The U.S. has made a lot of progress in the past few decades when it comes to air pollution. You only have to look at photos of Pittsburgh during the 1940s to clearly see that... But the more research we do on the health effects of air pollution, the more we realize that even very small amounts of it are more harmful than we thought.
The U.S. Environmental Protection Agency (EPA) is the government body that has the mandate to regulation air quality, via the seminal 1960's Clean Air Act. Part of that mandate is to review the standards every 5 years; the last time was in 2008, so we're overdue for an update.
That's not negligible! Imagine how much better things would be if we got even more serious about air quality and started truly designing our civilization (power plants, transportation, buildings, etc) around this issue.
Unfortunately, there is a lot of political resistance to the new standards - which, according to some medical and scientific sources, should be closer to 60 ppb - and it's not clear how easy they will be to get through.
But Republicans in Congress object to the proposed standards as too expensive for industry. Senator James Inhofe, an Oklahoma Republican who will chair the Senate Environment and Public Works Committee when Congress reconvenes in January, said, “EPA’s proposal to lower the Ozone National Ambient Air Quality Standard to between 65 parts per billion and 70 ppb will lower our nation’s economic competitiveness and stifle job creation for decades.” (source)
A lot of people have a vested interest in the status quo because tighter standards can mean redesigning products and machinery and installing anti-pollution devices and such, all of which are expensive (but so is the impact of air pollution -- how about the polluter pays rather than the innocent bystanders who are just breathing to live?).