Work has ground to a halt on the French superhospital for the third time in two weeks.
Quebec's workplace safety board (CNESST) closed the CHUM worksite on Thursday because of excessive heat.
On Friday morning the board told workers to go home, saying the results of air quality tests showed it was not a safe location.
About 2,600 employees will not be allowed to return until the air quality improves.
Several construction workers were treated for heatstroke on Thursday, similar to what happened last week, even though the employer has installed extra fans and other cooling devices on site.
