space where there is no ventilation supply to the area. The buildup of inert gas in such an area could generate an oxygen-deficient atmosphere.

A full face shield, loose fitting cryogenic handling gloves, apron, and cuffless slacks are recommended for transferring cryogenic fluids. Special vacuum jacket containers with loose-fitting lids should be used to handle small quantities. Vacuum jacketed containers should have overpressure relief devices in place. When plumbing cryogenic liquids, it’s very important to include a pressure relief valve between any two shutoff valves. Also, any space where cryogenic fluids may accumulate (consider leakage into enclosed equipment as well) must be protected by overpressure relief devices. Tremendous pressures can be obtained in enclosed spaces as the liquid converts to gas. For example, 1 cm3 of liquid nitrogen will expand to 700 times this volume as it converts (warms) to its gaseous state. Lines carrying liquid should be well insulated. Containers to be filled with cryogenic liquids should be filled slowly to avoid splashing. Cryogenic containers showing evidence of loss of vacuum in their outer jacket (ice buildup on the outside of the container) should not be accepted from the gas supplier. Contact with air (or gases with a higher boiling point) can cause an ice plug in a cryogenic container.

Should ice plugs be noted, obtain assistance.

___ 3.2 RADIO FREQUENCY (RF)

Some lasers contain RF-excited components such as plasma tubes and Q switches.

Unshielded and loose-fitting components may generate RF fields. RF leakage surveys should be conducted each time RF cables are reconnected. The appropriate protection guide for RF and microwave energy is that given in the American National Standard "Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 300 kHz to 100 GHz" (American National Standards Institute [ANSI] C95.1).

___ 3.3 PLASMA EMISSIONS

When high-power pulsed laser beams (peak irradiance of the order of 1012 W/cm2) are focused on a target, plasma is generated that may also emit collateral radiation.

The plasma may contain hazardous "blue light" ultraviolet (UV) emissions.

___ 3.4 UV AND VISIBLE RADIATION

When targets are heated to very high temperatures, as in laser welding and cutting, an intense bright light is emitted. This light often contains large amounts of short-wavelength or blue light, which may cause conjunctivitis, photochemical damage to the retina, and erythema (sunburn-like reactions) on the skin. Other sources of UV and visible radiation are laser discharge tubes and pump lamps.

The levels produced may be eye and skin hazards. They need to be suitably shielded to reduce exposure to levels below the ANSI Z 136.1 (extended source), the Occupational and Safety Health Administration (OSHA) PELs, and/or ACGIH threshold limit values.

___ 3.5 EXPLOSION HAZARD

Lasers and ancillary equipment may present explosion hazards. High-pressure arc lamps and filament lamps used to excite the lasing medium must be enclosed in housings that can withstand an explosion if the lamp disintegrates. In addition, the laser target and elements of the optical train may shatter during laser operation and should be enclosed in a suitable protective housing. Capacitors may explode if subjected to voltages higher than their rating and must be adequately shielded;

it is recommended that capacitors be equipped with current-limiting devices.

High-energy capacitors should be enclosed in one-eighth-inch thick steel cabinets.

___ 3.6 IONIZING RADIATION (X-RAYS)

X-rays can be produced from two main sources: (a) high-voltage vacuum tubes of laser power supplies, such as rectifiers and thyratrons, and (b) electric discharge lasers. Any power supplies that require more than 15 kV may produce enough x-rays to be a health concern. Although most laser systems use voltages less than 8 kV, some research models may operate above 20 kV.

___ 3.7 NOISE

Noise levels in laser areas rarely are a concern, outside of headaches from the constant pinging sounds from some pulse lasers onto targets. Noise can exceed safe limits because of high-voltage capacitor discharges and in laser peening operations.

In these cases hearing protection may be required. A noise-filled environment can mask the sound of alarms; in such cases visual alarm systems are advisable. An industrial hygienist should perform a noise evaluation if concerns exist.