The cold winter air may be crisp, but it's not clean.

For the second time this season, the Sacramento Metropolitan Air Quality Management District is asking the public to drive less and refrain from burning wood.

A "spare the air" advisory is in effect through tonight because a large mass of stagnant air has particulate pollution trapped over the region. Breathing air fouled by particles can lead to respiratory problems and heart disease.

Air district staff are concerned that freezing nighttime temperatures may drive people to warm homes by building fires.