It鈥檚 going to be another hot, dry summer for many Canadians. Most of the country is expected to experience above average seasonal temperatures over the next few months, according to Environment and Climate Change Canada. Less rain than usual is expected to fall on both the east and west coasts, potentially increasing the risk of summer wildfires.
This week, a 鈥渉eat dome鈥� has settled across parts of Ontario and Quebec, driving an extreme heat wave in the Ottawa-Gatineau area. While daytime temperatures are expected to reach the mid-30s, humidity could make it feel like 40 C or hotter in many areas.
Intense heat comes with health risks like high humidity, UV exposure and wildfire smoke. Already this year, more than 30,000 Canadians have had to leave their homes due to wildfires in Saskatchewan and Manitoba. Smoke from those fires blanketed much of Alberta earlier this month.
As the climate changes, some of those risks are becoming more intense or popping up with more frequency. Human-caused climate change makes heat domes more likely, according to a first-of-its-kind analysis by Environment and Climate Change Canada. In July 2024, 小蓝视频 and Alberta saw heat records smashed as temperatures soared to more than 40 C in several regions.
2024 was the fourth-hottest summer on record in Canada and 2023 holds the record for hottest ever 鈥� at least for now.
In other words, it鈥檚 time to get serious about heat.
The federal Environment Department concluded climate change, driven mostly by rising greenhouse gas emissions, is causing more extreme weather events like heat domes and contributing to increasingly frequent and ferocious wildfires.
Four years ago, in June 2021, 小蓝视频 experienced a heat dome with temperatures up to 20 degrees above normal and set the highest temperature ever recorded in Canada: 49.6 C in Lytton on June 29. (The town was destroyed by a wildfire the next day.) The extreme heat killed 619 people, according to the 小蓝视频 Coroners Service, making it the deadliest weather-related event in Canadian history. Most were older people living in the urban Lower Mainland.
All of this can make summer feel more like a time for fretting than for fun. It also raises questions: how do we measure the health risks that are becoming a feature of Canadian summers? What do tools like the humidex and air quality health indexes tell us? Read on.
How hot does it really feel? Check the humidex
People in Eastern Canada love to complain about the stickiness of humidity in the summertime. Sure, the West is having a heat wave too, they say 鈥� but it鈥檚 a dry heat. It鈥檚 not the same. And while heat is heat, there鈥檚 also a kernel of truth in there. Humidity can make high temperatures harder for human bodies to handle, causing us to feel even hotter and sometimes leading to health problems.
Why is humidity during a heat wave a health risk?
Humidity makes it more difficult to cool down. Normally, we do it by sweating. When we perspire, moisture seeps out onto our skin and evaporates, taking with it some of the heat from our bodies. But humidity short-circuits that process: sweat won鈥檛 evaporate if the air is already too wet, so the heat sticks around too. That raises the risk of health problems related to high temperatures, like heat rashes and heat-stroke. Humidity also plays a role in many of the deaths caused by heat waves.
Prolonged heat waves or heat dome events 鈥� when a large area of stagnant, hot air becomes trapped near Earth鈥檚 surface 鈥� can be even more stressful on the body because the sauna-like conditions can stick around for days at a time. If temperatures don鈥檛 dip below 20 C at night, your body can鈥檛 get respite from the stress of heat and humidity exposure, which is especially dangerous if you have underlying health conditions, like heart diseases or asthma.
What is the goal of the humidex?
Experts have come up with various formulas over the years 鈥� including the U.S. National Weather Service鈥檚 heat index 鈥� to try to quantify the danger of high moisture in the air. Some meteorologists factor in humidity for the 鈥渇eels like鈥� temperature listed in forecasts, which describes how we experience the weather rather than what the thermometer reads.
Environment and Climate Change Canada uses a tool called the humidex. Meteorologists began using it in 1965, combining measurements of heat and humidity to get a rough estimate of how hot it really feels outside so we can gauge how much the weather could affect us.
For example, during the June 2024 heat dome that hovered over Eastern Canada, meteorologists forecast temperatures of 30 to 35 C degrees in Toronto, with a humidex around 40. Although the mercury maxed out at 35 C, it felt about as hot as it would have been if the air were 40 C and dry.
A humidex rating between 20 to 29 means people might experience a bit of discomfort, though everyone responds differently to different weather conditions. At the other end of the spectrum, people should avoid exerting themselves when the humidex hits 40 to 45, according to Environment Canada. Anything above 45 is classified as dangerous, coming with a 鈥渃onsiderable risk鈥� of heat-stroke: that鈥檚 when core body temperatures exceed 40 C, affecting the central nervous system and causing symptoms like nausea, seizures, disorientation and confusion 鈥� and sometimes even loss of consciousness or coma.
What doesn鈥檛 the humidex tell us?
The humidex doesn鈥檛 account for everything 鈥� if a summer breeze by the water might cool you down, that鈥檚 not part of the equation. And many other factors, like the clothes you鈥檙e wearing or your age, might influence what a temperature feels like for you. Even so, experts say the humidex matters because it鈥檚 a way to warn people about humidity so they can find ways to cope with it and protect themselves.
People tend to be more familiar with the humidex in Ontario, Manitoba and Quebec. But once in a while, it will pop up on the forecast in parts of Alberta and Saskatchewan, when a mass of hot, wet air wafts north from the Caribbean or the Gulf of Mexico.
Trying to avoid sunburn? It鈥檚 time to learn about the UV index
The UV index measures the strength of ultraviolet rays from the sun. Scientists working for Environment and Climate Change Canada created it in 1992, and it quickly took off. Within a decade, a revised version of the scale officially became a global standard for weather forecasting, used to warn people to protect their eyes and skin from intense sunlight, which can age skin and cause sunburns, eye cataracts and skin cancer.
How to read the UV index
The UV index starts at zero and doesn鈥檛 technically have a top limit, though the maximum ever recorded in Canada was a 12 (other places to the south have recorded higher values). Southern Ontario tends to experience the highest UV values in the country, according to Environment and Climate Change Canada. In general, the higher the number, the stronger the UV rays and the more precautions you should take.
Experts generally recommend wearing sunscreen, sunglasses and a hat if you鈥檙e going to be outside, but the risks become much higher once the UV index hits six or seven. In the eight to 10 range, experts recommend reducing your time in the sun between 11 a.m. and 3 p.m. A UV index above 11 is considered extreme, with Environment and Climate Change Canada warning unprotected skin can be burned or damaged within minutes. Snow, white sand and other white surfaces can also reflect UV rays and increase your exposure, even if the UV index for the day isn鈥檛 very high.
What is the Air Quality Health Index and what does it measure?
In 2005, Health Canada and Environment Canada launched the Air Quality Health Index (AQHI), designed to help people better understand health risks stemming from air conditions. It was first used in 小蓝视频 and is now the preferred scale for communicating air quality in Canada.
The health index uses a colour-coded scale of zero to 10+, based on concentrations of ozone, particulate matter and nitrogen dioxide 鈥� the pollutants found to have the greatest health impacts. A 2021 Health Canada report estimated there were 15,300 premature deaths and 8,100 hospital visits due to air pollution in Canada in 2016.
The higher the health index score, the greater your risks and the more precautions you may need to take to stay safe and comfortable.
The health index also recognizes some people are more vulnerable to air pollution than others. People with cardiovascular and lung diseases are more likely to experience negative health effects, such as shortness of breath, at lower health index levels. Seniors and children are also more vulnerable to poor air quality conditions.
At 鈥渕oderate鈥� health index levels of four to six, most people won鈥檛 experience any health effects and can continue their regular outdoor activities. Those at higher risk may experience coughing or throat irritation and should consider skipping or rescheduling strenuous outdoor activities like hiking or running.
High health index levels range from seven to 10, signalling everyone should consider skipping strenuous outdoor activities. When the health index hits 10+, the risk of negative health effects is very high.
The worst health index reading ever recorded in Canada was in Kamloops during the wildfires of 2017, when the index hit 49.
As high-risk air quality events happen more often, Environment and Climate Change Canada has changed its communications to better reflect the risks of smoky air. In 2024, the agency began issuing a new type of air quality advisory to warn people about the potential health effects of 10+ AQHI scores and urge people to seriously consider cancelling outdoor events.
---
This story is available for use by Canadian Press clients through an agreement with The Narwhal. It was originally published in The Narwhal, a non-profit online magazine that publishes in-depth journalism about the natural world in Canada. Sign up for weekly updates at thenarwhal.ca/newsletter.
Shannon Waters and Emma McIntosh, The Narwhal
