Lehigh Portland Cement Co., Union Bridge, Maryland A mysterious gas with a bleach-like odor occasionally was encountered during kiln refractory and maintenance outages at Lehigh Portland Cement's Union Bridge operation. The Joint Safety & Health Committee and National Institute for Occupational Safety and Health industrial hygienists researched the problem on-site using elaborate testing equipment. The team safely recreated conditions to produce the smell, identify the agent, and measure its concentration. Ozone, a powerful oxidizer and irritant to the eyes and mucous membranes of the respiratory tract, was identified as the problem gas, generated by corona discharges in electrostatic precipitators. If that equipment is left energized or power-tested during maintenance, ozone can back-draft into the kiln and leak out of open equipment doors. The plant's Confined-Space Entry permit procedure was modified to ensure precipitators are de-energized, locked out, and positively ventilated. An ozone monitor was purchased for use before and during all kiln system entries, and is employed in conjunction with atmospheric testers that measure oxygen, carbon monoxide, explosive gases, and gaseous products or combustion. Hazard training was provided to all employees to recognize ozone and its effects.