Where does odor and the role it plays in human health fit in the hog industry puzzle?
Duke University professor Susan Schiffman recently published a report reviewing past and current research that looks at odor and health.
In the report, Schiffman discussed research showing how social influences can play a role in odor sensitivity. For instance, the suggestion of a bad odor can lead some people to believe they smelled the odor, even when one was not present.
Schiffman also pointed out that compounds in some odors can cause sensory irritation in the eye, nose and throat.
Overall, the effect of odors on humans is due to a complex interplay of physical and psychological factors. Schiffman suggested more research. Her research report looked at a number of areas including the following:
1. Common health complaints from odor - The main health complaints that occur involve an irritation to the eye, nose or throat. Schiffman also noted headaches and drowsiness are reported.
People reporting symptoms from odors usually have problems with a wide array of compounds. So she said any studies of adverse effects of hog odors must also examine other odors in the rural environment.
Schiffman reported on a study she conducted that compared sensitivity to odors among a group of self-described odor "sensitive" people to a group who called themselves "less-sensitive" to odor. The groups were asked to rate their sensitivity to different odors ranging from bleach and turpentine to animal odors.
Some 60% of the sensitive group reported 32 different odors would make them ill. Only 12 of those same odors would make the less-sensitive group ill. Animal odors were included in the list. In fact, 62% of both groups stated animal odors would make them ill if exposed for 30 min. or more. Schiffman found this surprising and later suggested that responses from the subjects "can be affected by the perceived purpose of the questionnaire as well as the duration of exposure." The subjects were North Carolina residents who do not live near agricultural operations.
2. Potential for odors causing rhinitis, asthma, bronchitis, etc. - Schiffman said irritants can set up a low-grade inflammation that will predispose some people to hyper-reactivity and allergy. Research studies on swine confinement workers show a high level of work-related coughs, indicating irritation.
Other research determined acceptable levels and exposure limits of total dust and ammonia for swine workers. These levels are generally higher than those experienced downwind from hog operations, Schiffman noted.
Overall, asthma and rhinitis have increased across the U.S. in the last 15-20 years. One study found a striking growth of asthma in both urban and rural children.
One compound found in hog odor, formaldehyde, has been linked to asthma-like symptoms. However, the levels of formaldehyde found downwind of swine operations is considerably less than that reportedly causing problems in humans, Schiffman said.
3. Toxic effects of hog odor - High levels of volatile organic compounds (VOC) can affect the olfactory system. Some VOC are found in hog odors.
But Schiffman stated research shows the concentrations of VOC downwind from hog farms are too low to cause toxic reactions. She did add that no work has been done on the results of mixing low levels of VOC.
4. Potential for odor to cause stress - "The perception of odor is dominated by the pleasantness-unpleasantness dimension," Schiffman reported. "Pleasant aromas such as cookies baking in the oven beckon us, whereas, unpleasant odors such as those from a garbage dump repel us."
She added that this aspect of odor can affect mood due to the overlap of the olfactory and emotional systems in the brain. The stress and moods caused by unpleasant odors can in turn influence health through immune changes.
Schiffman conducted a research project in 1995 comparing the moods of North Carolina residents living near hog operations to those not living near hog operations. She found more tension, depression and fatigue in people living near the intensive swine production units. However, she added that inherited physical traits and learned responses may add to these moods.
Pleasant smells, on the other hand, are shown to enhance moods. Ironically, two pleasant smells shown to improve immune status are bacon and ham.
5. The role of conditioning in odor responses - "Conditioning or learned associations can play a role in symptoms induced by odors," Schiffman said. One research project showed panic and hyperventilation symptoms were learned after an acute exposure to chemicals. Before the exposure, the chemical odors were tolerated.
6. Environmental and health concerns affect odor perception - Several research projects have shown that beliefs about the safety of an odor can have an effect on its perception.
One study in particular showed that how the odor was presented to participants in the research project affected the outcome. In the study, one group of participants, called the positive group, were told the odor was a natural extract used by aromatherapists. The negative group was told the odor was an industrial chemical that caused health effects after long exposure. A neutral group was told the odor was a common, approved stimulus for odor studies.
All groups received the same odor, same strengths. Yet the negative group found the odor to be more irritating, strong and complained more of health symptoms including light-headedness.
Another study showed people can develop psychological and health symptoms after the suggestion of a bad odor even though it was not present. In this study, people were given the suggestion that a room contained a malodor. They reported more negative moods and symptoms of discomfort than persons given the suggestion that the odor was pleasant. No odor was present in either case.
Schiffman concluded that people's expectations about odor will influence their perception.
7. Chronic exposure to odors - Several studies show people adapt to odors, especially in the workplace. Tests show these workers are not as sensitive to the odors as unexposed people.
"Long-term adaptation to animal odors occurs in persons who work daily in highly odorous environments," Schiffman reported. "It accounts for the finding that persons who work with livestock cannot fully understand the complaints from neighbors who only receive odors intermittently."