The ozone indicator in EJScreen reflects potential ozone exposure measured in terms of summer seasonal daily average maximum concentrations in an 8-hour period measured in parts per billion. Ozone information included in EJScreen highlights areas across the U.S. that are not meeting the national ambient air quality standard for ozone. In other words, the levels of ozone present in these areas are deemed harmful to human health. The ozone indicator in EJScreen is a measure of potential exposure but not a measure of risk. The raw ozone data is compiled by census tract, which is supplied for use in the tool by EPA’s OAQPS. For air toxics risk measures (as opposed to exposure) users can turn to EJScreen’s other three indicators: cancer risk, respiratory HI, and diesel PM.