Emissions laws change drastically depending on where you reside. Some states require all vehicles to pass a yearly emissions evaluation, but California requires cars, trucks and vans to pass a smog check biennially. Some states do not require emissions testing at all.

On days that are hot, pollutants emitted by vehicles, industry, and several products (nitrogen oxides and volatile organic compounds) react with each other to generate ozone, the main element of smog.