An air purifier unit at the Burnham plant caught fire Wednesday afternoon and kept fire crews busy for about two hours. According to the Falls township fire department, the blaze broke out just before 3:30 in the afternoon. The unit, which is located away from the main building keeps the air clean by collecting dust from the plant. Because it is in a different location, fire crews say there was not damage to the main building. Fire crews also say there were no injuries. They are still investigating why the air unit caught on fire.