Students at Dundas Valley Secondary School were evacuated Tuesday afternoon over a "haze" emanating from a washroom, the Hamilton Fire Department says.

When firefighters got there around 12:17 p.m., the school's students were already outside.

Fire crews found there was a "haze" coming from inside a first floor student washroom that they couldn't identify.

"Firefighters were informed that the haze was caused by water coming in contact with a powder in the washroom sink," a news release reads. That meant the hazmat team had to be brought in to investigate.

Tests showed there were no hazardous materials present, and students were allowed back inside at 1:57 p.m.