Meet the four-legged threat to the US environment

Drive into California's hot and dusty San Joaquin Valley in mid-summer and the chances are you will be greeted by a noxious cloud of air pollution hanging over the irrigated farm lands and sprawling suburban communities.

The valley is one of the three most smog-ridden areas of the United States, along with Los Angeles and Houston, and the reasons aren't too hard to see. The valley, the most productive agricultural region in the country, is effectively an air trap between two ranges of mountains where summer temperatures can easily soar above 40C. The population is growing and with that growth has come building development and an increase in car, lorry and tractor traffic.

Local authorities, however, say the biggest problem is the gas produced by the region's 2.5 million cows. Areport by the San Joaquin Valley Unified Air Pollution Control District estimates that each cow emits 19.3lb per annum of smog-inducing gases, or "volatile organic compounds", a daily pollution rate of 50 tons of the compounds - 20 tons more than the quantity produced by vehicle exhausts.

At first glance, this sounds like a joke. But to veterinary scientists, and more particularly to the $4bn (£2.25bn) dairy industry in California which now faces a flurry of new regulations, it is very far from funny. It has, in fact, triggered a monumental scientific and political debate in which all sides have been accused of working for various industry interests and betraying their professional integrity. One group of lobbyists says the air pollution authorities are chasing a phantom based on untested data, while another says the 19.3lb-per-year figure may in fact turn out to be low.

Five congressmen and 12 local state legislators - all of them with big farming interests in their constituencies - have banded together to urge the Air Pollution Control District to reconsider its numbers before the dairy industry is forced to invest millions of dollars on pollution-control technology and changes in the animals' diets (new regulations dictating exactly what farmers will have to do are due to be issued next summer).

Some of the scientists whose work was cited in the final report have disowned it, saying their findings were twisted and that the report's central contention is almost entirely unsupported by the known facts.

The big farming interests grumble that the state shows more interest in regulating them than it does in ordering further controls on fuel emissions in cars and trucks and that farmers are being unfairly targeted.

As one letter writer to a local paper, the Modesto Bee, wrote recently: "I'd like to challenge the people that came up with this information to enclose yourself in a shop with a cow, and at the same time have someone enclose themselves in a similar shop with a car or truck running. Then let me know the results."