Your hypothalamus, a bit of your brain the size of a pearl, will be activating your body's defences to keep you cool.
But it needs your help. And your mother or child might, too.
Heat is worse for the very old, the very young, and other vulnerable groups. With a summer forecast that could push even coastal temperatures into dangerous territory, health experts have a few things you should know.
1. Heat is worse for seniors
For seniors — at a physiological level — their bodies can't fight the heat as well, said Dr. Tim Takaro, a professor of health sciences at Simon Fraser University who studies the health impacts of climate change.
"Most studies seem to link this to your hypothalamus," said Takaro, who is also trained as a physician.
In a healthy individual, that part of the brain will be sending signals to dilate your blood vessels, moving blood closer to the skin where it can cool off, and activating sweat glands.
"Both those systems don't respond to the temperature stimulus as well in the elderly," he said.
2. Heat hits Vancouverites harder
The physiology isn't well understood, but what you're accustomed to does make a difference in how much heat your body can handle, said Takaro.
"In New Delhi, 40 happens every year, for days. [In Vancouver] we never see 40."
That's why the level at which authorities will issue a heat alert varies from location to location.
In Vancouver, the threshold for an Environment Canada warning is just 29 C over consecutive days, whereas it's 40 C in other parts of Canada.
"We do know that there is acclimatization, in that certain populations at a given temperature have no problem, and populations like ours have problems," said Takaro.
3. 'Everybody is at risk'
When it gets really hot, anyone who can't cool down — even a young healthy person working outside all day — is at risk, said Sarah Henderson, a senior environmental health scientist at the B.C. Centre for Disease Control.
That happened during the unprecedented heat wave in the summer of 2009, which killed 100 people, researchers learned later when they compared death rates during the heat wave to normal conditions.
"We observed a 40 per cent increase in mortality over a one-week period," said Henderson. "That's a public health emergency."
Research by Henderson and others on that 2009 heat wave led the local health authorities to set up special alerts for when the heat is dangerous for the general metro Vancouver population.
"Everybody is at risk when it gets really hot out. Do your best to stay cool and take it seriously."
4. Not a luxury but a health issue
Studies on deadly heat waves, including the one that killed more than 10,000 people in France in 2003, show that consecutive days and nights of heat are what take a toll, said Takaro.
"It's the third day when the bodies start piling up …. In the Paris heat wave it was the old people living in the 5th floor walk-ups whose family had gone to the beach."
That means finding a way to get cool isn't a luxury, it's a health issue.
Check on seniors and others living alone, and make sure they find a way to cool down, whether it's an air conditioned space, a cold bath, a swimming pool, or even a cooler part of the building.
"Four hours is a good break, eight hours is a better break," said Takaro. "Two hours may be enough. It does depend a bit on the level of vulnerability."
5. This could be hotter than 2009
The summer is expected to be hotter than normal in B.C., which is good for tourism but has risks for human health.
Henderson of the Centre of Disease Control will be watching it closely, especially as smoke from wildfires drifts into areas where people live, making it harder to cope with the heat.
"We're in for a hot, dry summer … and we might be seeing the kinds of conditions that are unprecedented."