In a June 8 Federal Register publication, the EPA proposed to retain the existing primary health-based NAAQS for sulfur oxides, a group of compounds that include sulfur dioxide (SO2). These NAAQS, set in 2010, are 75 parts per billion of daily maximum SO2 concentrations averaged over three years. This proposal comes at a time when the agency is already facing legal challenges to its decision concerning ozone NAAQS. EPA is accepting comments on this proposal until July 23.