Combustible Dust Atmospheres
Finely powdered dust from combustible materials such as wood, metal, or grain can be fuel for powerful explosions.
Combustible Dust
Dust clouds can develop as result of handling dusty materials or when solid materials are reduced to smaller particles from processes such as grinding, drilling, or crushing.
Airborne combustible dust at an explosive concentration would obscure vision at a distance of five feet (1.52 meters) or less. A direct reading instrument may be used to measure actual dust concentrations.
Important exposure guidelines include:
- No universal OSHA PEL: OSHA does not establish a single airborne exposure limit for combustible dust.
- Respirable nuisance dust: OSHA's PEL is 5 mg/m³ as an 8-hour time-weighted average (TWA) for the respirable fraction of particles not otherwise regulated.
- Total nuisance dust: OSHA's PEL is 15 mg/m³ as an 8-hour TWA for total dust particles not otherwise regulated.
- Material-specific dusts: Some combustible dusts, such as crystalline silica, coal dust, or certain metal dusts, have much lower OSHA exposure limits because of their toxic health effects.
From a fire and explosion standpoint, the most important concern is keeping dust concentrations well below the level at which an explosion can occur. Many combustible dusts can explode when suspended in air at concentrations of approximately 20 to 60 grams per cubic meter (g/m³), although the exact value varies by material. This concentration is known as the Minimum Explosible Concentration (MEC).
Toxic Atmospheres
Substances regarded as toxic in a confined space can cover the entire spectrum of gases, vapors, and finely-divided airborne dust in an industry. Toxic gases may be present in a confined space because:
- toxic substances are used as part of the production process, (for example, in producing polyvinyl chloride, hydrogen chloride is used as will as vinyl chloride monomer, which is carcinogenic).
- the biological and chemical "breakdown" of the product being stored in a tank, or
- maintenance activities (welding) being performed in the confined space.
Four common types of toxic gases encountered in confined spaces are:
- Hydrogen Sulfide (H2S): Also called "sewer gas," H2S is a colorless gas with the odor of rotten eggs. Excessive exposure has been linked to many confined space deaths. Hydrogen sulfide causes a loss of our sense of smell, causing people to mistakenly think that the gas has left the space. Hydrogen sulfide inhibits the exchange of oxygen on the cellular level
and causes asphyxiation.
- 10 ppm: OSHA's permissible exposure limit (ceiling) for most workplaces. Exposure above this level is not permitted except under limited conditions.
- 20 ppm: OSHA's maximum peak concentration allowed for a short duration under specific circumstances.
- 100 ppm: The sense of smell may be lost within minutes, making the gas impossible to detect by odor alone.
- 500-700 ppm: Can cause collapse within minutes and may be fatal.
- 1,000 ppm or higher: Can cause immediate collapse, respiratory paralysis, and death after only one or two breaths.
- Carbon Monoxide (CO): Carbon monoxide is a colorless, odorless gas produced when carbon-based fuels, such as gasoline, wood, natural gas, propane, diesel fuel, or charcoal, burn. It reduces the blood's ability to carry oxygen to the body's tissues and organs.
- 35 ppm: OSHA's permissible exposure limit as an 8-hour time-weighted average.
- 100 ppm: Can cause headache, fatigue, and dizziness after prolonged exposure.
- 400 ppm: May cause severe headache within a few hours and can become life-threatening with continued exposure.
- 800 ppm: May cause unconsciousness within about two hours and can be fatal.
- 1,600 ppm or higher: Can cause collapse and death in less than an hour.
Carbon Monoxide
Carbon monoxide enters the bloodstream through the lungs and attaches to hemoglobin in red blood cells much more readily than oxygen. This reduces the blood's ability to carry oxygen to the brain, heart, and other vital organs. As a result, the body's tissues can quickly become oxygen-starved, even when the surrounding air contains normal oxygen levels.
- Methane (CH4): Methane is a colorless, odorless, flammable gas produced by the natural decay of organic matter. It can accumulate in confined spaces and create two serious hazards. First, methane can form an explosive atmosphere if an ignition source is present. Second, it can displace oxygen, creating an oxygen-deficient atmosphere that can cause dizziness, unconsciousness, and asphyxiation.
- Oxygen below 19.5%: Methane may displace enough oxygen to create an oxygen-deficient atmosphere.
- 5% methane in air: This is the Lower Explosive Limit (LEL). At this concentration, methane can ignite if an ignition source is present.
- 5% to 15% methane: This is the flammable range where an explosion can occur.
- 10% of the LEL (0.5% methane): Many employers require workers to stop work or investigate before concentrations reach hazardous levels.
- Solvents: Many solvents, including kerosene, gasoline, paint strippers, and degreasers, are not only flammable but can also affect the central nervous system (CNS) when inhaled at high concentrations. CNS effects may include dizziness, drowsiness, difficulty concentrating, confusion, headaches, loss of consciousness, coma, and death.
- Vapor concentrations approaching 10% of the Lower Explosive Limit (LEL): Many employers require work to stop until the hazard is controlled.
- Exposure above the OSHA permissible exposure limit (PEL) or the ACGIH Threshold Limit Value (TLV): Can increase the risk of harmful health effects.
- High vapor concentrations: Can quickly cause dizziness, confusion, unconsciousness, and death, especially in poorly ventilated confined spaces.
Remember, atmospheric changes may occur due to the work procedure, the product stored, or a nearby gas line leak. The atmosphere may be safe upon entry, but can change very quickly.
- The work performed within the confined space (such as welding, degreasing, painting, or sanding) may produce toxic atmospheres.
- Toxic gases and vapors from adjacent areas can migrate to and collect in the confined space.
- Vapors may be released from the sludges on the bottom or scales on walls of emptied confined spaces, such as storage tanks, that previously contained flammable or toxic chemicals. Vapor release may be accelerated by wall scraping and sludge removal from confined spaces.
Knowledge Check Choose the best answer for the question.
4-6. Which of the following gases inhibits the body's ability to transport oxygen to all parts of the body.
You forgot to answer the question!