Avoiding Chemical and Thermal Dangers
The two previous installments of this series addressed safety administration and inertial hazards in the optics shop. This installment covers chemical and thermal issues.
Chemicals present hazards in several ways: contact, ingestion, inhalation or reactive combination. Their results can be acute, chronic or delayed – even by decades, in the case of accumulated exposure to carcinogens or heavy metals.
Backsplash from acetone or alcohol is the most frequent incident involving the eyes. Safety glasses are a good idea – anywhere and anytime. If any chemical splashes in your eyes, have someone call the safety officer immediately and flush your eyes with plenty of water. When handling acids and other dangerous chemicals, wear a rubber apron, long gloves and a face shield. Know ahead of time where the nearest emergency shower is.
Regulations have largely removed benzene, toluene, xylene, carbon tetrachloride, methylene chloride, MEK (methyl-ethyl-ketone), TCE (trichloroethane, trichloroethylene), perchloroethylene, green chrome and other toxic chemicals from frequent use; however, some stores and some legacy procedures may remain. Methanol, acetone, nitric acid, hydrochloric acid, sulfuric acid, phosphoric acid, ammonium hydroxide, potassium hydroxide, sodium hydroxide, chlorine bleach, pitch, terpenes, limonenes and epoxy resins are still used almost daily.
Some materials from which optical components are made are dangerous in certain forms. Beryllium dust, zinc selenide dust or burning vapors, gallium arsenide, AMTIR (amorphous material transmitting infrared radiation), cadmium sulfide, cadmium selenide and other substrate materials are toxic when ingested or inhaled. Even optical glass can contain substantial amounts of lead, arsenic and other heavy metals. Glass manufacturers have been reformulating (or obsoleting) many glass types to avoid the use of heavy metals, but much of this type of glass is still around. In general, if the glass seems particularly dense or heavy, it is likely to contain a lot of lead.
Check material safety data sheets and obtain any that are missing for all chemicals in use.
Hydrofluoric acid (HF) is worthy of special mention. Unlike most acids, which burn the skin and can be diluted with water, HF slowly works its way down to the bone where it destroys the bone’s structure. Most acids can be contained in glass jugs, but HF will even dissolve glass. When properly handled in the right environment by trained, aware personnel using the proper safety equipment, working with HF needn’t be any more dangerous than filling your car’s tank at the gas station. Never work with, or even get near, HF without special training and clearance from your safety department.
Ingestion, contact, absorption
Some chemicals don’t have to get to your mouth to get into your system. They can be absorbed directly through the skin. Be aware of what you’re touching, especially liquids. Acids and bases can cause burns directly. Adhesives can cause skin irritation.
Be aware of where you put your hands. Dust or liquid spilled on a countertop can be easily picked up and transferred to your mouth, nose or eyes. Use gloves when working with chemicals, and remove the gloves immediately afterward. Wash your hands frequently. Clean up areas when you are finished. Avoid leaning on countertops.
Solvents and acids, in particular, and pitch vapors can be hazardous through inhalation without direct contact. If it smells, avoid breathing it in. Of course, many chemicals have adverse effects even if you can’t smell them, and some chemicals can even suppress your sense of smell. You may be provided with respiration masks, but no one type of mask will work for every situation. Check with the safety officer.
The combination of two or more chemicals can cause adverse reactions.
One basic rule is: Never add water to acid! It can erupt in a boiling fountain and splash you with concentrated acid. Rather, add acid to water. If you’re ever in doubt about this, think about swimming pool maintenance: You pour the acid into the pool, but not the pool into the acid.
Another rule is: Never combine chlorine bleach with ammonia (such as mixing bleach with window cleaners or drain cleaner). The combination can form poisonous gases.
These two rules are perhaps the most important, but they are by no means sufficient. Basically, just don’t mix any two chemicals without specific instructions to do so, even if you are merely disposing of them. Proper disposal procedures are important. Consider something as apparently innocuous as pouring a single cleaning product down a drain: Someone else may in quick succession unwittingly pour a different product down the same or a nearby drain with unfortunate consequences.
Prevention is the best policy: Don’t smoke except in designated areas, avoid using matches, turn off torches when not in use, maintain equipment, use beam blocks for powerful lasers, and don’t overload circuitry.
Preparation is essential: Provide well-marked, well-maintained, sufficient and accessible fire extinguishers. Comply with sprinkler codes. Alarms – both automatic and manually activated – should be audible (and visible) in every area. Exit paths should be unobstructed, direct and marked for visibility even in the event of power failure. Practice drills should be scheduled, and their effectiveness must be reviewed.
Response includes education, notification of authorities, and accounting for all personnel. Education should include practice with extinguishers and an understanding of their dual purposes – to extinguish small fires before they become large, and to facilitate egress in an emergency. No one should ever attempt to stay behind to fight a growing fire. A small fire should be extinguished from its base upward.
Coordination with authorities must be thorough and open: Zinc selenide, for example, evolves highly toxic hydrogen selenide gas when heated to 500 °C, which can present a hazard for downwind neighbors. You should not expect the local fire department to know that you have many kilograms in inventory unless you tell them.
Thermal shocks to optics and equipment can cause breakage with unexpected collateral damage. Extreme temperature differentials caused by localized heating or cooling can stress pressure vessels, vacuum windows, glassware and optical components to fracture. Burst pressure vessels or vacuum windows can result in fatalities.
In addition, both extreme heat and extreme cold can damage tissue.
Solids, liquids and gases can all cause burns. Propane torches are frequently used for spot heating; standard safety precautions apply.
Hot plates are used to heat glass and metal tooling to about the temperature of hot coffee for blocking. But they can get much hotter, usually by mistake. The surface of the hot plate and anything that’s been on it can cause burns. Unfortunately, glass and metal don’t look any different when they’re hot – they don’t boil or smoke or glow.
Wax and pitch are melted to make them flow, for blocking and to make polishing laps. Unlike hot water, hot pitch or wax can’t be shaken off – they’ll stick and continue to burn, and then cannot be easily removed from the burn site. Hot pitch and cold pitch look the same – and unlike hot metal and glass, hot pitch and wax can splash. Pitch can get as hot as frying oil; treat it with great respect. Never poke a finger at pitch to see if it’s liquid – if it is, it’ll burn and it won’t let go.
An inexpensive pistol-grip spot IR thermometer is very helpful (I use one daily in our home kitchen to check whether cooking oil is ready). If one is not available, hold your palm or wrist close to anything you suspect of being hot as a quick check. Use oven mitts or other protection when picking up hot metal tools.
While ice cubes (about -10 °C) are merely uncomfortable to handle, dry ice (solid CO2 at less than -80 °C) and liquid nitrogen (LN2 at about -200 °C) can rapidly cause tissue damage. In addition to their thermal issues, their vapors displace oxygen, causing loss of consciousness. Metals cooled to these temperatures, due to their higher thermal conductivity and absence of phase change, chill skin far more aggressively – and moisture may make them stick to skin.
Biological hazards are an unfortunate fact of life. In this era of AIDS, hepatitis and other fluid-borne pathogens, you must follow a protocol when dealing with accidents and injuries. Blood, vomit and other bodily products should be treated as hazardous waste. If you are somehow brought into direct contact with these substances, contact the safety officer for further directions, which may include off-site testing.
Signs of chemical toxicity
If you suffer any of these symptoms, or notice them in co-workers, get attention right away because they can be symptoms of an adverse chemical effect:
• Difficulty in breathing.
• Irritation of the eyes, skin, throat or respiratory tract.
• Changes in skin color.
• Headache or blurred vision.
• Clumsiness or lack of coordination.
• Cramps or diarrhea.
- optical glass
- A glass that, during manufacture, is carefully controlled with respect to composition, melting, heat treatment, and other processing, to render its optical characteristics, such as its index of refraction, dispersion, transmittance, spectral transmittance, homogeneity, freedom from birefringence, permanence, etc., sufficient for its optical application.
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