Despite this finding, scientists believe that controlling cars -- and not cows -- appears to be the best way to blunt wintertime air pollution spikes in the Cache Valley.
And the toughest chore may be finding the small number of local vehicles that make up the biggest part of the problem.
These observations surfaced on a sparkling summer day earlier this month as local officials and air-quality regulators gathered at Utah State University to size up last winter's alarmingly high pollution levels, readings made worse by the livestock in this bucolic mountain valley.
Officials fear Cache County might breach federal limits as soon as December. So, they are considering measures to reduce the pollution before next winter.
One solution might be trying to control the ammonia from livestock waste, as some California communities are doing. But a more likely, more effective, option probably would be launching a vehicle inspection and maintenance program in Cache County like those in the Wasatch Front.
Officials are stepping cautiously.
"I think we need to make sure we are on firm ground when we go there," said Cache County Executive Lynn Lemon.
More information is needed before livestock curbs are seriously considered, he added.
"The agricultural community gets really offended when we blame it on the cows."
The federal Environmental Protection Agency (EPA) does not count Cache County on any of its lists of most-polluted places. However, readings for PM2.5, microscopic soot particles 1/40 the diameter of a human hair, lingered at unhealthy levels for more than two weeks last winter during weather inversions.
Inversions trap pollution in valley basins, like a towel over bread rising in a bowl. The unique weather, geography and economy of the deep valley make conditions perfect for pollution to grow.
Last season's readings regularly exceeded those in Utah's most populated areas. In fact, they were among the highest ever seen in the nation in the seven years PM2.5 has been measured.
Just a few more too-high PM2.5 readings next season and the EPA can add Cache County to its polluted list, opening the door for tighter federal regulation and possible sanctions.
Equally troubling to regulators and local leaders is knowing that increased PM2.5 hurts the youngest, the oldest, and those with heart and lung ailments. Studies show pollution increases lead to more school absences, lost work days and even higher death rates.
"We know there's a problem," said meteorologist Tess Davis. "It's undeniable."
Livestock may or may not factor into Cache County's solution.
Data presented at a Utah State University symposium earlier this month showed that the urine and manure of the Cache Valley's 75,000 cows releases about 5.3 tons of ammonia vapor into the air each winter day. Trapped by the inversion, it concentrates to five or 10 times the normal level, and second only in potency to concentrations usually measured at the nation's stockyard center on the panhandle of Oklahoma.
But other tests run by USU researchers, led by engineering professor Randal S. Martin, suggested faster and better results could be achieved by limiting the hydrocarbons and nitrogen oxide typically found in vehicle exhaust.
The thinking is: since the tiny pollution particles of PM2.5 are created by the mixing of vehicle exhaust gasses and ammonia vapor from the livestock waste, the key to controlling the pollution is controlling the waste gasses, and it would be far easier to control car exhaust than cowpies.
By tracking pollution from cars passing through four check points, researchers were able to determine that half the cars produce about 90 percent of the nitrogen oxides and 10 percent of cars can be blamed for about half of the hyrdocarbon emissions.
They estimate that repairing the county's high-exhaust vehicles -- about 15 percent of those on the road -- would reduce the pollution-causing emissions by as much as half. The challenge would be identifying vehicles that pollute most because newer cars do not necessarily run cleaner than old ones.
But there are problems with instituting such a program. It would cost Cache County drivers an estimated $1.8 million. It's also not clear that the program would actually cut pollution soon enough to keep Cache air within EPA standards.
In short, local officials said they have not decided what to do next.
Said Sarah Skanchy, a Logan resident and member of the Bear River health board, "You have to make an informed decision."
"We may be pushed" by an EPA clampdown, she added, "but I don't think we are there yet."
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