The hazardous substance was reported when it leaked from a package at the sorting office in Maxted Road, Hemel Hempstead industrial estate, at around 9.21pm yesterday (September 24).
Herts Fire and Rescue sent crews from the Hemel Hempstead, Berkhamsted, and St Albans and firefighters contained the chemical.
Staff were evacuated and the East of England Ambulance Service treated two people for chemical burns.
It is being treated as non-suspicious and no criminal investigation has been required.
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Herts Fire and Rescue assistant chief fire officer Darren Cook 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.”
Herts Fire Control confirmed that the chemical had been contained at 11.22pm.