The heat index measurement, calculated by meteorologists to indicate how hot it feels, underestimates the temperature perceived by people on the most sweltering days, a new study reveals.

Climate scientists, including those from the University of California (UC) Berkeley in the US, say the apparent temperature perceived by individuals is sometimes off by over 20°F.

Humans adapt to hot temperatures by sweating as well as flushing, a natural process where blood is diverted to capillaries close to the skin to dissipate heat.

The heat index, researchers say, is a measure of how the body deals with heat when the humidity is high, and sweating becomes less effective at cooling us down.

While the measure has been adopted as an indicator of people’s comfort, scientists say, the index has remained undefined for many extreme conditions that are now becoming increasingly common due to climate change.

They say accurate indication of people’s comfort using the measurement breaks down when people perspire so much that sweat pools on the skin.

The findings, published recently in the journal Environment Research Letters, have implications for those who suffer through heat waves.

When the heat index is high, the human body is more stressed during heat waves than public health officials often realise, researchers warn.

A heat index above 103 could be dangerous, and above 125 can be extremely dangerous, scientists say.

The new study expanded the old model to accurately represent the apparent temperature for regimes outside those previously calculated.

“Most of the time, the heat index that the National Weather Service is giving you is just the right value. It’s only in these extreme cases where they’re getting the wrong number,” David Romps, UC Berkeley professor of earth and planetary science, said in a statement.

“Where it matters is when you start to map the heat index back onto physiological states and you realize, oh, these people are being stressed to a condition of very elevated skin blood flow where the body is coming close to running out of tricks for compensating for this kind of heat and humidity. So, we’re closer to that edge than we thought we were before,” Dr Romps, a co-author of the study, said.

In the new research, scientists applied the extended heat index to the top 100 heat waves that occurred between 1984 and 2020.

They found that seven of the 10 most physiologically stressful heat waves in the US over that time period were in the Midwest – mostly in Illinois, Iowa, and Missouri – not the Southeast, as meteorologists assumed.

Citing an example of one of the conclusions drawn from the study, scientists say the maximum heat index reported during the July 1995 heat wave in Chicago, which killed at least 465 people, was 135°F when it actually felt like 154°F.

Doctors say the body generally starts going haywire when the skin temperature rises to equal the body’s core temperature of about 98.6°F.

Past this point, the core temperature begins to increase, and the maximum sustainable core temperature is about 107°F – the threshold for heat death.

For the healthiest of individuals, researchers say, that threshold is reached at a heat index of 200°F.

While the heat index reported at the time at the Midway Airport in Chicago of 124°F, implied only a 90 per cent increase in skin blood flow, the new study suggests people in the shade would have experienced blood flow to the skin that was 170 per cent above normal to adapt to the very high temperature and humidity by flushing.

“Diverting blood to the skin stresses the system because you’re pulling blood that would otherwise be sent to internal organs and sending it to the skin to try to bring up the skin’s temperature. The approximate calculation used by the NWS, and widely adopted, inadvertently downplays the health risks of severe heat waves,” Dr Romps said.

“But now that we’ve got this model of human thermoregulation that works out at these conditions, what does it actually mean for the future habitability of the US and the planet as a whole? There are some frightening things we are looking at,” he added.

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