When the temperature drops, we can warm up by adding layers of clothes, but with the mercury soaring, cooling down isn't as easy in the heat.

The human body needs to work hard to maintain a core temperature of about 37 degrees Celsius.

Key points: If you do not cool down with rest or hydration then you should seek medical attention

If you do not cool down with rest or hydration then you should seek medical attention Pregnant women, the elderly and young children are most likely to be affected by extreme heat

Pregnant women, the elderly and young children are most likely to be affected by extreme heat Experts say people should cease any outdoor exertion when the temperature hits 38 degrees Celsius

Liz Hanna from the Climate Change Institute at ANU says hot weather can be deadly and can affect you at lower temperatures than you might think.

The early warning signs of heat stress are feeling hot, flushed in the face and fatigued. Those are the body's attempts at self-preservation.

"If resting, going to a cool place and rehydrating does not make you feel better, then you need to be closely monitored," Dr Hanna says.

"Some people stop sweating and go cold and clammy, while some people go very hot and dry."

If rest, hydration and cooling don't work, you need to go to hospital.

Dr Hanna describes the effect of heat on your health as being like frying an egg, when you see it go white.

"We are pretty much made up of protein," she says.

"With extreme heat, the cells at a cellular level stop functioning, and if you have got a lot of cell death then you have organ failure."

How hot does it have to be to get heat stress or dehydration?

The impact of heat on your health depends on your body type, how humid it is, how physically active you are, and how acclimatised you are to hot weather.

"When the temperature is in the early 40s, and up to 46C and 47C, then getting out of bed is dangerous," Dr Hanna says.

Humidity, in particular, makes the effect of heat much worse.

International guidelines for working in the heat suggest you should stop work when the temperature hits 38C with 20 per cent relative humidity.

You should cancel any "outdoor exertion" at 38C and 30 per cent relative humidity.

How to avoid heat-related health problems: Avoid the heat of the day by staying indoors, keep cool by using air conditioners and fans, and close blinds and curtains

Avoid the heat of the day by staying indoors, keep cool by using air conditioners and fans, and close blinds and curtains Keep hydrated by drinking plenty of water

Keep hydrated by drinking plenty of water Check on vulnerable neighbours, friends and family by telephone or in person if it is safe for them to do so

Check on vulnerable neighbours, friends and family by telephone or in person if it is safe for them to do so Plan ahead for hot days

Sweating is the body's way of keeping cool. Imagine the difference between heat in places such as humid Darwin and Melbourne, where the heat tends to be drier.

"What cools us is evaporation and sweat," Dr Hanna said.

"If the air is humid, it's not going to absorb that water so we don't have that cooling effect."

So if it's humid, the temperature threshold for heat stress and dehydration is much lower.

"That's why 33C in Darwin can feel horrible, whereas 33C in Melbourne, you can still walk if you stay in the shade."

NSW Health Medical adviser Adi Vyas says staying indoors during the heat of the day is the best way to keep cool. If the air is smoky, as it has been recently in New South Wales, then that rule of thumb goes double.

Sydney has been trapped in an orange haze due to the number of fires surrounding NSW ( AAP: Paul Braven )

"Minimising physical activity, staying well hydrated are also important ways of reducing the risk of heat-related illness," Dr Vyas says.

"Staying indoors and reducing activity are also the best ways to reduce exposure to smoky air.

"We know that combined effects of bushfire smoke and extreme temperatures have potential to cause severe illness, hospital admissions and even death."



Which parts of the body are most affected by the heat?

If you spend too long in the heat, your thinking can become cloudy and confused.

"Once people are confused, it means the brain is not operating properly and they need to get to hospital quickly," Dr Hanna says.

"At the very serious level, people can become unconscious."

Your heart can be affected by the heat, because when we are very hot blood vessels to our skin dilate.

When the environment around us is hot, we absorb heat rather than shedding it.

"The heart has to maintain blood pressure, so it has to work harder to keep the blood pressure up when all those vessels are dilated," Dr Hanna says.

It's made worse by dehydration, and when we sweat we lose fluid.

"The heart has to beat much faster and harder, which is why if you have cardiac problems your heart is not very strong. [If] you are old or unfit, then your heart can peter out in the heat," Dr Hanna says.

The kidneys are also at risk.

When it's hot, the kidneys try to conserve water and stop you losing fluids.

"The kidneys act as a filter, so if you have cellular damage and your body is excreting that through your kidneys it can get obstructed and you can clog up your kidneys," Dr Hanna says.

The kidneys can become blocked and you can have acute renal failure.

Who is most at risk of heat-related health problems?

Those people most at risk include the very young, as they don't sweat as efficiently as adults.

The elderly are at risk because their cardiac function isn't as good. Additionally, their brain function and ability to sweat can also be impaired.

"Older people don't perceive that they are thirsty or that they are hot, so we find little old people sitting in their cardigans on a hot day," Dr Hanna says.

"Young people, school kids exercising in the sun, people who feel like they need to keep going in the heat, even if they are feeling hot but feel like they need to keep going, they are very much at risk."

Pregnant women are also more at risk of heat-related health problems, as growing babies don't like being hot.



"Extra degrees of heat can be damaging for the foetus so pregnant women need to stay out of the sun and outdoors," Dr Hanna says.