The bus was at West Warren and Grandmont avenues when a male passenger pulled out powder from his pocket and threw it on the bus. He had gotten into an argument with the bus driver.
The man, who was wearing a mask, then ran off the bus.
The driver and four passengers were quarantined on the bus while police investigated. Police and firefighters treated it as a hazmat situation strictly as a precaution.
Police are searching for the man who threw the powder.