Published Tuesday, November 18, 2003 in the San Jose Mercury News

Weather Corner

Not just blowing smoke: Old fireplaces, wood stoves belch dangerous pollutants
Special By Jan Null

On these cool fall nights it's tempting to light a fire in the fireplace to take some of the chill off. But this year, you might breathe a little easier if you don't.

Under the right meteorological conditions, smoke particles from fireplaces and wood stoves can make the area's fall and winter air unhealthy to breathe. And just as there are Spare the Air Days in summer, when local air quality officials feel unhealthy conditions may develop because of high pollution concentrations, the cooler seasons bring Spare the Air Tonight advisories.

In summer, the key air pollution problem is ground-level ozone, or smog, formed when sunlight hits chemicals in emissions from cars, factories and other sources. With its harmful health effects, ground-level ozone is referred to as the ``bad ozone'' -- set apart from the naturally occurring ``good ozone'' in the stratosphere that protects Earth by blocking ultraviolet light.

Winter brings a new pollution problem in the form of particulate matter, or PM. Particles 10 micrometers or less in diameter (PM10), and a subset of even smaller particulates that are tinier than 2.5 micrometers (PM2.5), are categorized by the U.S Environmental Protection Agency as unhealthful when they are concentrated in the atmosphere and are small enough to pass through the human respiratory system and lodge deep in the lungs. Increases in PM10 and PM2.5 have been linked to serious health effects such as asthma symptoms, decreased lung function, increased hospital admissions and even premature death.

These solid and liquid particles result from man-made and natural sources, including all types of combustion -- such as motor vehicles, power plants and wood burning -- and some industrial processes. In the Bay Area, a major source of PM is from wood smoke that emanates from older fireplaces and wood stoves.

Particulate matter builds up on cold, stagnant evenings when there is an inversion that traps cold air near Earth's surface. In the Bay Area, about 30 percent of the PM comes from wood smoke, largely because of older unregulated stoves and fireplaces of a 1970s or 1980s vintage that expel about 60 grams of particulate matter an hour. Newer models are required to produce less than eight grams an hour, and some models emit as low as two or three grams an hour.

To help reduce the wintertime PM problem in Santa Clara County, the Bay Area Air Quality Management District is offering as much as $500 in rebates when a wood-burning appliance is replaced with one that uses natural gas. The funding for this program comes from the new Calpine-Los Esteros Power Plant in Alviso and a planned power plant in Santa Clara as part of their air quality mitigation programs. More information can be found at

When stagnant conditions are forecast, Bay Area residents are urged to minimize automobile use and to refrain from burning wood in fireplaces and old-fashioned wood stoves. See for the latest conditions.

Q.  I understand that on the equinox, day and night are equal. So as the equinox approaches, I keep an eye on the ``hours of daylight'' on the Mercury News weather page. On Sept. 22 I expected that night would exceed day, but that didn't happen until Sept. 27. Was this a series of misprints in the paper? If not, perhaps you can explain. Lyle Settle - San Jose

A.  This is a common question around both the fall and spring equinoxes. The answers lies in different methods used to calculate the equinox and sunrise and sunset.

The equinox occurs when the geometric center of the sun's disc crosses the equator. On this day, the center of the sun is above the horizon -- everywhere on Earth -- for 12 hours.

But sunrise is calculated from when the upper portion of the disc first becomes visible. Sunset is timed when the upper edge of the sun's disc slips below the horizon. Therefore, day and night aren't exactly equal on the equinoxes.

In the Northern Hemisphere, the date of equal day and night is several days before the March equinox and several days after the September equinox. At our latitude of about 36 degrees north, night and day are equal about March 16 and Sept. 26.


Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o WeatherCorner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone questions at (510) 657-2246, fax them to (510) 315-3015 or e-mail, or fill out a form online at Please indicate in your e-mail what city you live in.