By studying existing research on the human health impacts of hydrogen sulfide (H2S) from paper mills, refineries, geothermal sources and meat packing plants, researchers hope to learn more about the health impacts of swine odors.

“We are going to try to compare the numbers we have measured to what is known, medically, about certain levels of hydrogen sulfide,” says Susan Schiffman, psychologist and director of the Taste and Smell Lab at Duke University Medical Center, Durham, NC.

The ambient level of hydrogen sulfide in a hog building ranges from 1 to 5 parts per million (ppm), according to Schiffman's research and data from other universities. At the property line, measurements average between 20 and 50 parts per billion (ppb).

“There are six highly responsible community studies that found that chronic exposure to low levels of hydrogen sulfide can cause health impacts,” she explains.

Two of those studies, one studying health symptoms in Finnish communities near paper mills and another studying brain function of residents near a refinery, sulfur plant and cooking units, show health effects from average daily exposure of 10 to 11 ppb of H2S, she says.

The health impacts generally included headache and respiratory problems, Schiffman says. The brain function study found neuro-physicological abnormalities such as slower reaction times, color discrimination and mood alterations. See Table 1 for detection, health impacts and regulation of increasing concentrations of H2S.

Table 1. Effect of H2S at Increasing Concentrations
H2S Concentration Reported Effects
0.02-0.05 ppb Concentration of H2S in undeveloped areas.
0.5 ppb Detectable by 2% of population; U.S. EPA finds no health effects at this level.
2 ppb Detectable by 14% of population; 2% of population is annoyed by odor.
4 ppb Detectable by 30% of population, 5% annoyed.
5 ppb World Health Organization recommends exposure to this level not exceed 30 minutes.
10 ppb Detected by 56% of population; 17% annoyed; when 10 ppb is daily average, more reported eye irritation and coughs.
30 ppb Detected by 85% of population; 40% annoyed; 30 ppb averaged over one hour exceeds California Ambient Air Quality Standard.
0.32 ppm over 1 hour Nausea, diarrhea, sleep disturbance, shortness of breath, respiratory irritation and headache.
2-5 ppm Coughing and throat irritation after 15 min. of exposure.
20 ppm Occupational Safety and Health Administration ceiling.


“In my opinion, hydrogen sulfide is one of the main problems from animal facilities,” she says. “The main conclusion we (she and fellow researchers) came to about hydrogen sulfide is that the levels of hydrogen sulfide in animal facilities exceed those in other settings, and can cause health problems from chronic exposure.”

The most common health complaints by hog unit neighbors are eye, nose and throat irritation, headache, sinus problems, nausea, hoarseness or cough, nasal congestion, shortness of breath, stress and drowsiness.

Schiffman and her colleagues offer three conclusions about why some neighbors to hog units feel swine odors affect their health:

  1. That symptoms are caused by irritant properties of the compounds or gases, rather than the odor itself;

  2. That the odor is part of a mixture with a co-pollutant, such as bacteria, that causes the health impacts; or

  3. That there may be non-toxicological symptoms — where there is no medical reason for symptoms.



Some responses may be learned effects, where people have learned to have a negative physical reaction to odor, based on previous experience. Or, negative health impacts to odor may be caused by a primitive biological drive in human beings.

“Studies show that when you smell something like hydrogen sulfide, it illuminates one part of the brain. When you smell something pleasant, like Christmas cookies in the oven, it lights up another part of the brain,” she explains. “This may be a binary issue, which is likely primitive, where the brain is telling us to get away from bad things.”

More research is needed to address how H2S, ammonia, dust and the more than 400 volatile organic compounds found in manure all impact swine odors. Plus, more study is needed to find out what role environmental factors, for example, pollen, play in the effects of odors on human health, Schiffman says.