A Student Loans Company office was evacuated after a member of staff opened an envelope and white powder came out.
Emergency services were called to the offices in Darlington, Co Durham, when an employee reported feeling unwell after opening the A4 envelope.
Durham Police said the staff member was "recovering well" following medical treatment at the scene. Police said the offices in the Lingfield Point business park were evacuated as a precaution while the powder was analysed and the incident investigated. Staff were later allowed back in the building.
Durham Police Assistant Chief Constable Michael Banks added: "This incident is under control and I must reassure people that we are not aware at this stage of any risk to the wider public."