Abstract: Utah Power & Light, the employer, did an air load test on an electrostatic precipitator after an extended shutdown. The testing was done on the weekend when the contractor was not on site. After the testing was completed, no one turned off the equipment. With no smoke load, the unit became a very efficient ozone generator. The contractor went to work Monday morning without checking the system. Shortly after the start of the work shift, employees started to experience acute respiratory problems. Employees #1, #2, and #3 passed out and were transported to a medical facility, where they were hospitalized. Three others showed chronic respiratory symptoms and were removed with the other employees to clean air.