Colder temperatures in fall and winter draws friends and families to the warmth of the fireplace, but maybe not in San Francisco anymore--not on nights with high smog pollution, at least.

A new proposed ban on wood burning fires on high smog days in the San Francisco Bay area highlights a surprising source of air pollution. Because wood fires are a natural way to heat the house, most people don't associate them with bad air quality. But burning wood increases pollution by throwing particles in the air, especially inside the house since the particles have no way to escape.

In fact, research shows wood smoke is just as harmful to lungs as cigarette smoke and can cause both respiratory and cardiac problems. The harmful effects of an indoor wood burning fireplace are, however, lessened if you follow steps like cleaning your chimney clean and burning only clean, dry, seasoned hardwood.

Residents of San Francisco and other areas in the state are currently asked not to use wood burning fireplaces on days with bad smog levels, but the Bay Area Air Quality Management District is calling for a regulation that would allow fines to be levied against offenders. Instead of the fine, residents who violate the ban for the first time could choose to attend a two-hour school about the effects of wood fires on air pollution. The district will give the public the chance to voice their opinions about the proposed ban in workshops over the next several weeks. Currently, Sacramento County, the San Joaquin Valley and some Bay Area cities already have similar bans in place.

According to the board, the new regulation would result in banning fireplace use on approximately 20 nights a year.