Air pollution can be both natural - such as fires or volcanic eruptions - and man-made. Most of the air pollution in the Bay Area is man-made, and results from industrial processes and everyday activities like driving, boating, using household products or lawn and garden equipment, burning wood, painting, barbecuing, etc.
In the summertime, ground-level ozone causes the most serious air quality problems. Commonly referred to as "smog", ozone is a gas formed when sunlight reacts with oxides of nitrogen and volatile organic compounds emitted by cars, buses, trucks, construction equipment, gas stations, refineries, and many other sources.
While some ozone is produced every day, cooler temperatures and steady winds normally keep it from accumulating to unhealthy concentrations. However, on long, hot, stagnant days, ozone can build up to levels that violate federal and state health-based standards.
In the wintertime, especially between the months of November and February, particulate matter, or soot, is the major air quality concern. Tiny particles, either solid or liquid, can be produced by a variety of natural and man-made sources. Particles less than 10 micrometers in diameter can cause significant health problems, with those under 2.5 micrometers, known as "fine" particles, considered to be the most dangerous. These latter are produced by any kind of combustion (motor vehicles, wood burning, power plants, etc.) and some industrial processes.