To be honest, the emissions of individual cars are relatively low compared to other sources of pollution. But when thousands of people sit in the car and pack highways into capacity, the cumulative impact is huge. When you are sitting in a traffic jam, gazing at the smoke coming out of the tailpipe in front of you and rising to join the smog in the sky and think about what the smoke is. It may not look like a lot, but the smoke has gone a long way since it just launched the car's engine.
Concern about our automobiles' effect on the environment is nothing new. The 1970 version of the Clean Air Act gave the US Environmental Protection Agency extensive control over vehicle emissions. As technology advances, EPA drives more stringent standards. Although new cars are cleaner than they were 40 years ago, people tend to travel longer distances and tend to be more frequent, contributing to a lot more pollution. The Clean Air Act was revamped and increased in scope in 1977, and then again in 1990.
Automotive manufacturers and parts suppliers have had to keep pace with these changes, and as you can imagine, a car's exhaust system has had to evolve considerably, too.