What is regulated, what is not

The federal government regulates seven air pollutants: ozone, carbon monoxide, nitrogen oxides, sulfur dioxide and lead, and two types of particles.

Known as "criteria" pollutants, the Dirty Seven contribute to acid rain, respiratory illnesses and other health problems. For years, the government has regulated particles 10 microns and smaller but recently began regulating fine particles no larger than 2.5 microns, the size of very fine dust. In particular, fine particles penetrate deep into the lungs and are associated with lung damage, heart attacks and premature death, MPCA literature says.

Criteria pollutants have drawn more attention because of the potential cleanup cost. The Minnesota Chamber of Commerce estimates that would cost between $189 million and $266 million to install the emission controls if the area failed to meet federal limits, said Bill Droessler, director of Clean Air Minnesota, a public/private collaboration.

In the Twin Cities metro area, ozone and fine particles are the two criteria pollutants that could most easily exceed federal limits, said Rick Strassman, the agency's air quality monitoring supervisor.

Ozone needs heat and sunlight to form and is a summer-only problem, he said. Fine particle pollution is a year-round problem.

In 2003, the metro area had 10 health alerts for fine-particle levels. So far in 2004, it has had seven.

The federal government does not set standards for all air toxins, such as cancer-causing formaldehyde and benzene found in car exhaust. Downtown formaldehyde levels are among the region's highest, according to the MPCA, and exceed state Health Department's health risk values.

(The health risk values are conservative standards very protective of public health, according to a health official who worked on them.)

With some key pollutants trending downward, the MPCA is trying to understand why formaldehyde remains high, Strassman said.

"It seems to be more persistent and stubborn. I don't have a good scientific explanation for it," he said.