It was extremely hot and humid as Toronto hosted its final World Cup match Thursday, between Portugal and Croatia. In fact, it was near the “wet bulb” temperature threshold where world soccer union guidelines recommend that games be delayed.
That had already happened for a number of matches in the tournament, raising concerns about the health risk to players.
But how hot is too hot? And what actually happens to players and their matches at that temperature?
How hot and humid was it in Toronto?
Portugal beat Croatia 2-1 in their World Cup Round of 32 match at Toronto Stadium on Thursday, which kicked off at 7 p.m. ET.
By that time, the temperature in downtown Toronto had cooled to 27 C, with 67 per cent humidity making it feel like 35, Environment Canada reported.
That was an improvement from the afternoon, when a heat dome that has settled over southern Ontario and Quebec sent Toronto temperatures to a high of 35 C, with the humidity making it feel like 42.
It was so hot that the city cancelled one of its most popular public viewing events for World Cup matches – the broadcast at Nathan Phillips Square.
A screenshot from X shows an announcement from the City of Toronto that its World Cup watch party at Nathan Phillips Square has been cancelled due to heat. (City of Toronto/X)
What do world soccer guidelines say about heat?
FIFPRO, the global organization representing professional footballers and an umbrella group for 70 player unions, has guidelines for extreme heat.
The guidelines are based on a measure called the “wet bulb globe temperature” (WBGT). They say that if the WBGT is above 28 C — or the ambient temperature is above 36 C — games should be delayed or postponed “until conditions for players and officials (and fans) are safer.”
According to a Canadian Armed Forces chart that uses temperature and relative humidity to estimate WBGT, the WBGT at the start of Thursday’s game in Toronto would have been close to 28 C, although the exact number would have depended on the amount of sun and wind.
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Environment Canada has issued yellow alerts for heat from central Ontario to Nova Scotia and orange alerts for southern Ontario, including Toronto, where the city has cancelled the World Cup watch party for the last game played there, between Portugal and Croatia.
Kate Hutchins is a University of Ottawa postdoctoral researcher who studies how extreme heat affects the human body, including its effects on older people or those who work outdoors.
“It’s definitely a positive step having these types of guidelines in place,” she said.
She said the concern with soccer players is that running produces “a massive amount of metabolic heat,” limiting the body’s ability to release heat.
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What is WBGT?
Wet bulb globe temperature takes into account the cooling effect people experience when they sweat.
Sweating helps cool the body through evaporation, which is why pseudo sweat also works. But the body’s ability to cool itself this way depends on relative humidity. When the air already contains a lot of moisture, sweat does not evaporate as easily.
CBC meteorologist Johanna Wagstaffe explains it this way: “The more humidity in the environment, the slower the evaporation, and the harder your body is to cool down.”
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The wet-bulb component of WBGT is measured using a thermometer with a wet cloth wrapped around its bulb and a wick dipped in water, allowing evaporation to occur.
That helps WBGT approximate the heat stress experienced by the body, which is why the measure is used in occupational health guidelines such as FIFPRO’s.
Although WBGT can be estimated using temperature and relative humidity, sun and wind exposure also affect the reading. As a result, WBGT can vary by several degrees even when the ambient, or dry-bulb, temperature remains the same.
Hutchins said WBGT “gives you a bigger picture around what the environmental conditions are and how the body is going to tolerate those conditions.”
Once the wet-bulb temperature is the same as or higher than body temperature — about 35 C — the body can no longer effectively cool itself through sweating, which can be life-threatening.
How does WBGT relate to the humidex?
Humidex is intended to measure how hot conditions feel. Environment Canada says a humidex above 40 can cause great discomfort and that people should avoid exertion. A humidex above 45 is dangerous and could lead to heat stroke. The agency provides a chart that uses temperature and relative humidity to estimate the humidex.
“Direct comparison between WBGT and humidex is not possible — there are no conversion tables or mathematical formulas to make such conversions,” the Canadian Centre for Occupational Health and Safety says.
Both readings are extremely high in communities across southern Ontario and Quebec this week.
How is the body affected?
When the body can’t cool itself properly, that causes a rise in core body temperature.
Hutchins says that leads to a decline in performance for athletes and workers long before other symptoms.
“It’s not just performance which is the concern, it’s also the person’s safety,” Hutchins said. “That elevation in core temperature can lead to heat illnesses.”
Heat-related illness can include heat stroke or heat exhaustion, cardiovascular emergencies, confusion, memory loss and respiratory and kidney problems.
Hutchins said people should watch for symptoms such as dizziness or lightheadedness, which may be signs of overheating, and seek shade and water.
“Listen to your body,” she said.
People should also check on those who are more vulnerable to heat, including older friends and family members.
Here are some precautions to help you stay safe, whether you’re playing or watching soccer.
At what temperature are World Cup matches actually postponed?
FIFA has not announced a temperature threshold at which it would delay or postpone a match. No matches have been delayed or postponed because of the heat since the World Cup began on June 11.
As of Tuesday, nine matches in this year’s tournament had already been played amid potentially dangerous heat and humidity, the Guardian reported.
Hutchins said it is difficult to set a definitive threshold for postponing or cancelling games because many factors affect how the body responds to heat stress and there are other practical considerations.
FIFPRO told CBC News it will discuss heat and other issues affecting players in more detail at a news conference on July 17.
When FIFA announced the match schedule in December, a FIFPRO spokesperson said it was clear the organization had adjusted the schedule to address concerns about player health. However, the spokesperson noted that some matches still carried risks and that competition formats and “infrastructure limitations” made it difficult to eliminate those risks entirely.