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Air quality regulators have banned wood-burning fires on eight of the past fifteen days. Last year, there were only four such "Winter Spare the Air" alerts all season. 

One reason for the increase?

“We don’t have any rain,” said Jennifer Jones, a public information officer for the Bay Area Air Quality Management District, which issues the alerts.

“In general, storms bring good air quality, because they mix air that’s closer to the ground with air that’s high up,” Jones said. “It helps dilute or disperse pollution."

One Bay Citizen reader wondered why regulators are so concerned about wood smoke in the winter.

“I actually saw a park ranger make a group of people put out their Ocean Beach bonfire last week while a gazillion cars sped by on the Great Highway and everywhere else,” Maxime Stadlen wrote in a comment on a Quality of Life blog post.

“Taking care of our environment is the utmost priority to me, but seriously, is a ban on wood fires on selected days in SF a solution?”

Wood smoke is the largest source of wintertime air pollution, regulators say.

“In the wintertime, we’re more concerned about particulate matter,” Jones said. In winter, the smoke from fires accounts for 39 percent of that pollution in the Bay Area, while only 7 percent comes from vehicles.

In the summer, smog is a bigger problem. It occurs when the chemicals emitted from the exhaust systems on cars and trucks react with warm temperatures. “Without those high temperatures, [smog is] not really a concern in the winter time,” Jones said.

When the weather is dry and cold, and there is little or no wind, the small particles from wood-burning fires become trapped under a layer of warmer air in the atmosphere. This particulate matter can irritate the lungs, increasing the risk of serious health problems like pneumonia and blood clots. The elderly and children are especially at risk from such pollution.

So far, this month has been third-driest December on record in San Francisco, with just .12 inches of rain, according to the National Weather Service. In 1876 and 1989 there was no rain in December in San Francisco.

The northern Sierra Nevada, where most of the state’s water comes from, is seeing the second-driest December since records were first kept in 1921, according to Maurice Roos, a scientist with the state’s Department of Water Resources. "We would be concerned if the month comes in dry, but it's not the end of the rainy season," he said. December, January and February are typically the wettest months. "The odds are that things will improve as we go further into winter."

The current weather conditions are expected to hold through next Tuesday, according to Diana Henderson, a forecaster for the National Weather Service. “It’s pretty much mostly clear and sunny for the next several days, at least through Tuesday,” she said. “Mornings are going to be quite chilly.”

In the Bay Area, violators of a Winter Spare the Air alert no-burn edict may receive warning letters for a first-time offense with a $400 fine imposed for an additional violation.

Since the program began in 2008, the Bay Area Air Quality Management District has issued only 12 of the $400 fines, but it has sent 623 warning letters.

This season, the district has received about 1,400 complaints about wood smoke on Winter Spare the Air days. It is currently investigating 200 of those complaints, according to Jones.

There is some evidence that the wood-burning bans are having an effect: about 20 percent of Bay Area households with stoves or fireplaces have reduced their wood burning because of the Winter Spare the Air program, according to a survey by the Air District published in March.