Despite dramatic population growth, the Bay Area’s air quality has improved significantly over the past three decades thanks to cleaner motor vehicles and fuels, air-pollution control measures implemented by industry, and California’s vehicle inspection and maintenance program. In fact, the region enjoys cleaner air than most major metropolitan areas in the nation. However, the region continues to fall short of state standards for some pollutants. Plus, the Bay Area, along with the rest of the nation, is producing more climate-changing greenhouse gases. Both problems are directly linked to our escalating car use.

According to the Bay Area Air Quality Management District, motor vehicles produce nearly half of the emissions that combine to produce ozone, the principal component of smog. Cars are also a major source of carbon monoxide, fine particulates and certain toxic contaminants that, along with ozone, are detrimental to human health.

Cars also produce a significant amount of carbon dioxide, which contributes to global warming. However, unlike air pollutants, carbon dioxide can’t be removed easily or inexpensively by attaching emission-control devices to vehicles. The only way to cut carbon dioxide is to improve fuel economy, reduce the number of vehicles on the road, or switch fuels, none of which have much popular support.

To the extent that our land use increases—or decreases—auto dependence, our air quality and the world’s climate will be affected. Bay Area homes, job sites, schools, shops, and services are frequently scattered over great distances. They may be bordered by vast parking lots and wide, high-speed roadways that make public transit, walking, and cycling between sites impractical. Newer developments are often built at densities too low to support frequent, convenient transit service. It is clear that the way we choose to accommodate population and economic growth will have a profound effect on future air quality.