G3G5 Effects of animal waste on the air resource

Livestock production facilities can be the source of gases, aerosols, vapors, and dust that, individually or in combination, can create such air quality problems as:

• health problems for animals in confined housing units,

• corrosion of materials; and

• the generation of deadly gases that can affect animals and humans.

Different gases are produced as animal waste is degraded by micro-organisms. Under aerobic conditions, carbon dioxide is the principal gas produced. Under anaerobic conditions, the primary gases are methane and carbon dioxide. About 60 to 70 percent of the gas generated in an anaerobic lagoon is methane, and about 30 percent is carbon dioxide. However, trace amounts of more than 40 other compounds have been identified in the air exposed to degrading animal waste. Some of these include mercaptans (this family of compounds includes the odor generated by skunks), aromatics, sulfides, and various esters, car-bonyls, and amines.

The gases of most interest and concern in manure management are methane (CH4), carbon dioxide (CO2), ammonia (NH3), and hydrogen sulfide (H2S). Table 3-9 provides a summary of the most significant characteristics of ammonia, carbon dioxide, hydrogen sulfide, and methane.

Methane is flammable, and in recent years interest in using it as a source of energy on the farm has increased. Because methane is also explosive, extreme care is required when attempting to generate and capture this gas for onfarm use.

Carbon dioxide can be an asphyxiant when it displaces normal air in a confined facility. Because CO2 is heavier than air, it remains in a tank or other well-sealed structure, gradually displacing the lighter gases.

Ammonia is primarily an irritant and has been known to create health problems in animals in confinement buildings. Irritation of the eyes and respiratory tract are common problems from prolonged exposure to this gas. It is also associated with soil acidification processes. (See chapter 2.)

Table 3-9 Properties and physiological effects of the most important gases produced from animal wastes in an anaerobic environment


Lighter than air






Sharp, pungent


Irritation of eyes and throat at low concentrations. Asphyxiating, could be fatal at high concentrations with 30- to 40-minute exposure.

Carbon dioxide




<20,000 ppm=safe level; increased breathing, drowsiness, and headaches as concentration increases; could be fatal at 300,000 ppm for 30 minutes.

Hydrogen sulfide


Rotten eggs


Headaches, dizziness at 200 ppm for 60 minutes. Nausea, excitement, insomnia at 500 ppm for 30 minutes; unconsciousness, death at 1,000 ppm.




Asphyxiant, flammable

Headaches at 500,000 ppm.

Hydrogen sulfide is deadly. Humans and farm animals have been killed by this gas after falling into or entering a manure tank or being in a building in which a manure tank was being agitated. Although only small amounts of hydrogen sulfide are produced in a manure tank compared to the other major gases, this gas is heavier than air and becomes more concentrated in the tank over time.

When tanks are agitated in preparation for pump out, hydrogen sulfide can be released to the area overhead. Where a tank is located beneath the animals in a building, forced-air ventilation in the building is imperative before operating the agitation equipment. An exhaust system should also be provided within the tank during agitation and pump out.

Hydrogen sulfide has the distinct odor of rotten eggs. At the first hint of this odor, the area around the tank should be immediately evacuated of all humans. H2S deadens the olfactory nerves (the sense of smell); therefore, if the smell of rotten eggs appears to have disappeared, this does not indicate that the area is not still contaminated with this highly poisonous gas.

A person should never enter a manure storage tank even to help rescue someone else who has succumbed to the hydrogen sulfide. Several lives have been lost attempting such rescues. If a tank must be entered, the air in the tank should first be evacuated using a forced-air ventilation system. Self-contained breathing apparatus, safety lines, and sufficient personnel to man the lines are needed in all cases. A mechanical hoisting device would be preferable.

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