A bus had to be evacuated when its driver was sprayed by a fire extinguisher.
Emergency Services were called to reports of a powdered substance being thrown at a bus in Whitton Road, Hounslow, at 11:00 BST.
Police say two people on a moped discharged the fire extinguisher into the bus. No arrests have been made.
The driver is being treated at the scene. His injuries are not life-threatening or life-changing.