The Sacramento region complies with the federal air quality standard for fine particle pollution, the U.S. Environmental Protection Agency determined this month.
After the federal agency tightened its fine particle standard in 2006, Sacramento became one of the few areas in the country with dirty air under EPA rules.
The Sacramento region met the new standard largely due to winter regulations on wood burning, said Larry Greene, executive director of the Sacramento Metropolitan Air Quality Management District.
"That's a major part of the reduction because the days we exceeded (standards) were largely days in the wintertime when we had a lot of wood burning," he said.
Since 2007, the district has imposed wood burning limits between November and February through its "Check Before You Burn" program. The region had 13 days last season in which residents could not burn at all.
Fine particles can come from direct sources such as forest fires, or they can form in the air from power plant emissions and auto tailpipe exhaust. But about half of Sacramento's problem during fall and winter when particulate levels are highest comes from wood burning, according to the Sacramento Metropolitan air district.
While state and local air quality officials largely credited the burning restrictions, they said the region's air also benefited from efforts to cleanse diesel emissions from trucks and farm equipment, cleaner passenger vehicles and stricter regulations on pollution from buildings such as power plants and factories.
The EPA-designated Sacramento area includes all of Sacramento County, western El Dorado and Placer counties, southern Sutter County and eastern Yolo and Solano counties.
The only areas left in California that still do not have clean data to pass the 2006 standard are based in San Joaquin, Los Angeles and Imperial counties.
The Sacramento region had until 2014 to meet the 2006 standard, facing the potential threat of sanctions such as more stringent business permitting requirements and loss of federal transportation funds.
"This is good news for people who live and breathe in Sacramento," said Kerry Drake, an air division assistant director at the U.S. EPA.
Although wood smoke has other harmful pollutants such as carbon monoxide, particulate matter is the principal pollutant of concern due to the health risk it poses.
Fine particles are tiny pieces of matter suspended in the air. Each particle is more than 30 times smaller than a grain of fine beach sand.
Concentrations high enough to harm human health are not necessarily visible. "It can still look clear down here," said Roblee Allen, a pulmonologist at the UC Davis Medical Center in Sacramento.
Because of their small size, fine particles can lodge deeply into the lungs and even penetrate red blood cell walls, Allen said.
Once in the bloodstream, the particles can overwhelm the lungs and heart, aggravate asthma, trigger cancers and hasten death.
The Sacramento area had previously met the EPA standard of 65 micrograms of fine particles per cubic meter of air over a 24-hour period when it was originally set in 1997, according to Sacramento Metropolitan air district spokeswoman Lori Kobza.
However, after extensive studies showed exposure to fine particles posed a greater risk to human health than previously thought, the EPA tightened the standard to 35 micrograms per cubic meter.
Sacramento was designated in December 2009 by the EPA as a "nonattainment area" for exceeding the standard. Greene said the region had readings hovering around 45 micrograms per cubic meter.
That has since fallen below 32 micrograms, he said. To meet EPA requirements, the area showed three years of clean readings from five monitoring sites throughout the area.
"Make no mistake, they worked hard to make sure they got clean data," said Drake, regarding the Sacramento area's progress. "They were very responsive and targeted what they need to do. And they did it. That's the way it should work."
While the EPA has determined that Sacramento met the stringent 2006 standard, the area must continue to maintain its clean air levels to shed its "nonattainment" designation.
The San Joaquin and Los Angeles areas have yet to meet the less stringent 1997 EPA standard.
"Not for lack of trying," said Drake. "L.A. and San Joaquin have more challenges, mostly due to meteorology and geography."
The bowl-shaped Sacramento Valley faces unique challenges in fighting pollution. The area is prone to "inversions," which occur when hot air sits on top of cold air like a lid, inverting the normal relationship of temperature to altitude.
Delta breezes from the coast and cold air flowing downslope from the Sierra both funnel in colder air at ground level and contribute to the inversion layer.
Since cold air is denser, it cannot rise above the hot air unless it is heated, so the polluted cold air gets trapped where it is generated at ground level.
The Check Before You Burn campaign prohibits burning in all indoor or outdoor fireplaces, wood stoves, and fire pits on days with a particularly unhealthy particle pollution forecast.
First-time violators are required to take a course on wood burning or pay a $50 fine.
On less polluted days, the Sacramento air district allows burning with EPA-certified equipment or advises against burning but does not impose penalties.
The 2012-2013 winter saw only 13 days of complete burning bans. But during the prior season, a complete burning ban was in effect for 29 out of 38 days between December 2011 and January 2012.
35 micrograms of fine particles per cubic meter of air EPA standard
45 micrograms: Sacramento's average for December 2009, knocking the region out of compliance
32 micrograms: Sacramento's average for the past three years, bringing the region back into compliance
Call The Bee's Ellen Le, (916) 321-1031. Follow her in Twitter @ellenble.