The nine counties that surround San Francisco Bay are home to almost seven million residents and an estimated 1.7 million fireplaces and woodstoves. The particulate matter (PM) in the wood smoke from these fireplaces and wood stoves has been a health concern in the Bay Area for many years.
Particulate matter is a mixture of solid and liquid particles in the air. The smaller-sized particles – those 10 micrometers or less in diameter (PM10) – are of greatest health concern because they can pass through the nose and throat and lodge deep in the lungs. Included in PM10 is a subset of very tiny particles 2.5 micrometers or smaller (PM 2.5), sometimes called “fine” particles. For comparison, a particle 10 micrometers in diameter is about one-seventh the diameter of a human hair.
Since the 1980’s, many scientific studies have been published that correlate rising PM levels with serious health effects, such as asthma symptoms, decreased lung function, increased hospital admissions and even premature death.
In response to these concerns and the proliferation of wood heaters in the 1970’s and 80’s, the U.S. Environmental Protection Agency (EPA) set a national PM emission standard for woodstoves at 7.5 grams per hour. Since July 1, 1990, all woodstoves manufactured in the United States have been required to meet this EPA standard. Previously, unregulated woodstoves averaged 60 grams of PM in an hour.
Under certain meteorological conditions –
cold, stagnant winter evenings – surface based radiation inversions
form quickly in the Bay Area and PM levels rise rapidly. By the 1980’s,
wood smoke became the largest area-wide stationary source of particulate
matter in the Bay Area. Studies by the Air District indicated that wood
smoke was responsible for an average of one-third of the PM10 in the air
basin during the winter months and almost 70 percent of the PM10 in Santa
Rosa. In addition, wood burning generates carbon monoxide and toxic air
pollutants such as benzene and dioxin.
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