The workers at the Stobhill Depot in Gorebridge, Midlothian, were checked by ambulance crews after they came into contact with a bag containing a strong-smelling substance.
An investigation by specialists found it was ammonia chloride sulphate and posed a low to medium risk, similar to a cleaning product. The centre was closed following the incident but later reopened.
A spokeswoman for Midlothian Council said: "We take these types of incidents very seriously and as soon as they occur we work with our colleagues in emergency services to put our contingency plan into place. We would like to thank everyone for their patience while the fire service undertook their investigations. Thankfully it turned out to be low risk with no health implications."