That number is expected to grow exponentially after this summer, when the EPA's stricter standards take effect. Hundreds of communities will learn for the first time that they have a problem. Here's a primer on what you need to know. The standards EPA's current standards say an area cannot exceed 120 parts per billion of ozone for any one-hour period. Under the new standard, an area may not exceed an average of 80 parts per billion over an eight-hour period.
Ozone is a molecule made of three oxygen atoms linked together.
By itself it is odorless and colorless. It is not emitted from smokestacks and tailpipes, but forms from pollutants emitted by them. The pollutants, volatile organic compounds and nitrogen oxide, combine in the presence of sunlight and stagnant air to form ozone. Mix in sooty particles, known as particulates, and you have smog. High in the atmosphere, the natural ozone layer is good, because it filters out harmful ultraviolet rays.