As the saying goes: it’s not the heat, it’s the humidity.

As large parts of southern Ontario and Quebec continue to suffer under uncomfortably high temperatures, the added risks from humid conditions are putting some people at higher risk of health problems.

One of the ways the human body cools itself down is through the evaporation of sweat from the skin.

But humidity -- the amount of water vapour in the air-- impedes the process, because the more moisture there is in the air the less the sweat evaporates, Environment Canada says.

In Canada, meteorologists use a humidex value – a calculation which combines the temperature and humidity-- to describe how hot weather feels to an average person.

Environment Canada says that because the humidex takes into account both the temperature and the humidity, it is a better measure than either factor alone.

When the humidex falls between 35 to 39, the weather agency advises residents to limit the amount of outdoor physical activity.

When the humidex is 40 or higher, the weather agency advises limiting all unnecessary activity. A humidex value above 45 is considered dangerous and poses a risk of heat stroke.

CTV Medical Consultant Dr. Marla Shapiro told CTV News Channel that high temperatures and humidity can cause a number of illnesses including heat cramps, heat exhaustion, heat stroke and sometimes even death.

“The combination of heat and humidity make it very difficult for a body to cool itself down,” she said Tuesday. “It’s very easy to become overcome… that can happen very quickly.”

In extreme cases, high body temperature can damage vital organs, including the brain, the U.S. Centers for Disease Control and Preventions says.

Shapiro said the young and the elderly are less heat-adapted, as are individuals who have chronic diseases.

Other risk factors for heat-related illness include: obesity, fever, dehydration, heart disease, mental illness, poor circulation, and prescription drug or alcohol use.

Illness and death caused by heat are preventable, yet extreme summer weather conditions contribute to a number of deaths every year.

Health Canada recommends the following to stay cool during an extreme heat event:

  • Go to indoor air-conditioned places, including shopping malls, community centres and local libraries
  • Drink lots of water even before you feel thirsty
  • Limit physical activity
  • Check in on elderly family members and neighbours
  • Never leave seniors, children or pets in unattended cars
  • Seek shady areas or use an umbrella when outdoors
  • Take cool showers or baths, or use wet towels to cool down
  • Wear loose, light-coloured clothing and wide-brimmed hats when outdoors
  • Limit the amount of time spent outdoors or reschedule outdoor activities until the cooler evening hours

Shapiro said it is especially important during the summer to limit the amount of the outdoor activities for young children.

“You really want to look at small bites, in terms of the windows of being outside and making sure that you’re having frequent indoor cooling,” she said.