It was first reported just before 11 am at the Fresno Housing Authority apartment complex on Blackstone at Illinois Avenue.
Fire officials say there were reports of an odd smell coming from the building.
Crews arrived and found two people vomiting and not feeling well.
One of them was taken to the hospital for treatment.
The hazmat team was called out, and four units had to be evacuated.
Officials believe fentanyl or other drugs could be a factor.