The health hazard posed by tiny particles from vehicle exhaust is well established. But now a new pollution worry may have emerged: reduced snowfall.

Don A. Griffith, president of North American Weather Consultants in Sandy, thinks winter snowfall in the mountains east of the Salt Lake City-Provo urban complex may be decreasing because of pollution. His 2005 study, "Is Air Pollution Impacting Winter Orographic Precipitation in Utah?" was published in the Journal of Weather Modification last year.

Griffith's company is involved in weather modification efforts to increase precipitation, so they have an interest in finding out if any human-caused factor other than their own efforts are influencing the weather.

The study, co-authored by the company's Mark E. Solak and David P. Yorty, examined weather record stretching back to 1949 and 1956, and found a surprising trend. In mountains to the east of Salt Lake City and Provo, snowfall has been decreasing.

Meanwhile, for the same period, November-March, that didn't happen in Salt Lake City. An explanation is that snow would begin falling on Salt Lake City as weather drifted in from the west, the flakes falling before weather systems absorbed pollution. In fact, snow seems to have increased slightly in Utah's largest city.

The study was inspired by work carried out in California and Israel indicating snowfall was decreasing at sites downwind from cities.

The theory is microscopic particles, possibly from diesel exhaust, "might serve as additional cloud condensation nuclei," Griffith said in a telephone interview. These nuclei are the grains upon which cloud water droplets condense.

If enough tiny particles are in the cloud, "you may produce more but smaller cloud droplets," he said.

Precipitation is produced through a mechanism called collision coalescence, he said. Tiny cloud droplets collide, forming larger ones. When they are massive enough, they precipitate out and rain or snow falls.

As snowflakes drift through polluted air, they may not collect these droplets as effectively as they would in an unpolluted environment, he said. They may not be as efficient in collecting smaller droplets that formed around microscopic particles.

The long-term trend looks "pretty dramatic," he said. Downwind of Salt Lake City and Provo, winter precipitation seems to have declined over the past 60 years or so.

Would the snow precipitate somewhere farther away from the city? The team could not find evidence of that, when they studied weather records for Heber City. "It looks like the negative effects, whatever they're being caused by" may extend to the western Uinta Mountains, he said.

They selected two rural areas to examine so they could compare precipitation in unpolluted regions against what was happening near Salt Lake City.

At Beaver, precipitation "had increased with time, instead of decreasing as we found in Salt Lake City," he said. Beaver's situation may be complicated because the company is carrying out a cloud-seeding program that may have increased precipitation in the nearby mountains.

Without such complications are Ely, Nev., and the mountainous region to its east called Berry Creek. There, records shows "no indications of decreases . . . but rather slight increases with time," over the period 1949-2004, he said.

After the North American Weather Consultants study was published, one of the authors of the research involving Israel and California took a look at Salt Lake City and "also found decreases in mountainous areas" to the east, he said.

That researcher, D. Rosenfeld, "also found declines in mountains areas east of Albuquerque, east of Seattle, east of Phoenix," he added.

What's next for the theory?

Griffith said a good study would be to examine weather patterns in comparison to pollution levels, especially particulate pollution of 2.5 microns or less in size.