It's hotter than world soccer guidelines recommend in Toronto today. What that means.

Extreme heat expected to exceed player safety guidelines at Toronto World Cup match

Toronto forecast to have a WBGT of 32 C and a humidex of 39 C when 7 p.m. Portugal-Croatia match starts

Extreme heat during today's World Cup match between Portugal and Croatia is expected to exceed world soccer union guidelines based on the "wet bulb" temperature. That has already happened for a number of matches, raising concerns about the health risk to players. Here's what it all means.

How hot is it today in Toronto?

Toronto is hosting its final game today of the FIFA men's World Cup today. It may also be the final World Cup match of Portugal captain Cristiano Ronaldo's legendary career.

But it is so hot that the city cancelled one of its most popular public viewing events for World Cup matches – the broadcast at Nathan Phillips Square.

Environment Canada forecast a high of 34 C around 5 p.m., expected to feel like 42 C with the humidity. Relative humidity was expected to range from 45 and 60 per cent for much of the day.

By 7 p.m., when the match begins, the temperature is forecast to drop to 32 C, with a humidex of 41.

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 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, tonight's Toronto match is expected to kick off with a WBGT of 30 C. The estimate is based on a temperature of 31 C and humidity of 45 per cent.

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Kate Hutchins is a University of Ottawa researcher who studies how extreme heat affects the human body, including its effects on older people or those who work outdoors.

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.

"It's definitely a positive step having these types of guidelines in place," she said.

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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.

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."

Johanna Wagstaffe Science Smart Wet Bulb Temperature

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 today are extremely high.

That puts people at risk of dangerous heat-related health effects, including 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.