A teenager stole a nitrous oxide gas canister from an ambulance while it was parked in Abbots Langley.
The theft happened just before 10pm on Tuesday (August 25) in Tibbs Hill Road while paramedics were tending to a patient.
Nitrous oxide is a form of pain relief for patients but is also the chemical used in ‘laughing gas’ - a dangerous legal high popular with revellers that can cause death through asphyxiation.
The teenage man, aged between 16 and 19 and described of slight build and wearing a light grey hoodie and light coloured trousers, made off with the canister down a nearby alleyway.
Police are appealing for anyone who saw the incident or may have information about the theft to contact officers via the 101 number quoting crime reference number C2/15/2011.
Alternatively contact Crimestoppers, anonymously, on 0800 555 111 or through their anonymous online form at www.crimestoppers-uk.org.