You can look at air quality in different ways. A number of pollutants have seen dramatic decreases since the 1970s. With sulphur dioxide, we've been extremely successful in reducing that pollutant. Ozone is a problem. It has stayed more or less the same, or in some cases has been increasing as an issue.
The key thing about air pollution is that we know a lot more than we did before; we know a lot more about its health effects. So there's the issue of how bad it is and the issue of how bad we know it is.
But for a number of pollutants, yes, if you look at the measurements, several pollutants have decreased since the 1970s. The pollutants of greatest focus in terms of smog are PM and ozone. PM has been more or less flat, though it depends on what part of the country you're in. So all of these things vary.