Think smog. This is what it looks like, the agency says, and I count six such advisories during July that included Milwaukee County:
Smog over the city of Milwaukee.
Ozone is a gas that occurs in both Earth’s upper atmosphere and at ground level. When ozone is at ground level, it is a major component of smog and can cause a number of health problems for humans and other living things. Because of this serious health risk, the DNR monitors ground-level ozone around the state and issues air quality advisories when levels become potentially harmful. Counties that do not meet federal clean air standards for ozone require businesses and residents in those areas to take special measures to reduce ground-level ozone.