Windshield washer fluid may contain bacteria that cause the deadly form of pneumonia known as Legionnaires’ disease, according to researchers who say the study is the first to detect high levels of Legionella in automobiles or aerosolized by washer fluid spray.

Legionella bacteria, which are naturally occurring and typically found in water, are transmitted to people through mist or vapor from water containing the bacteria. For example, air conditioners and hot tubs can be sources of infection. The bacteria do not spread from person to person.

Most people exposed to Legionella bacteria don’t become ill, but some develop Legionnaires’ disease. The bacteria can also cause a milder illness similar to the flu.

Researchers found Legionella bacteria in the windshield washer fluid of 75% of school buses they tested in one central Arizona school district. The investigators also discovered that Legionella bacteria can grow in windshield washer fluid and maintain stable populations in the fluid for up to 14 months.

“Washer fluid spray can release potentially dangerous numbers of these bacteria into the air. These results suggest that automobiles may serve as a source of transmission for Legionella infections,” study author Otto Schwake, a doctoral student at Arizona State University, said in a society news release.