Animal Confinement Gases and Other Gases

Animal confinement areas and larger confinement animal facility operations (CAFOs) consist of indoor areas that confine and feed animals and do not grow or store grain. Animals are typically gathered in large numbers to maximize efficiency of space and labor. This practice first became widespread in poultry farms but eventually has been used in other animal confinement areas, such as for raising swine, sheep, and young beef cattle. Animals typically receive all required care in the confinement areas, including feeding, washing, and veterinary services, and may spend their entire lives in these areas. The density of animals in these areas can vary, but generally they are very crowded. A concentrated animal feeding operation can house over 1000 animals (1).

Toxic inhalation exposures in animal confinement facilities are possible with exposures to gases produced from the manure pit. High levels of gases are generated as a by-product of animal waste, especially in high-density confinement facilities such as with swine. The major gases include ammonia, hydrogen sulfide (H2S), carbon dioxide, and methane, which are all produced in manure pits (14,15).


Ammonia is highly water-soluble and associated with upper airway irritation producing immediate symptoms of burning of eyes, nose, and throat, accompanied by coughing. The odor of ammonia is detectable at 3 to 5 ppm and respiratory irritation at 50 ppm. Sinusitis, mucous membrane inflammation syndrome, with massive inhalation exposures, and noncardiogenic pulmonary edema can result from exposure. Tolerance to ammonia can occur over time, leading to less irritant symptomatology with greater exposure. Possible effects of long-term exposure (i.e., greater than 2 hours per day for up to 6 years) include sinusitis, mucous membrane inflammation syndrome, chronic bronchitis, and asthma-like syndrome (7,16,17).

Anhydrous Ammonia

Anhydrous ammonia (NH3) is also common in agriculture and is stored as a liquid and then injected into soil to add nitrogen as fertilizer. It is a highly irritating gas that is very water-soluble. Exposures have resulted in severe burns, laryngeal edema, as well as pulmonary effects including bronchiolitis obliterans and reactive airways dysfunction syndrome (18,19).

Hydrogen Sulfide

Hydrogen sulfide (H2S) gas is produced from sulfur-containing compounds in manure contained within an anaerobic environment. It is a respiratory irritant at low concentrations and a chemical asphyxiant at high concentration. A concentration of 20 ppm produces mucous membrane irritation; levels of 100 ppm can cause lung injury and bronchiolitis. Higher concentrations may cause asphyxia via inhibition of cytochrome oxidase, similar to the effects of cyanide. Levels of 250 ppm can cause pulmonary edema, and unconsciousness and death can occur at 500 ppm. However, at levels of 150 ppm or higher, olfactory fatigue and paralysis occur. Exposed persons are not able to detect the presence of the gas, leading to fatal exposures. Agitation of manure during emptying of manure pits can generate concentrations of H2S as high as 1000 ppm into breathing zones of humans and animals. Open-storage manure pits and lagoons are less dangerous than deep pits that are enclosed. Accidental death due to H2S asphyxiation or cardiogenic pulmonary edema, although rare, can occur with exposures in swine or dairy confinement buildings with under-building manure pits (7,20,21).

Treatment of acute H2S exposures is with nitrites, which facilitates removal of sulfides by inducing methemoglobinemia. However, nitrates may not be helpful after the acute injury period. Complete recovery may occur after exposure to H2S although some have suggested the possibility of residual central nervous system toxicity (14,22,23).

Carbon Monoxide

Carbon monoxide (CO) is generated from the operation of gas-powered equipment such as kerosene heaters in insufficiently ventilated buildings. As the gas is invisible and odorless, toxic levels may develop in as little time as 3 to 5 minutes, resulting in poisoning. Higher-level exposures can result in coma, cardiac toxic-ity, respiratory arrest, and long-term neurologic sequelae and death (20,24).

Carbon Dioxide and Methane

Carbon dioxide (CO2) and methane (NH4), also generated from animal wastes, are simple asphyxiants. Unlike H2S, they are generally not primary causes of adverse health effects. However, methane and carbon dioxide are hazardous when they displace enough oxygen to cause asphyxiation. At levels above 5%, methane can be a potential explosive hazard. CO2 is also produced by animal respiration. Co2 levels serve as an indicator of ventilation with acceptable levels typically below 5000 ppm (7,17).


Fumigants are chemicals used to eliminate pests and are applied to crops, grain, or grain storage facilities. Since they are volatile, rapid dissipation occurs and little or no trace is left on the crops or grains. Methyl bromide and phosphine are two common fumigants. Methyl bromide is very toxic and may cause pulmonary edema and hemorrhage after acute exposures. Phosphine is produced from aluminum phosphide pellets that are added to grain and is very reactive, unstable, and toxic. Some have suggested that fumigant exposures may lead to chronic lung disease (14,25,26).

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