Arsonists set fire to public toilets in St Neots on Tuesday.
Cambridgeshire Fire and Rescue Service crews were called to a toilet block in Tebbuts Road at about 6.45pm.
A spokesman for the fire service said: “The toilet block was heavily smoke logged and two firefighters wearing breathing apparatus used a hose reel to extinguish the fire.
“A thermal imaging camera was used to check for hot spots and positive pressure ventilation fan cleared smoke.”
Crews returned to station by 7.30pm.
An investigation revealed that the cause of the fire was deliberate.
Anyone with information should contact police on 101 or Crimestoppers anonymously on 0800 555111.