The World Health Organization advises people to not rely too much on electric fans when temperatures rise above 35°C (95°F) and the US Environmental Protection Agency (EPA) strongly advises against them at temperatures over 37°C (99°F).

Researchers from the University of Sydney were curious to know if these guidelines had any scientific backing. In a small study involving 12 healthy male volunteers, they found that the advice appears to be too clean-cut and that the actual picture is more nuanced. The team found that in environments that were very hot and humid, fans were more beneficial that in dry and hot environments. The findings are reported in the Annals of Internal Medicine.

The team used a measurement known as the heat index, which combines air temperature and humidity to provide a parameter for what a temperature “feels like” to humans. The EPA states that using electric fans when the heat index is higher than 37.2°C (99°F) can actually increase the heat stress a person is experiencing.

The data collected from the 12 participants showed that in hot, humid conditions, fans were still effective at reducing core body temperature up to a heat index of 56°C (133°F). When the heat was dry, however, the cut-off point was much lower at a heat index of 46°C (115°F).

“Our results suggest that under environmental conditions that represent the vast majority of peak heatwaves in the United States and Europe fans should be recommended and the guidelines issued by most public health authorities are unnecessarily conservative,” senior author Associate Professor Ollie Jay said in a statement. “It is only when the air temperature is very high and humidity is very low that fans are detrimental, which can be seen in arid conditions such as Phoenix or Las Vegas in the US, or Adelaide in South Australia.”

The 12 men that took part in the study were placed in a room for two hours where the conditions of certain past heatwaves were recreated. The July 1995 Chicago and July 2017 Shanghai heatwaves served as models for the hot and humid scenario. Meanwhile, the California heatwave of July 2018 was used to create the hot and dry scenario.

Evidence that fans can be used at higher temperatures is good news for the many that can’t afford air conditioning but the team stresses that this is a limited study and larger ones are necessary to confirm the results.