A security guard is believed to have had a substance sprayed in his face as he was attacked during a cash in transit robbery.
The man was injured with what is thought to have been an ammonia-type substance during the incident shortly after 1pm on Tuesday in Brighouse, Calderdale, West Yorkshire police said.
The male suspect made off with the cash box the security guard had been holding.
The victim has been taken to Calderdale Royal Infirmary for treatment.
No-one has been arrested and police said inquiries are ongoing in Brighouse town centre.