for National Geographic News
Standing around chewing the cud, cows don't look especially threatening. But dairy herds in California are the latest livestock to be branded an environmental health risk on account of their flatulent behavior.
This month government regulators issued a report identifying dairy cows as the main source of smog-forming pollutants in the San Joaquin Valley, California.
The announcement highlights growing concern over the global impact of greenhouse gases produced by cattle and other livestock.
A dairy cow annually emits almost 20 pounds (9 kilograms) of smog-forming gases known as volatile organic compounds (VOCs)more than a car or light truck, according to the San Joaquin Valley United Air Pollution Control District.
The local government organization works to monitor and improve regional air quality.
Based on 15 separate studies, the district's figure almost doubles scientists' previous estimate for cow emissions. The finding will form the basis for stricter air quality controls on dairy farms in the area.
The San Joaquin Valley is home to a thriving dairy industry that includes some 2.5 million cattle. With more new dairy operations planned, an additional 400,000 cows are expected to arrive in the valley within the next few years.
But the dairyland is also known for its smog. Over the last six years the valley has violated the federal limit on smog levels more often than any other region in the country.
"The valley air basin harbors some of the worst air quality in the entire country, and on far too many days air quality is unhealthy," said Kelly Hogan Malay of the San Joaquin Valley Air Pollution Control District.
Local environmental groups blame air pollution for health problems, such as high rates of childhood asthma, in the region. But dairy farmers say their cows are being made into scapegoats for a gas-guzzling society.
Malay says motor vehicles are a much bigger source of air pollution overall, because they emit pollutants such as nitrous oxides and carbon monoxide in addition to VOCs.
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