Public health guidelines that recommend turning off fans when temperatures exceed 35 degrees are likely flawed and may unnecessarily expose vulnerable people to health effects during heatwaves, researchers at the University of Sydney say.

Ollie Jay, director of the university's Thermal Ergonomics Laboratory that simulates a range of heat extremes and how humans respond, said advice in many jurisdictions for coping with heatwaves was "not really based on a lot of scientific evidence".

By adjusting the laboratory's special heat room – originally set up three decades ago by the late Professor Martin Thompson to help athletes on hot days – the researchers have been able to study how people handle precise heatwave conditions.

These include mimicking a 2009 Adelaide heatwave when the mercury climbed to 48 degrees with humidity of just 5-6 per cent, and a 2015 "heat dome" over the Pakistani city of Karachi when residents endured 45 degrees at about 50 per cent humidity.