Some parts of the United States are hitting temperatures "too hot for safe fan use" twice as often as they did decades ago, new research shows.

Analyzing hourly weather data from the past 20 years and between 1950 and 1969, Luke Parsons, a climate scientist at Duke University, and colleagues found that more US residents are being exposed more often to temperatures that are too hot for electric fans to cool people down – and might even be dangerous.

"We find that the geographic extent of temperatures too high for safe fan use is expanding, and the number of safe hours is decreasing," Parsons and colleagues explain in their published paper.

"In particularly hot locations, most afternoon hours during the hottest months of the year can exceed [safe] thresholds," they add.

The study, although focused only on the US, provides new insights at a time when much of North America, Europe, and China has sweltered through extreme summer heat made more likely by climate change.

It also follows efforts from scientists to refine estimates initially set by the World Health Organization of conditions where fans might exacerbate heat stress, pushing more hot air onto people than is wicked away.

Electric fans, research now shows, can still be safely used above 35 °C (95 °F) but their limit depends on who is using them and where.

In exceptionally humid or arid areas, electric fans tend to add to heat stress. Their use is also not recommended above 37-38 °C for older adults, especially those taking medications that have side effects of reduced sweating. Young, healthy adults can keep using fans up to 39 °C.

With those new recommendations at hand, Parsons and colleagues combined population data and temperature records to map out heat exposure across the continental US.

As a cooling strategy, electric fans are more affordable than air conditioning and use up to 50 times less electricity; helping to reduce power demand during heatwaves.

However, many locations across the US and northern Mexico "already experience ambient climate conditions that exceed the recommended temperature thresholds for safe fan use, even for the least sensitive demographic of healthy, younger adults," Parsons and colleagues write.

Hotspots of concern include inland areas of California along its border with Arizona, much of central Texas and Oklahoma, and along the US-Mexican border through Texas.

Communities in those locations now face more than 1,000 hours or over 44 days per year where temperatures are too hot for safe fan use. That's 200 more hours – or about 8 additional days each year – where temperatures are dangerously hot for fan users compared with 50-70 years ago.

Some of the hottest places were rural areas, where healthcare options may be thin on the ground. Low socioeconomic areas have also experienced the fastest increases in temperature extremes over time.

"These results highlight the need to direct resources to some of the most vulnerable communities who are the most impacted by climate change and the least likely to have the resources to cope with climate change," the team concludes.

The study has been published in GeoHealth.