The dense, choking cloud of sulfur dioxide and sulfur trioxide pollution that billowed across Interstate 80 Sunday night points up the need for tough air quality monitoring and enforcement.

Kennecott's smelter at Magna released the plume around ground level when the pollutants failed to go up the smelter's tall stack. Exactly why that happened is being investigated by state officials.One possibility is that the stack's fans stopped because of a power failure, so the gases spewed out as low-level leakage.

Another possibility - that may or may not be true, according to Burnell Cordner, director of the Utah Bureau of Air Quality - is that "they have a system in their stack, some sensors that when the stack gets to a certain temperature, close down the fan."

Accepting the possibility that such a system is in place, it would be designed to prevent the stack's liner from burning up if the gas is too high, he said. But until he gets a report from Kennecott he can't confirm whether this is what happened.

What is certain is that vehicles drove through the dense cloud, and at least one Tooele family was treated in a clinic because of the exposure. The cloud was so dense that the Highway Patrol and Salt Lake County Sheriff's Department had to direct traffic.

One report said the cloud persisted for three hours.

"I think we as a health department are very concerned," Cordner said.

"Whatever happened, for whatever reason, from our viewpoint is totally unacceptable."

Nothing in the state's air quality regulations says the public must be warned against such a hazard, he said. "There are rules that when breakdowns occur, the Health Department has to be notified."

Kennecott did notify the state's emergency response coordinator. Cordner and Kenneth Alkema, director of the Utah Division of Environmental Health, are attempting to trace the state's response.

"This is something that should not happen again," said Nina Dougherty, chairman of the Air Quality Committee of the American Lung Association of Utah.

The incident points up failings in the new PM 10 particulate rules that are about to go out for public comments, she said. The rules are "much too fuzzy on the low-level emissions," Dougherty said.

Kennecott's pollution, both from the tall stack and from low-level emissions, has a significant impact on the Salt Lake Valley, he said. Although the SO2 and SO3 in the cloud released Sunday are gases, they are precursors to particulates.

Particles form when the gas reacts with the atmosphere, she said. PM 10 particles, bits of junk 10 microns in size or smaller, are dangerous because they can lodge in the lungs and bronchial passages.

Sulfates, the particles that SO2 can form upon reacting with the air, are even finer than 10 microns. "It's more like 2.5 or something.

"And sulfate is considered to be one of the worst of the fine particulates, one of the more harmful."

A tracer study in March 1990, in which special gases were released both at the stack and with low-level emissions, showed "impacts in the (Salt Lake) Valley from both sources," she said.

"Draper is where the main impact was seen. It was quite significant, even though Kennecott was not even emitting much during that study."

According to Dougherty, when state and Environmental Protection Agency officials analyzed the study, they said Kennecott put out only 3,000 pounds per hour of pollution. "They very often are emitting 20,000 to 50,000 pounds per hour," she said.

Also, the study was performed in March but not during the worst conditions, a severe inversion. At that time, "you get a higher rate of conversion of SO2 to sulfate," she said.

Sunday's incident "shows that there's a real need for a lot more monitoring, a lot more state involvement, oversight of what's happening," she said.

It's unfortunate that people were subjected to this pollution. But at least the timing was fortunate. When the public hearings are held on PM 10 rules, the exposure of these innocent people is certain to be a main point brought up by many. And state regulators will listen carefully to what they say.