Despite bans on fireplace wood-burning, concentrations of soot and other dangerous particles are getting worse in the valley's air — already the nation's worst for this kind of particle pollution.
It's an odd twist because the wood-burning bans have improved air quality around Thanksgiving and the winter holidays in Fresno and Bakersfield.
Yet the year-round average level of the microscopic debris — called PM-2.5, which is linked with lung problems, heart disease and early death — has increased more than 5 percent over the last several years.
Local air officials say the uptick is connected to weather and record-setting wildfires in summer, not fireplace smoke in fall and winter. They point to a long-term decline of PM-2.5 over the last decade.
Never miss a local story.
"What you're seeing is normal variation, and it's minimal to me," said Scott Nester, planning director of the San Joaquin valley Air Pollution Control District.
Readings averaged over 3 years
Valley air has an average of 45 percent more PM-2.5 particles than the federal health standard allows, according to the U.S. Environmental Protection Agency. The agency averages readings in three-year increments to establish concentration levels.
High concentrations of PM-2.5 appear as part of the valley haze, passing easily into the lungs and the bloodstream.
Wood and diesel smoke are the biggest direct sources of PM-2.5 specks. Wind-blown dust, debris from paved and unpaved roads and commercial meat-cooking also contribute.
But about half of the valley's PM-2.5 forms as oxides of nitrogen from vehicle exhaust and ammonia from dairies combine. Air officials say huge plumes of the resulting pollutant, called ammonium nitrate, drive up the valley's concentrations of PM-2.5.
Nobody knows for sure if large amounts of ammonium nitrate are harmful, because there has not been a lot of research on it.
But ammonium nitrate is not considered as big of a health threat as wood and diesel smoke, because it changes to nitric acid in the body. Nitric acid occurs naturally in humans.
Same chemicals in diesel, fires
Smoke from fireplaces and wood stoves is the bigger threat to valley residents, air officials say, because many people in city neighborhoods are exposed to it. The smoke is more dangerous than generally known a few years ago.
The latest medical research shows the tiny specks in the smoke carry many chemicals into the bloodstream. Some, such as benzopyrene and chrysene, are considered carcinogenic. The same chemicals are found in toxic diesel exhaust.
"Your fireplace smells a lot better than diesel does, but as far as chemical content is concerned, they're very similar," said researcher Tim Tyner of the University of California at San Francisco-Fresno Medical Education Program.
PM-2.5 pollution is blamed for 800 premature valley deaths each year. A study from California State University, Fullerton, last year found two-thirds of valley residents are exposed to unhealthy levels of PM-2.5.
David Lighthall, district science adviser, said the worst seasons for PM-2.5 are fall and winter, when wood smoke sometimes hangs for hours in neighborhoods.
The district's rules have resulted in particle pollution reductions of more than 12 percent in Bakersfield and Fresno over the last several winters.
"We're using a lot more burning prohibitions," he said.
But summer wildfires are adding to the year-round average, as the federal government's year-round figures might indicate. In 2008, there were hundreds of wildfires and smoke entering the valley from all directions.
Scientists predict an increase of such fires as the climate warms over the next century.
Air officials are working with the U.S. Forest Service to allow more prescribed burning, which would eliminate excess brush and reduce the size of the fires.
But the local air district can't control wildfires, weather or the valley's bowl shape that traps a lot of air pollution.
"We have inherited the worst possible natural scenario here," said planning director Nester. "That's why we pursue the toughest rules."