But the bullet probably will hit its target in 2006, says Grant Koford, environmental health scientist for the Bear River Health Department.

If the pollution since early 2004 - combined with probable pollution next year - doesn't trigger increased regulation, newly proposed Environmental Protection Agency rules will.

Warm and rainy weather has kept down pollution levels in recent weeks. "If it [the weather] stays unsettled throughout the winter, we could make it through," Koford said. "Chances of that are not good."

The bowl-shaped Cache Valley is prone to severe winter pollution when inversions trap cold air against the snow-covered ground. It takes a storm or strong winds to blow the trapped cold, polluted air out of the valley.

So far this winter, the valley has experienced moderately high fine particulate matter (PM 2.5) pollution for only two or three days.

An extensive public information campaign, including portable billboards on Main Street, kicked in to encourage less driving, and the inversion lifted quickly because there was not much snow on the ground.

But it will take only eight days of similar pollution (PM 2.5 levels of at least 32 micrograms per cubic meter of air) in 2006 for Cache County to be declared out of compliance with the Clean Air Act.

If that happens, the Utah Division of Air Quality will mandate changes to clean the air, said director Rick Sprott.

Mandatory vehicle-emission tests and repairs probably would be part of the plan, but given the Cache Valley's topography, such a program would reduce PM 2.5 by only 15 percent to 20 percent, Sprott said.

PM 2.5 pollution is extremely fine dust and soot particles that lodge in the lungs and make it hard for those with breathing problems.

It's not yet clear how the rest of the pollution would be cleaned up, Sprott said.

The solution will depend, in part, on what ongoing research reveals about the Cache Valley's unique pollution problem.

A state meteorologist is working with Utah State University researchers to determine whether fumes from fuel, paint and other sources - volatile organic compounds - or combustion from engines and wood stoves - nitrogen oxide - are the bigger source of pollution.

How those pollutants combine with ammonia from the valley's cattle herds to form PM 2.5 also needs further study.

The state is working to devise a better inventory of all emission sources in the valley, including vehicles, homes, businesses and farms, Sprott said.

An added layer of difficulty, Sprott said, is the new health standard for PM 2.5 proposed this month by the EPA.

If it becomes final in September, it could lower the bar for PM 2.5 pollution from 65 micrograms per cubic meter of air to 35 micrograms, a level that Cache County surpasses quite regularly in winter.

"It's hard to imagine how one could meet that health standard," Sprott said. "We've got challenges now but, looking forward, it's going to be a major challenge."

kmoulton@sltrib.com