Two people have been treated for chemical burns after an incident at a Royal Mail depot in Hemel Hempstead.
Emergency crews, including the fire service were called to the site on Maxted Road last night, after corrosive liquid leaked from a package there.
Staff were evacuated - the incident's not being treated as suspicious.
Darren Cook, Assistant Chief Fire Officer at Hertfordshire Fire and Rescue Service, said: “Our firefighters are trained to handle a wide range of complex incidents, including chemical-related emergencies both large and small. Crews responded swiftly and effectively, and we’re pleased that the situation was resolved safely.”