Medical Evaluation and Risk Assessment

December 7, 1998

The Association of Health Effects
with Exposure to Odors from Hog Farm Operations

North Carolina Department of Health and Human Services
State Health Director
A. Dennis McBride, MD, MPH


The purpose of this paper is to address the public health risks posed by exposure to odors from hog farm operations.


The number of hogs in North Carolina has increased from around two million in 1989 to greater than 11 million in 1998. These hogs are grown in industrial style facilities that have confinement buildings with each holding several hundred hogs. These facilities include multi-acre lagoons for storage and anaerobic digestion of manure and spray fields for application of the liquid from the lagoons. The confinement buildings, lagoons, and spray irrigation are sources of odors.

As the number of hogs produced has increased greatly and as the human population has increased steadily in North Carolina, more people are living near hog farms. Health and environmental agencies have received numerous complaints from individuals living near hog operations. Their primary complaints are annoyance with the odors and concerns about health risks from exposure to the odors. Many North Carolina citizens are upset that they are exposed to odors and do not have any control over their environment. In many cases, citizens have received very little communication from hog farm owners and from state government agencies about the odors. The authors of "Control of Odor Emissions from Animal Operations" report:

". . . the attitudes and beliefs of some residents in southeastern North Carolina counties were probed through five focus groups held during June of 1998. Results showed that while some odor emission from animal operations is recognized as an inevitable part of rural life, the current situation is unacceptable, has caused controversies that divided communities and should be addressed. All groups agreed that state government should enforce current regulations and that there is a need to implement more technology to better control odor emissions."

A review of the current scientific literature on the association of health effects and exposure to odors from hog operations is presented. There are several articles available addressing odors in general and a few articles addressing health effects reported by people living near hog farms. The paper will cover briefly the vast amount of data on the health risks to people working in and around hog confinement buildings.

Health Effects from Exposure to Odors - General

Exposure to environmental odors results in physiological stresses that may result in a variety of symptoms including headache, nausea, loss of appetite, and emotional disturbance. Odors may exacerbate stress-related illnesses. The symptoms may result from odor annoyance, stress associated with odor exposure, and conditioned responses to odors. The literature also reports that exposure to odors may exacerbate asthma symptoms. The following are excerpts from articles that address human response to environmental odors.

N. P. Shukla (1991) "In the case of humans, the immediate physiological stresses produced by odours can cause loss of appetite and food rejection, low water consumption, poor respiration, nausea, and even vomiting, and mental perturbations. In extreme cases, offensive odours can lead to deterioration of personal and community well-being, interfere with human relations, deter population growth and lower its socio-economic status."
Dennis Shusterman (1992) "Environmental odor pollution problems generate a significant fraction of the publicly-initiated complaints received by air pollution control districts. Such complaints can trigger a variety of enforcement activities under existing state and local statutes. However, because of the transient timing of exposures, odor sources often elude successful abatement. Furthermore, because of the predominantly subjective nature of associated health complaints, air pollution control authorities may predicate their enforcement activities upon a judgement of the public health impact of the odor source. Noxious environmental odors may trigger symptoms by a variety of physiologic mechanisms, including exacerbation of underlying medical conditions, innate odor aversions, aversive conditioning phenomena, stress-induced illness, and possible pheromonal reactions. Whereas relatively consistent patterns of subjective symptoms have been reported among individuals who live near environmental odor sources, documentation of objective correlates to such symptoms would require as yet unproven research tools. Therefore, given our current state of knowledge, any differential regulatory response to environmental odor pollution, which is based upon the distinction between community 'annoyance reactions' and 'health effects', is a matter of legal not scientific interpretation."
Shim and Williams (1991) "Many patients complain that some odors worsen their asthma. Perfume and cologne are two of the most frequently mentioned offenders. Four patients with a history of worsening asthma on exposure to cologne underwent challenge with cologne, and their pulmonary function was tested before, during, and after the exposure. Forced expiratory volume in one-second declined 18 to 58 percent below the baseline period during the 10-minute exposure and gradually increased in the next 20 minutes. Saline placebo pretreatment did not affect the response to subsequent challenge. Single pretreatment with metaproteronol and atropine prevented decline in one-second forced expiratory volume in three of four patients and blunted the response in the other. Cromolyn sodium prevented decline in one of four, and occlusion of nostrils prevented decline in one of three. A survey of 60 asthmatic patients revealed a history of respiratory symptoms in 57 on exposure to one or more common odors. Odors are an important cause of worsening of asthma. From a practical standpoint, sensitive asthmatic patients should be advised to eliminate odors from their environment as much as possible."
Susan Knasko (1993) "The effects of intermittent bursts of pleasant, unpleasant, and no experimental odor on human task performance, mood, and perceived health weretested in this study. Odors did not influence any of these measures; however, subjects who had been exposed to the malodors reported retrospectively that they thought the odors had a negative effect on all of these factors."
Pierre Caralini (1994) "With regard to general health complaints, it was found that when exposed to odorant concentrations, some people are annoyed and of these people, only some report general health complaints. Exposure in itself does not directly cause general health complaints. Annoyance is the intervening variable between exposure and general health complaints. A possible explanation for the relation between annoyance by malodor and general health complaints might be found in the personality and attitudes of the exposed individual. Finally, we found confirmation for the appraisal hypothesis, i.e., the extent to which individuals regard malodor as threatening is positively related to odor annoyance."
Shusterman, et. al. (1991) "Retrospective symptom prevalence data, collected from over 2000 adult respondents living near three different hazardous waste sites, were analyzed with respect to both self-reported 'environmental worry' and frequency of perceiving environmental (particularly petrochemical) odors. Significant positive relationships were observed between the prevalence of several symptoms (headache, nausea, eye, and throat irritation) and both frequency of odor perception and degree of worry. Headaches for example, showed a prevalence odds ratio of 5.0 comparing respondents who reported noticing no such odors and 10.8 comparing those who described themselves as 'very worried' versus 'not worried' about environmental conditions in their neighborhood.

. . . Potential explanations for these observations are presented, including the possibility that odors serve as a sensory cue for the manifestation of stress-related illness (or heightened awareness of the underlying symptoms) among individuals concerned about the quality of their neighborhood environment."

Sources of Air Contaminants and Odors in Hog Confinement Buildings

A University of Iowa researcher, Kelly Donham, and his co-workers have extensively studied air pollutants and odor sources, their components, and health impacts on people working in hog confinement buildings. Numerous sources of air contaminants in indoor confinement facilities have been identified. The hogs themselves shed various proteins from their saliva, skin, urine and feces. Feed particles, pollens, molds, bacteria, fungi, endotoxins, other various microbial proteins and grain mites and other insect body parts can be detected in confinement houses. (Donham, 1993). Many of these particles are of respirable size (<10um) and may be inhaled into the small airways and alveoli of the lungs (Hill, Kenworthy, 1970). Dust particles may absorb toxic irritating gases such as ammonia, as well as bacteria, endotoxins and other proteins and transport these to the alveoli and small bronchioles. A literature review by O'Neill and Phillips (1992) on odorous chemicals in livestock waste and the air around them identified 168 different compounds. The most commonly reported compounds in the literature review were volatile fatty acids (acetic, propionic, butanoic and pentanoic), phenol, p-cresol and ammonia. Clayton and Clayton, (1993) have identified some of these compounds as respiratory tract, skin or eye irritants. These biological and chemical compounds and dust particles comprise the potential agents of exposure to humans working in hog operations and possibly living adjacently to hog operations.

Health Effects of Exposure to Air Contaminants and Odors Generated by Hog Confinement Facilities

Many studies have reported the health effects experienced by people working in hog confinement buildings. Donham et. al. (1989) reported the following acute symptoms and prevalence rates in a study of hog confinement workers:

  • cough (67%)
  • phlegm production (56%)
  • scratchy throat (54%)
  • runny nose (45%)
  • burning and watery eyes (39%)
  • headaches (37%)
  • chest tightness (36%)
  • shortness of breath (30%)
  • wheezing (27%)
  • muscle aches and pain (25%)

Schwartz et. al. (1990) reported that chronic effects are manifested as bronchitis, where airway obstruction was present affecting up to 25% of hog house workers. Long term lung damage may occur as pulmonary function tests indicate air trapping in lungs and a persistent leukocytosis.

A carefully designed five year prospective study conducted by Donham et. al. (1990) compared respiratory symptoms in hog confinement workers with sex-race-age-matched comparison groups of non-confinement hog farmers and blue collar (postal) workers. As a consequence of this design, the association between prevalence of chronic respiratory symptoms and the degree of exposure to airborne substances associated with hog production (confinement operations vs. non-confinement operation vs. blue collar) could be examined. The results showed an exposure-related response with chronic cough reported by nearly 20% of the confinement workers, 14% of the non-confinement workers, and 8% of the blue collar workers. Chronic phlegm production was reported by nearly 25% of the confinement workers, 11% of the non-confinement workers, and 6% of the blue collar workers. There is a statistically significant increase in the prevalence of two chronic symptoms in the confinement workers over that of the postal workers (p<0.001). In addition, for work-related symptoms, there was also a very high prevalence of acute respiratory symptoms in hog operation workers working outside of hog houses.

The fact that some of the non-confinement workers experienced the same symptoms as some of the confinement workers indicates that exposure to substances associated with hog production may cause symptoms in open-air situations. This raises the question as to whether gases and/or particles from swine operations occur in sufficient concentrations to induce similar respiratory symptoms in neighboring residents. The fact that plumes of odor from large hog operations travel for several miles indicates that neighbors are being exposed to some unknown extent to odor causing substances from hog facilities.

Human Studies - Neighboring Residents

Little data are available about residential exposure to air contaminants generated by hog confinement facilities. While there are qualitative data describing the nature of the contaminants, work to determine quantitative levels of exposure to these agents is only in a rudimentary stage and will require many resources and much time as environmental conditions differ from site to site and within a site over time. Temperature, precipitation, wind direction and speed, and varying terrain are just a few environmental conditions contributing to the difficulties of assessment of exposure to environmental odors.

A recent study in Iowa by Thu et. al. (1997) collected mental and physical health information by personal interviews from a random sample of 18 residents living within two miles of a 4,000 sow operation. These data were compared to those collected from a demographically comparable sample of 18 rural residents living in an area with minimal livestock production. The results of the comparison indicated that "neighbors of the large-scale swine operation reported experiencing significantly higher rates than the controls of four clusters of symptoms that are known to represent toxic or inflammatory effects on the respiratory tract. These clusters of symptoms have been well documented among swine confinement workers."(Thu et al, 1997) The specific symptoms reported are listed below and are quite similar to the list of symptoms reported by hog farm workers: (Thu et. al., 1997).

  • cough
  • increased sputum production
  • shortness of breath
  • chest tightness
  • wheezing
  • nausea
  • dizziness
  • headaches
  • runny nose
  • scratchy throat
  • burning eyes
  • muscle aches and pains
  • skin rash
  • fever

However, among the control group, symptoms of skin rash, muscle aches, and fever were more frequently reported. Additionally, there was no difference in the frequency of reported symptoms and distance from the swine facility as one might suspect. This study found that neighbors did not suffer higher rates of psychological health problems such as depression or anxiety when compared to controls. Thu et. al. also state that all responders felt the owner of the farm was creating social and class divisions within that community.

Another study was conducted in Michigan (Warner et. al., 1990). It was designed to assess the impact of a 50,000 animal population swine-growing facility as an odor source and potential health problem. In parallel with the measurement of odor intensity, the Michigan Department of Public Health conducted a health survey to obtain information regarding the pervasiveness of the odor in the community and its possible health implications. Citizens' complaints reported included physical symptoms such as breathing difficulties, burning sensations in the nose and throat, nausea and vomiting, and headaches. A survey of residents within 0.5 miles of the center of the facility (58 households/89 persons) and those between 0.5 and 1.25 miles away (176 households/225 persons) resulted in response rates of 55% and 49%, respectively. The authors concluded the following: "These responses contained complaints of symptoms attributable to the swine facility. As with any population, symptoms as general as those which relate to complaints as noted are difficult to correlate to specific health problems. However, the clear excess of complaints stands as a fact of record. Perhaps further study is needed to surface a better understanding of individual health effects and symptoms as these relate to perception of odor." (Warner et al, 1990). However no information is provided on the type of complaints nor was there any report on frequency of symptoms or whether they differed by proximity to the farm.

Schiffman et. al., (1995) evaluated 44 individuals living near hog operations (experimental group) and 44 control subjects. These two groups were matched on the basis of age, gender, race, and educational level. Compared to controls, the experimental group had statistically significant increases in tension, depression, anger, fatigue, and confusion scores from the standardized Profile of Moods State questionnaire (McNair et al., 1992). Schiffman states that the mood alteration could be caused by "a) the unpleasantness of the sensory quality of the odor; b) the intermittent nature of the stimulus; c) learned (via conditioning) aversions to the odor, and are well documented in the scientific literature; d) potential neural stimulation of immune responses via direct neural connections between odor centers in the brain and lymphoid tissue; e) direct physical effects from molecules in the plume including nasal and respiratory irritation; f) possible chemosensory disorders; and g) unpleasant thoughts associated with the odor."

Summary and Conclusions

The significant increases in the number of hogs and the steady increase in the number of people in North Carolina have resulted in more contact between people and hog odors. People are reporting to health and environmental agencies that odors from hog farms are a nuisance and that they are concerned about the health risks posed by exposure to the odors. The Odor Control Task Force of the North Carolina Agricultural Research Service at North Carolina State University reported, in regards to odors from hog farms, that "the current situation is unacceptable, has caused controversies that have divided communities, and should be addressed." The sources of odors are confinement buildings, lagoons, and spray irrigation.

The State Health Director is aware of three studies in the scientific literature reporting on health studies of people living near hog farms. These studies indicate that people living near hog farms report more adverse health effects (including respiratory and irritation symptoms and emotional disturbance) than people living away from hog farms. In addition, the literature on exposure to odors in general documents that exposure to odors in specific situations results in adverse health effects. In addition, hog farm workers are known to experience a high prevalence of adverse health effects including acute and chronic respiratory symptoms. The air in hog confinement buildings has been found to contain many gaseous and particulate chemicals some of which are known to be respiratory irritants. These findings support the premise that exposure to hog farm odors may result in adverse health effects. It should be noted that the odor exposure/health effects association exists for people with frequent exposure to the odors and does not apply to people with one-time or infrequent exposure to odors. At present, it is difficult to quantify or establish a dose-response relationship for environmental exposures to hog farm odors. The State Health Director recognizes the need for more epidemiologic studies of the association of health effects with exposure to hog farm odors. Nevertheless, as a preventive public health policy, the State Health Director considers exposure to hog farm odors as a public health risk and recommends that efforts be made to minimize odor exposures. The State Health Director encourages farm owners/operators and regulators to take actions to minimize odor and inhalation exposures for hog farm workers and hog farm neighbors.


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