There was a time when air pollution was a much more visible problem, with smokestacks belching brown soot into the air. These days, pollution is more subtle, but still a big concern. Near the top of the list is ozone, a colorless, odorless gas that can irritate your respiratory system and aggravate asthma and other diseases.

Unlike many other pollutants, ozone is not emitted into the air. Instead, ozone forms when other pollutants already in the air chemically react in sunny, warm weather. Some of these ozone-making ingredients come from the burning of fuels, so emissions from vehicles do set the stage for ozone to form. The other pollutants that help form ozone include the vapors you see and smell when working with paint thinner or when you fill up the gas tank. When a stew of pollutants like these bakes in the air on a sunny, hot day, ozone pollution can build to unhealthy levels.

You may be surprised to learn that all of Delaware, New Jersey, and southeastern Pennsylvania, and much of Maryland, don't meet the federal health standard for ozone pollution. And and it's not just a city problem. Unhealthy ozone levels occur regularly in suburban and even rural areas.