The Environmental Protection Agency (EPA) is approving, under the Clean Air Act (CAA), a revision to Indiana's State Implementation Plan (SIP) to address changes to its air emissions monitoring rules for Portland cement plants. Indiana revised its rules for Portland cement plants to update the monitoring of particulate matter (PM) emissions to allow an additional monitoring option. This additional monitoring option is consistent with EPA's recent revisions to Federal requirements for Portland cement plants. EPA proposed to approve this action on March 25, 2021 and received no comments.