Nearly every state has a different standard when it comes to auto emissions. Some are more lax, whereas others are quite rigorous. Depending upon what city you live in, you may or may not have to get these kinds of tests done. The Clean Air Act requires local municipal governments to enact testing standards in cities whose air quality level does not meet federal standards. Other states simply mandate periodic testing regardless of what city you live in. There are 28 states and the District of Columbia that order periodic emissions testing, and two states that demand testing upon the sale or transfer of any vehicle. As you can see, the majority of Americans have to submit to auto emissions testing at some point(s) in their lives. While these may seem like an unnecessary pain, these tests help control pollution levels to improve air quality. Cars release harmful toxins into the air each time they are used, which negatively impacts the entire planet. In fact, auto emissions are a leading contributor to global warming. If left unregulated, vehicle emissions would enhance what is already a serious problem worldwide.