The geography of smog

The U.S. Environmental Protection Agency wants to tighten its standard for ozone, the lung-damaging gas in smog. It’s considering a range of 65 to 70 parts per billion, any part of which would put a significantly broader swath of the country out of compliance, requiring government agencies and industry in those places to step up pollution-control efforts. This map shows which counties measured ozone levels below and above the proposed limit, including which areas topped the current 75 ppb standard, during 2011-2013. Click on counties for more information.

Ozone levels in parts per billion

65 ppb or less
Between 66 and 75 ppb
Greater than 75 ppb

Source: U.S. Environmental Protection Agency