Chlorofluorocarbons are reducing this layer, but that is a different issue. Ground-level ozone pollution is more commonly called smog. Ozone days occur during the long sunshine-filled days of late spring, summer, and early fall. Since ozone formation requires sunlight, ozone is not produced until after the sun rises. Ozone causes breathing problems and can cause asthmatic episodes.
Ozone also causes harm to some crops.
Recently, the EPA set a new eight-hour health standard of 0.08 parts per million to replace the previous standard after determining that human health was not adequately protected. All states are required to attain the new ozone standard. Unfortunately, some states will not be able to attain the new ozone standard, partly because of pollution not originating in their state but from other states. The EPA determined that about 20 upwind states were significant contributors to the ozone problem of northeastern states. If those states reduce their NO x emissions, downwind states should be able to attain the standards.