An unknown substance was thrown over the security guard in the attack.
The search for a suspect in relation to a robbery in Brighouse in January is being stepped up by police.
A male security guard had an unknown substance thrown at him in the attack outside a bank on Briggate, just after 1pm on Tuesday January 23.
He was taken to Calderdale Royal Hospital for treatment on what's being described as 'mild skin irritation' - and remained there overnight as a precaution.
The suspect, who escaped with a cash box being held by the guard at the time of the attack, is yet to be identified.
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."
Police are urging anyone with any information about the incident to contact Calderdale CID on 101, using the reference 13180037476.
Information can also be passed to Crimestoppers anonymously and in confidence on 0800 555 111.