Living a half-mile downwind from 12,000 hogs has been hard on Sharlene Merk, of Audubon, Iowa. It's beyond smelly; it's a health hazard, says Mrs. Merk, a longtime farmer who, with her husband, once raised hogs. Ammonia and hydrogen sulfide fumes are a natural byproduct of the animal farms that supply America's meat. But as farms expand - some housing close to 100,000 livestock - so have concerns about air quality and the impact on people nearby. Studies near bigger farms, for example, have documented high rates of respiratory illness in the human population.