The smog in California's San Joaquin Valley has puzzled scientists for years. Even though the region is largely rural and agricultural, its smog levels exceed those of densely populated cities like Los Angeles.
Some have speculated that animal waste or pesticides are the cause: both emit ozone, a primary ingredient in smog. But a recent study published in the journal Environmental Science and Technology suggests that the primary culprit is actually cattle feed.
Researchers found that animal feed is the largest emitter of ozone in the valley, at 25 tons per day, followed by motor vehicles at 14 tons.
The smog presents health concerns because it is known to be a factor in triggering asthma, bronchitis and emphysema as well as other heart and lung ailments.
The Environmental Protection Agency has said that if smog levels were more strictly controlled, 12,000 deaths from heart or lung diseases could be avoided annually. In January, the agency proposed stricter standards for smog-causing pollutants.
The main issue with feed seems to be the way it is stored. Cattle feed ferments when kept in an oxygen-poor environment, producing organic compounds, said Michael Kleeman, a professor of environmental engineering at the University of California at Davis and one of the study's authors.
When the feed is then placed in front of animals outside, those organic compounds are released to the atmosphere, contributing to ozone formation.