BOSTON -- Hazmat crews were called to a South Boston building on Thursday after a green powder was found in an envelope.

Boston Fire's Steve MacDonald said a worker at the Massachusetts Rehabilitative Commission office on the fifth floor of 27 Wormwood St. opened an envelope and powder fell out.

Three of the building's six floors were evacuated.

EMS treated one person for exposure to the unknown substance, and 10 other people in the area were being evaluated.

A decontamination unit was also called to the scene.