Toxicities Caused by Gases

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Five gases (ammonia, carbon dioxide, carbon monoxide, hydrogen sulfide, and methane) are associated with swine environments, especially confinement rearing.

Ammonia (NH3) and hydrogen sulfide (H2S) are normally generated from the decomposition of swine excrement. Carbon dioxide (CO2) can cause deaths but is perhaps a lesser threat than carbon monoxide (CO) because it is heavier than air and usually only accumulates in slurry pits below pig housing and working areas. Methane (CH4) is an explosion hazard. All five gases represent potential threats to both swine and people but ammonia, carbon monoxide and hydrogen sulfide toxicities are by far most common and are discussed below.

Ammonia Toxicity

Ammonia gas (NH3) is formed by the decomposition of animal waste and is present at some level in most animal facilities. The odor can be detected by most people at concentrations of around 10 ppm; a level which appears to have little detrimental effect on pig health. However, when the concentration reaches 50 ppm or more, NH3 may act as an irritant of the mucous membranes of the eyes, nasal passages, and lungs and cause ocular and nasal discharge. Concentrations of NH3 that are high enough to be irritating to mucous membranes of pig farm workers likely have a similar effect on animals continuously exposed.

Research on swine suggests that toxic concentrations of NH3 (over 50-100 ppm) reduce growth rate, reduce bacterial clearance from the lungs (interferes with mucociliary apparatus), exacerbate nasal turbinate lesions in pigs infected with Bordetella bronchiseptica and may influence the course of infectious diseases.

Carbon Monoxide Toxicity

Carbon monoxide (CO) has a much greater affinity for hemoglobin than does oxygen, effectively displacing oxygen from the blood. It combines with hemoglobin to form carboxyhemoglobin, reduces oxygen exchange, and causes mortality. Carbon monoxide is produced by incomplete combustion of any carbonaceous fuel, but most poisonings occur because of improperly functioning (yellow versus blue flame) space heaters or furnaces. Carbon monoxide is a colorless, odorless, and tasteless gas.

Recognition of malfunctioning heaters should alert one to the possibility of poisoning.

In late pregnancy sows, a high level of stillbirths and neonatal mortality is associated with CO poisoning. The sows themselves may show no other signs. Affected piglets usually show no lesions other than a pink to bright red color imparted to their blood and tissues by carboxyhemoglobin. Blood from affected piglets and fetuses with CO poisoning is usually characterized as “cherry red.” The concentration of carboxyhemoglobin in fetal thoracic fluid or blood of an affected pig can be used to confirm CO poisoning.

Hydrogen Sulfide Toxicity

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.