A Montreal-area hospital has been warning staff and patients not to drink unfiltered tap water since Monday, after mysterious black particles were found by an employee.

The Sainte-Justine hospital said testing done in conjunction with the city and a local university has shown the water is not toxic or dangerous, but there is still no known cause.

Medical procedures like surgeries are unaffected because those use sterilized water, but patients and staff are being asked to drink only filtered or bottled water.

An update on the situation is expected Friday afternoon.

With a report from CTV Montreal’s Annie DeMelt