Police, firefighters and paramedics were called to the Barclays back next to Town Hall Dental in Briggate on Tuesday afternoon.
The police helicopter was also mobilised as officers closed off Bethel Street and diverted traffic.
West Yorkshire Police have confirmed rumours that a chemical was used in the attack, saying that an “ammonia type substance” was used in the attack.
A spokesperson said: “Police were called to Briggate, Brighouse at about 1.07pm today to a report of a cash in transit robbery.
“Officers attended to find a security guard had been attacked outside a bank by a male who then ran off with the cash box the guard was holding.
“It is believed the victim had an ammonia type substance sprayed in his face, and he is currently receiving medical treatment at Calderdale Royal Infirmary.
“Enquiries are ongoing in Brighouse town centre by Calderdale CID. Anyone who witnessed the incident is asked to contact Calderdale CID on 101 referencing police log 870 of 23/01.”