Doctors and health-care experts are warning Canadians to not only look after themselves but also check on others who could be vulnerable as large parts of central Canada face a sweltering heat wave.

Global News chief meteorologist Anthony Farnell says a large heat ridge or dome will build over the Ohio Valley in the U.S. and expand into southeastern Canada by the end of the weekend.

Temperatures are expected to reach the low to mid-30s C and the humidex could make that feel much higher, with humidex readings potentially into the mid-40s.

When heat hits those levels, the body has to work harder to keep itself cool.

“The hypothalamus acts as the thermostat and sends signals to the heart to start beating faster, sends signals to our body to breathe a little faster and that helps the blood flow to the extremities so that we can dissipate heat through radiant heat loss,” said Dr. Samantha Green, a family physician at Unity Health Toronto with expertise in climate change and health.

“It also sends signals so that we can cool down through evaporative cooling through sweating.”

Prolonged exposure to heat can impact the human body in a variety of ways.

One of the most common is heat exhaustion, which can lead to heat stroke.

Heat exhaustion is characterized by headache, dizziness, extreme thirst, heavy sweating, muscle cramps and nausea and vomiting, according to Health Canada, and people are advised to immediately move to a cool place and drink water if experiencing these symptoms.

If left untreated, that exhaustion can become heat stroke, which is defined by Johns Hopkins Medicine as the body being overwhelmed by excessive heat.

Those suffering from heat stroke often have an internal temperature of 40 C or higher, warm, dry skin, confusion and nausea, and it can lead to worse conditions, including seizures, coma and even death.

While people will keep an eye on the temperature, studies show that people should also be aware of the “wet bulb” temperature — when both humidity and air temperatures are high, it can be harder to shed heat and keep yourself cool.

A 2010 study found that a wet-bulb temperature of 35 C that lasts longer than six hours could cause serious health impacts or even death.

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A study in the Journal of Applied Physiology in 2022 found that at 100 per cent humidity, the threshold could be as low as 31 C, while at 38 C, just 60 per cent humidity could be too much for the body.

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If you are suffering from heat stroke or you notice someone with similar symptoms, move to a cool place to rest and call 911, as it is a life-threatening medical emergency.

“If you don’t feel well from the heat, seek medical attention or call 911. Don’t be afraid to ask for help,” said Caroline Metz, managing director of climate resilience and health at the Intact Centre on Climate Adaptation at the University of Waterloo.

Vulnerable populations, such as youth and older adults or those with chronic conditions, may find it more difficult to beat the heat.

Nemours Children’s Health in the U.S. notes that children’s bodies are still developing and may sweat at a lower rate than adults and start sweating at a higher temperature, putting them more at risk of heat-related illness.

Older adults face higher risks as they may have trouble accessing cool spaces due to mobility or transportation challenges, and those living alone may be more vulnerable as a result, according to B.C. Interior Health.

Older adults, as well as other Canadians, may also have chronic underlying health conditions or be taking medication that could interfere with the body’s ability to stay cool.

The U.S. Centers for Disease Control and Prevention notes that those with these conditions may be less likely to respond to changes in temperature, and that those with heart disease, mental illness and obesity have a higher risk of heat-related illness.

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“It’s also why people with chronic health conditions are more at risk of developing heat exhaustion, heat stroke, where the body’s natural cooling mechanisms break down and the core body temperature exceeds 40 degrees,” Green said.

“That’s where we end up with massive inflammation and failure of organs in the body.”

People taking chronic medications can also be affected. Green said beta blockers, for example, prevent the heart from beating faster in response to heat and keep the body from naturally being able to cool down.

It’s not just health conditions or age that can pose vulnerabilities, however.

“Anybody who’s working outside or people experiencing homelessness, they’re more exposed to heat and that’s a group or a population that’s more at risk,” Metz said.

Metz notes that during heat waves, it’s important for people to watch out for others and check in on neighbours because those living alone or socially isolated can also be vulnerable to heat.

A B.C. coroner’s report following the 2023 heat dome noted that 98 per cent of the 619 deaths identified as being related to heat occurred indoors, with 56 per cent of those who died living alone.

“Social cohesion or social connectedness has been shown to be a very important factor for resilience and for health during heat waves,” Metz said.

“In fact, in some cultures where there are multi-generational families living together, it’s just very natural for people to care about others … so they don’t often experience the same kind of situation that we might have here in North America, where we have one generation living in a household.

“The ability to reach out, call someone, or stop by has been shown to greatly support health and well-being for people who are isolated.”

She added that older adults and those living alone should reach out to other people to let them know they’re on their own and are open to having someone check on them during periods of extreme weather.

As a heat wave enters Ontario later this weekend, Metz said people should stay in the coolest part of their home if possible or go to a community cooling centre, wear lighter-coloured clothing if you have to go out, keep your doors and windows closed during the hottest parts of the day, and turn off items that may produce extra heat in your home.

“If you need to be out, you want to do that during the parts of the day that are the coolest,” she suggested, noting this means early mornings and late evenings.