Dr. Susan Schiffman holds up a bag used to collect odor samples near swine operations. (N&O photo by Scott Sharpe)

Here are some of the latest findings on hog odor, as reported by Dr. Susan Schiffman of Duke University and the Swine Odor Task Force:

• Hog odors are caused by a soup of about 150 gases that result from the bacterial decomposition of manure. The most commonly cited suspects in hog odor, hydrogen sulfide and ammonia, aren't even the worst offenders.
• The odors come from three sources: from the barns themselves, from croplands where the wastes are sprayed and from the lagoons where the wastes are stored.
• The gases that cause hog odor tend to travel in a plume, or clump, and they can remain potent for considerable distances if not dispersed by wind or other forces. Schiffman found that the gases were sometimes just as concentrated 1,500 feet from the barn as they were just outside the barn.
• The number of odor-causing molecules in a typical air sample is quite small -- in the parts-per-billion or parts-per-trillion range. But even at they level, they can induce a strong reaction in human subjects in blind studies. Schiffman measured odor levels at some farms that were 27 times higher than the threshhold of perception.
• Roofing shingles, siding, fabrics and other materials can act as a "sink" for hog odors, trapping the compounds at night and releasing them during the day as the sun beats down. Fatty tissues in the human body absorb the compounds in the same way, Schiffman said.

"That's why some people say they can smell the odor on their breath long after they left the farm," she said.

• Hog farmers and employees aren't lying when they claim to be unable to detect odors. Schiffman says workers quickly become desensitized to the smell because the odor molecules tie up the olfactory receptors in the brain, just as a phone call ties up a receiver.
• Hog odors tend to be intermittent rather than constant, affecting some neighbors perhaps no more than a few times in a week.

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