Four people were given medical checks after breathing fumes from a liquid which leaked at a Royal Mail building.
Firefighters were called to the delivery office in East Square in Basildon at about 15:00 BST on Wednesday.
Three people went to hospital after falling ill and a fourth presented themselves for treatment to paramedics.
The fire service said the chemical involved was a diesel fuel additive called Cetane.
A fire service spokesman said firefighters put on breathing equipment, went into the building, sealed the package and ventilated the building.
All of the staff affected were given the all-clear by medical staff.