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667 HAZWOPER for General Site Workers VIII
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Common Toxic Gases

Four common types of toxic gases encountered in confined spaces are:

  • Hydrogen Sulfide (H2S) - or "sewer gas," is a colorless gas with the odor of rotten eggs, but because H2S causes a loss of our sense of smell, people may think that the gas has dissipated. H2S causes asphyxiation by inhibiting the exchange of oxygen on the cellular level.
Carbon Monoxide Educational Outreach
  • Carbon monoxide (CO) - is an odorless, colorless gas that is formed by burning carbon based fuels (gas, wood). Carbon monoxide inhibits oxygen transport by displacing oxygen in blood vessels.
  • Methane (CH4) - is a flammable, explosive, colorless, and odorless gas produced by the decay of organic matter. It can displace oxygen in the confined space and cause dizziness, unconsciousness, and asphyxiation.
  • Solvents - such as kerosene, gasoline, paint strippers, degreasers, etc. are flammable, and high concentrations can cause central nervous system (CNS) effects including dizziness, drowsiness, lack of concentration, confusion, headaches, coma, and death. (Source: University of South Carolina)

The atmosphere inside a confined space can change rapidly and unexpectedly. Any ignition source (such as sparks from grinding or welding equipment, static electricity, etc.,) can initiate an explosion. Be sure to only use non-sparking electrical tools in confined spaces.

Knowledge Check Choose the best answer for the question.

2-7. Which is a flammable, explosive, colorless, and odorless gas produced by the decay of organic matter?