Hydrogen Sulfide Toxicity

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Hydrogen sulfide gas (H2S) inhaled at toxic levels is dangerous and fatal to both pigs and people. The danger of high concentrations (greater than 100 ppm) of H2S should be recognized, respected, and avoided but the usual, low level of the gas in closed confinement facilities, (less than 0.2 ppm) is not toxic and of little consequence. A “sewer gas odor/rotten egg smell” detectable by humans from 0.1 to 5 ppm, is sometimes offensive but is not toxic. Levels from 10 ppm to 100 ppm can cause eye and respiratory irritation. Humans cannot detect the odor of H2S at levels greater than 150-200 ppm because of olfactory paralysis induced by the gas. Levels greater than 200 ppm affect the nervous system; immediate collapse and respiratory paralysis occurs at levels greater than 1000 ppm.

Hydrogen sulfide is heavier than air and accumulates in liquid manure holding pits below confinement buildings. The gas usually remains dissolved in the liquid component of swine effluent and remains below the toxic level in air unless the effluent is agitated. Effluent storage pits are often agitated just prior to and during the emptying process at which time high levels of H2S can be released. If inhaled in high concentration, H2S can cause instant fatal systemic intoxication of exposed swine or people by directly suppressing the respiratory center in the brain. High levels of the gas paralyze a worker’s sense of smell and may give a false sense of security. Workers trying to rescue affected swine or co-workers are at a very high risk of asphyxiation.

Exposure to toxic levels of H2S can be avoided by emptying and cleaning the pit when the building is empty or when pigs and people have been moved out of the building temporarily. Adequate ventilation should be provided (fans on, curtains down) whenever pits are agitated to keep H2S at a nontoxic level.