The stench of a large hog farm may seem nauseating, but a study now suggests that hog farm emissions—which include dust, irritants, allergens, hydrogen sulfide, ammonia, and hundreds of volatile organic compounds—could have a measurable impact on human health. Neighbors of such farms experience a rise in blood pressure when the farm odor is strong, researchers found.
Industrial-scale farms that raise animals for food—called concentrated animal feeding operations (CAFOs), in the parlance of the U.S. Environmental Protection Agency—have a number of documented impacts on the environment, mostly from the massive quantities of manure they produce. That waste, which contains microbes that can make humans sick, is collected in open pits or sprayed on fields as fertilizer, risking contamination of the air, water, and soil.
But the waste is just one part of the problem—there's also the smell. Previous research in North Carolina, where the growth of hog farms has been so staggering in the last 25 years that now there are more hogs than people, found that farm odor caused stress and negative mood states  in neighboring residents, according to a 2009 study in the American Journal of Public Health.
Steve Wing, an epidemiologist at the University of North Carolina, Chapel Hill, School of Public Health, sought to find a link between the odors from CAFOs and these symptoms. If odor was a stressor, Wing hypothesized, could that be measured by a rise in blood pressure?
Using mobile monitoring equipment, the researchers measured the air quality, including particulate matter concentration and hydrogen sulfide, of neighborhoods adjacent to hog CAFOs in North Carolina. Participating residents sat just outside their houses for two 10-minute periods each day, reporting what they thought was the strength of the swine odor based on a graded scale from 0 to 8 and taking their own blood pressure with an automated device.
Stronger odors and higher measured hydrogen sulfide concentrations both correlated to higher blood pressure in the residents , the researchers found. (The concentration of particulate matter didn't seem to have an effect.) The most dramatic effects were a nearly 2 mmHg rise in diastolic (the low number) blood pressure when the odor was rated an 8 compared with no odor, and a nearly 3 mmHg rise in systolic (the high number) blood pressure when hydrogen sulfide concentrations reached 10 parts per billion compared with no detectable level, the researchers reported last week in Environmental Health Perspectives.
Although self-reported data are inherently variable, making the diastolic-odor correlation less robust, "I was impressed by the strength of the relationship," Wing says. The most sensitive detector of hog farm air pollution, he adds, is still the human nose. "Odor can be present without detectable hydrogen sulfide. It's made up of volatile organic compounds at very low concentrations and in a particular combination that people recognize as 'swine.' "
Wing suspects that residents' lack of control over the odor might account for the rise in blood pressure. Unlike, say, dog poop that can be easily washed off a shoe, people have no way to avoid the smell wafting from a giant hog farm—and animal studies suggest that the impact of stressors, which can cause, for example, feelings of depression and anxiety, can be modulated by a sense of control over that stressor .
Wing's study highlights a persistent topic in community health, says Susanna Von Essen, an internist at the Nebraska Medical Center in Omaha. "The industry is under fire from hog farm neighbors," Von Essen says. "But it's been difficult to find specific health problems." In 2010, Von Essen co-authored a systematic review on CAFOs and community health in PLOS ONE that examined the relationship between hog farms and allergies and asthma, but found the evidence for such a relationship to be inconsistent . As for Wing's study, she says, "the blood pressure rise was teeny tiny. I'm not sure this is alarming in and of itself—unless one can show repeated spikes lead to sustained hypertension or earlier onset of hypertension."