When are fans safe or harmful during extreme heat?

Fans are likely safe up to 43°C (109°F) in young healthy adults, ~3 – 11°C (~5-20°F) above current guidelines from various public health agencies, based on findings from our recent publication.

During extreme heat events, avoiding heat is the best way to protect yourself. One way we avoid heat is by cooling the local environment with air conditioners, but their use comes with several negative consequences. While it provides benefits to those who have access to it, it exacerbates greenhouse gas emissions and urban overheating, worsening global warming. Concerningly, recent projections predict that ~4 billion people globally may lack air conditioning in 2050, which will disproportionately impact those from a lower socio-economic status. Thus, we urgently need an alternative, accessible, low-cost solution for protecting people from heat stress in our warming world.

A simple and market-ready technology for protecting individuals during extreme heat can be an electric fan. However, their use has been cautioned by various public health agencies globally, with some advising to turn them off when the air temperature is as low as 32.2°C (90°F).  This temperature threshold, and various others from different agencies, have little supporting evidence backing their advice. A 2012 Cochrane Review concluded that no evidence exists to “…support or refute the use of electric fans during a heatwave”. More recently, the World Health Organization revised their threshold for fans to 40°C (104°F). However, this threshold has been recently criticized too since it was largely based on biophysical modelling.

How does a fan work for human cooling? A fan is a powerful cooling solution when air temperature is less than skin temperature, however once air temperature exceeds skin temperature (for example, roughly higher than 35°C (95°F)), the fan begins to promote heat gain when you only consider dry heat exchange. However, that is only one of the ways we humans transfer heat. During hot conditions, we can counterbalance that dry heat gain with the secretion and evaporation of sweat on the skin surface to improve heat loss. Sweating is the largest physiologically modifiable way we lose heat in warm conditions, and for this mechanism to be effective, we need the sweat to evaporate off the skin. While evaporative efficiency is already high at a low ambient humidity, the benefits of a fan for reducing thermal and cardiac strain become more pronounced in humid conditions as fans enhance evaporative efficiency compared to still air. This is why a fan can be safe or beneficial at temperatures higher than skin temperature (higher than 35°C (95°F)) in comparison to still air, despite the added dry heat gain. However, at some air temperature, the rate of heat gain from a fan will exceed what we can lose via evaporation, making the fan harmful, especially in low humidity conditions.

At what air temperature are fans not safe? To answer this, we simulated a 3-h indoor overheating scenario that would mimic the temperature rise during an extreme heat event, specifically at the point when air temperatures would exceed skin temperature. We purposefully chose dry conditions (~25%RH) as we know from previous work in young adults that fans are beneficial at high humidity and detrimental in hot dry conditions, and modelling suggests for a given air temperature a fan can be harmful or safe at low humidity, but beneficial as humidity rises.

Our objective was to find the indoor air temperature where the electric fan will no longer be safe in a dry indoor environment, defined as greater rises in heart rate and core temperature relative to still air. Contextually, a single air temperature as a metric is easy for people to understand and measure in their homes (for example, using thermostats or basic thermometers) and is how the current threshold is defined in public health guidance.

Our Key Findings:

• Fans are safe… up to a point. Fans were safe until the room temperature hit ~43°C (109°F). That’s 3 – 11°C (5-20°F) higher than suggested by various heat health guidance globally.

• Heart stress came first. The body’s cardiovascular system began to show signs of strain with fan use starting at 44°C (111°F), before core body temperature began rising at 45–46°C (113-115°F).

• More sweat, but manageable. People using fans produced more sweat, but the extra fluid loss could be offset with about 250 mL (1 cup) of water per hour at 43°C (109°F), a manageable amount.

Our findings parallel what has been shown in longer duration heat exposure studies (more than 2 hours) at various ambient temperatures, too.

In conclusion, we suggest that for young adults at rest, the added heat gain from a fan can be managed by the evaporation of sweat from the skin surface up to an indoor air temperature of ~43°C (109°F). The fan will be safe at low humidity, and be beneficial at higher humidity, like we have previously shown at 42°C (108°F). We didn't test below 25% relative humidity at 43°C (109°F), but it is unlikely to be that dry indoors as various items in the home already increase ambient water vapor pressure, including you.

What do we not know?

• These results are only for young healthy adults. More work is needed in older adults and people with various health conditions.

• We only tested people at rest. Doing light physical activity may change the results.

• The air speed used was extremely high (~5 m/s) — stronger than what many household fans can produce. Lower fan speeds may be even safer.

• We used a highly controlled environment. More work is needed in more ecologically valid conditions to truly determine the safety of fans in high-heat stress scenarios.

Reference: Emily Mihalcin, Stefano Schiavon, Nicholas Ravanelli. 2025. Examining the physiological strain with electric fans during high indoor heat stress. Building and Environment: 113261. Open access https://doi.org/10.1016/j.buildenv.2025.113261.