Particles are everywhere, but some kinds cause more harm than others. Larger particles are less likely to cause health problems, because they can't make it through our nasal passages and upper airways. But smaller particles can evade the respiratory system's defenses. In general, particles less than 10 microns in diameter pose this problem. (A micron is smaller than you might imagine -- the period at the end of this sentence is hundreds of microns across.)
These small particles lodge in our lungs and can cause short-term effects, like wheezing and coughing, as well as chronic respiratory problems. People with asthma have worse symptoms, and take more medications, when particle levels are high, and emergency-room visits for respiratory complaints also increase. Breathable particles are especially harmful to elderly people and those with existing respiratory or cardiovascular problems. Recent studies show that chronic exposure to breathable particles is associated with a higher rate of death from respiratory and cardiovascular disease.
Sources of Particle Pollution
Combustion is a major cause of breathable particles. In the Bay Area, vehicles and wood burning are the major combustion sources. (In other regions -- the Central Valley, for example -- forest fires and agricultural burning are also big contributors. But wind patterns blow pollution away from the Bay Area, not toward it.)
Particle pollution is usually at its worst on winter days with little wind. The main offender in these cases is household fireplaces. On a still day, the particles in smoke from just a single fireplace can cause serious problems for neighbors with asthma or other respiratory diseases.
At other times of the year, vehicles are the principal source of breathable particles. Although both gasoline and diesel vehicles emit particles, diesel buses and trucks emit much more for each mile they're driven. Diesel engines account for approximately 79 percent of the particle pollution from all on-road vehicles. More than 98 percent of the particles they emit are less than one micron in diameter, way below the level at which particles lodge in the lungs.