“We have the toughest air regulations in the nation, but we have no control over the geography and the climate,” he said. “The Valley is basically a bowl with a lid on top for most of the year.”

Dr. Vipul Jain, a pulmonologist who runs a chronic lung disease program in Fresno, said the effect of the drought and fires was likely to get worse this week, with temperatures expected to soar to 106 degrees. He said that hospitals could have as many as double the number of patients for acute lung problems in the coming days and weeks.

“It’s kind of a worst-nightmare situation,” said Dr. Jain, an associate professor at the University of California, San Francisco. “And the worst is still yet to come. We can see it, but anyone with a lung problem, they feel it, and there is no way to prevent an exacerbation of their problems. People have to go outside and work, and they will suffer.”

For years, California has had the most polluted air in the country, with cities here dominating national lists of the dirtiest urban areas. And the Central Valley has had the worst air in the state for decades: More than 20 percent of all children in the Central Valley have asthma, according to the American Lung Association.

Many counties throughout the state and the San Joaquin Valley have successfully reduced the amount of ozone in the air. But levels of soot, or particle pollution known as PM-2.5, have started to increase after years of decline, according to state figures. Officials are blaming the drought.

At local asthma clinics, doctors have had an explosion in business since the drought began. Dr. Malik N. Baz said his asthma and allergy practice had grown at least 20 percent each of the past three years, and he has opened five new clinics around the Valley in the past two years — all of them overflowing with patients.