Lehigh Cement Company's (Lehigh) Mitchell, Indiana facility violated its Title V permit limit for carbon monoxide (CO) at two of its three cement kilns. The emissions exceedence was discovered during required stack testing of the kilns exhaust gases. U.S. EPA discovered the emission limit exceedences during a Clean Air Act (CAA) compliance inspection three months after the testing was conducted. A violation notice was subsequently issued to Lehigh. Lehigh conducted a testing investigation at the kilns to determine the source of the CO emissions. Lehigh concluded that the use of foundry sand (a supplement to the raw materials) was contributing to the generation of CO. Through an agreed upon testing program, Lehigh will conduct stack tests with varying quantities of foundry sand to determine the upper usage limit of the feed supplement while complying with the company's Title V permit limit. Once determined, a foundry sand usage limit will be established in Lehigh Title V permit.