What happens to the human body in 49C heat? Australians are finding out

1. What effect does extreme heat have on the body?

In warm environments, the body dissipates heat through the skin, which is responsible for about 90% of heat loss. It does so by sweating and by increasing blood flow to the extremities.

“That involves increasing the amount of work that the heart does,” says Dr Arnagretta Hunter, a cardiologist and senior lecturer at the Australian National University.

She says extreme heat – especially warm temperatures in the evening and “the loss of overnight cooling” – has wide-ranging impacts on both physical and mental health. “You will see an increase in [hospital] presentations that have a cardiac problem, a heart problem, a kidney problem, geriatric admissions … people falling over [from dizziness].”

Experts have warned that the degree of heat people experience could far exceed forecast figures, because the Bureau of Meteorology’s weather stations record ambient temperature in the shade.

“Depending on what surface you’re standing on and the radiant heat, it could be up to 10C hotter,” Dr Kim Loo, the NSW and ACT deputy chair of the Royal Australian College of General Practitioners, told Guardian Australia earlier in January.

2. What happens during exposure to temperatures close to 50C?

“How long is it safe to be outside at 49C? I think the answer is: it’s not safe to be outside at 49C for any protracted period of time,” Hunter says.

Extreme heat in parts of India and Pakistan, where temperatures have exceeded 50C, has led to hundreds of deaths. “Even short periods of time in that 50C-plus environment can lead to dehydration; the heart’s working hard,” Hunter says. “This sort of heat stress can provoke a heart attack, it can provoke an arrhythmia – rhythm abnormalities, it can provoke circulatory collapse.”

In Victoria, the 2009 Black Saturday bushfires killed 173 people. “That was catastrophically bad,” Hunter says. But, she points out, the heatwave that preceded it – during which a new state temperature maximum was set – killed more than twice as many, resulting in an estimated 374 excess deaths.

“There are biological limits to temperatures that we can survive,” Hunter says.

3. How to prevent heat stress

Public health authorities suggest staying well hydrated – drinking six to eight glasses of water a day, and avoiding alcoholic, hot or sugary drinks. People with heart failure and other medical conditions that may require limiting fluid intake should consult their medical practitioner.

Older people, babies, people with chronic illness or who are pregnant or breastfeeding are more susceptible to the effects of extreme heat.

Hunter suggests keeping houses and work environments as cool as possible – whether through air conditioning or fans, which can safely be used in combination with other measures in indoor temperatures up to 37C.

If it’s not humid, wetting the skin can also help, because most of the heat the body loses in warm environments is due to sweat evaporating.

Because of the impacts on both physical and mental health, Hunter suggests “taking it quietly through the day”. She says: “It’s not a day for exercise, it’s not a day for spending time outside. It’s not a day for  complex decision-making.”