Organizers of the Rock 'n' Roll Oasis Montreal Marathon say while they understand some runners are disappointed this weekend's full marathon has been cancelled, the forecast heat forced them to make that call.

The cancellation of the 42.2-kilometre race was announced Wednesday afternoon, days before Sunday's race was to be held.

It will be unseasonably warm on Sunday — the high will reach around 28 C, but with the humidity it will feel like upwards of 36 C. The average high for this time of year is 17 C.

Louis Malafarina, executive director of the race, said once they realized the forecast for Sunday wasn't trending downward, it became clear that the marathon would have to be cancelled.

"There's real danger for participants, especially on the marathon side, after five, six hours of running," he said.

He said this is the first time in 27 years that they've had to cancel the marathon. About 5,000 people were signed up for that event.

Dr. François de Champlain is the trauma team leader at the Montreal General Hospital and has been advising the marathon for last five years.

He said their priority was ensuring the safety of the people participating in the event.

"For me, when the temperature was that high, it was pretty clear that this was the right thing to do."

He cited recommendations by the American College of Sport Medicine, which considers humidex readings of over 28 to be a red flag — and says the risk of heat-related illness increases exponentially as the humidity increases. 

De Champlain said in 2014, it felt like 31 C during the race, and 1,100 people visited the medical tent at the finish line, many of them suffering from different degrees of heat exhaustion.

Challenges of running in the heat

Ryan Ortiz was planning to run the full marathon for the eighth time this year, but he'll now settle for running the half instead.

Ryan Ortiz

Runner Ryan Ortiz said he was disappointed at the decision to cancel the marathon, but knows how tough it can be to run in the heat. (Submitted by Ryan Ortiz)

He said he was disappointed at first, but after thinking it over, he acknowledged it may not have been a terrible decision to cancel the race.

He pointed out in 2011, it was similarly hot and he finished 45 minutes later than he thought he would.

"It really takes a serious toll, for sure," Ortiz said.

Montreal resident Jean-Francis Presseau, 32, is believed to have gone into cardiac arrest about one kilometre from the finish line that year and died.

Ortiz runs year-round, but ramps up his training in the spring, usually running in the mornings when it's cooler. He said while it's not impossible to run in high heat and humidity, people who live in hotter places are more accustomed to those conditions.

Jean-François Thériault, out on a morning run in La Fontaine Park Thursday, said while he understands why the organizers made their decision, runners should know the risks when they sign up for a race.

He will be running the 10-kilometre race on Sunday, which is going ahead as planned. He said he will take steps to make sure he's hydrated, but isn't too worried about it.

"If ever I run and I feel the heat too much, I will stop and that's it."

Jean-François Thériault

Jean-François Thériault said he knows many runners are disappointed by the decision to cancel the race. (Matt D'Amours/CBC)

If you're going to run, hydrate properly

Malafarina said planning the race takes about a year, and with the logistics involved, including finding hundreds of volunteers, it would have been "almost impossible" to reschedule the race a week before it was set to go.

Anyone registered in the half or full marathons can get a full refund if they don't wish to participate. Another option is to transfer the bib to another race in the Oasis series.

He said the cancellation will certainly affect their bottom line.

"It's a hit, we can't deny that, but really what's paramount is the health of our participants," he said.

The first cohort of half-marathon runners will leave at 7:30 a.m. Sunday, an hour earlier than usual in order to keep them out of the brunt of the heat. They will have three hours to finish the race.

There will be a number of first responders along the course as well, and De Champlain says he has advised his trauma team that it may be a busy day.

He said the key for those running in the half is to arrive well-hydrated, and hydrate during the race.

Water Stations

Make sure to hydrate with drinks that contain electrolytes while running in the heat, not just water. (CBC)

But there has to be a balance between water and electrolyte intake. Electrolytes are key to staying hydrated, and drinking too much water can dilute them, he explained.

Some runners opt to bring their own salt tablets or other products that help with electrolyte consumption. Runners are being advised to drink a cup of water with salt before the race.

And of course, participants should always be aware of how they feel and slow down if they aren't feeling well.

With files from CBC Montreal's Daybreak and Matt D'Amours