West Yorkshire Police today appealed for information after the offence on Tuesday, January 23 shortly after 1pm.
A male security guard was attacked by another male outside a bank on Briggate with a substance that is still to be identified, police said, before fleeing with the cash box that the guard was holding at the time.
The victim was taken to Calderdale Royal Hospital for treatment for his injuries which were described as "mild skin irritation". He remained in hospital overnight as a precaution.
Detective Inspector Gary Stephenson of Calderdale CID said: “Extensive enquiries in to this incident are continuing. Whilst the victim did not sustain serious injuries, clearly the incident was incredibly distressing for him, and people who witnessed what took place.
“We have spoken to a number of witnesses already who were around the location when the incident took place, but would ask anyone who has not yet spoken to the police who has any information to come forward.
"I would seek to reassure the public that this appears to have been an isolated, targeted incident, however we always would encourage anyone with any concerns to speak to their local neighbourhood policing team.”
Anyone with any information is asked to contact Calderdale CID via 101 using the reference 13180037476. Information can also be passed to Crimestoppers anonymously and in confidence on 0800 555 111.