(AP) -- This is the time of year when residents who often live with the nation's worst pollution often can draw a breath of fresh air. But this winter has not been kind to people who want to play outside in California's Central Valley.
A dry December and January has stagnated air across California, but nowhere is the situation more serious than between Modesto and Bakersfield, where nearly every day dirty air has exceeded federal health standards.
It's the worst air quality recorded in a dozen years, and it's the unhealthiest kind- microscopic, chemical-laden particles that can get into lungs and absorbed into the bloodstream to create health risks in everyone, not just the young and infirm.
The southern San Joaquin half of the valley stretches 200 miles from Stockton to Bakersfield and is home to 4 million people. It traditionally records the highest level of particulate matter and ozone pollution in the United States and has a rate of asthma three times the national average, according to the American Lung Association.
Air quality advocates have argued for years that the local air district's focus on fireplace burn bans ignores other major sources of industrial pollution, such as dairies, feed lots and oil rigs. "The air board's strategy is failing," said Kevin Hall, executive director of the Central Valley Air Quality Coalition.
Air officials say their policies are sound, but there is little they can do with La Nina conditions in the Pacific creating stagnant air.
Fighting air pollution in the Central Valley is a task that so far has not succeeded in meeting federal health standards. Surrounded on three sides by mountains, the valley opens in the north toward San Francisco and Sacramento, where weather patterns suck emissions south.
Cutting through the valley are the state's two main north-south highway corridors, the routes for nearly all long-distance tractor trailer rigs, the No. 2 source of particulate pollution in the valley. Also in the mix are millions of acres of plowed farmland and 1.6 million dairy cows and the flatulence and ammonia-laden manure they create.
Without wind and rain, the air sits, trapped as if in a pot with a lid.
Since 2003, the San Joaquin Valley Air Pollution Control District has targeted fireplace soot as biggest source that is easiest to end and calls "no burn days" based on weather forecasts.
Fires were banned on nearly every day in December, including Christmas Eve and New Year's, and the 60 people who patrol neighborhoods writing citations to offenders have been busy. Violations doubled in some areas and were up to five times higher in others last month as the district cracked down during unseasonably cold weather.
"When we have weather conditions like this, there is nothing we can do really to meet the federal standards," said Seyed Sadredin, executive director of the district. "Even if we shut down I-5 and (U.S. Highway) 99 and shut businesses we would still violate the standard because there's no dispersion. The best we can do is to minimize the damage, and the best way to do that is with the fireplace rule."
The struggle with particulate pollution comes after the district failed during the summer months, despite a publicity campaign, to keep ozone emissions under EPA limits to avoid ongoing federal fines.
Warnings about the potential adverse health effects of air pollution become a year-round event in the valley. And those warnings are about to start coming more furiously. This week district officials lowered by nearly half the level of pollution they say is safe for outdoor activities.
The air district helped fund a study of 1 million residents in 2011 that found that emergency room visits for asthma and heart attacks went up when particulate pollution went up. That convinced officials that the federal government's standard, which relied on a 24-hour average of air quality, was too high. Small particulates in the bloodstream can break off plaque in the coronary artery, creating a logjam and a heart attack.
"The old level may work for Beijing, China, but we need to bring it down to where it really belongs," said David Lighthall, the district's health science adviser. "We are recognizing that the air quality is different from one time of day to another and we're trying to give people the information they need to make decisions about outdoor exercise."
The district sends advisories to schools and those signed up for email alerts, called "Real Time Outdoor Activity Risk" warnings, whenever the air reaches the "unhealthy" level so that teachers know whether to call off recess and residents can decide to postpone a jog or a bike ride. On Friday morning, for instance, some Fresno residents received an email alert at 10 a.m. working that the air was "Level 5 Very Unhealthy" for everyone, indicating the highest levels of pollution.
"We can give people a tool, whether an athlete or school manager, and ensure they do stay indoors at particular times when air quality is threatening, and also find out when a better time to go out would be," Lighthall said.
Just before Christmas, the Center for Race, Poverty and the Environment sued the U.S. EPA on behalf of Central Valley residents alleging it has not pressed California for a viable, enforceable plan to improve air quality.
"We are going to need far tighter rules coming out of the air district if we are really going to make progress in meeting federal standards," said Tom Franz of the Bakersfield-based Association of Irritated Residents, one of the groups suing.
Air pollution officials say the technology doesn't yet exist to lessen the valley's pollution and bring the region into compliance, though the district is investing in research and giving grants for things such as the new generation of battery powered leaf blowers and lawn mowers.
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