It's hard to know exactly how many athletes have been affected by the heat, since some may have been treated by their own team staff rather than the Games medical team, organizers said.
But even those who didn't seek medical attention said the sweltering conditions made for a particularly gruelling day of competition.
"I think the toughest part was the hills and the heat, I think that was the biggest factor," said Canadian runner Rachel Hannah, who finished fourth in the women's marathon.
"When I ran Ottawa, it was about 11 degrees, flat course and it didn't even really feel too challenging, but this one definitely hit me at about 25 kilometres," she said.
Her teammate Catherine Watkins said the biggest hurdles were "humidity, then the hills, then the heat."
"I'm really proud to gut it out and finish, it's a really tough course and it was tough conditions — it was hot and humid," said Watkins, who finished ninth.
Dr. Julia Alleyne, the Games' chief medical officer, said that while many athletes competing at the Games are used to training in the heat, they may not be used to the humidity.
And she said the pressure of competing could drive them to push harder than they normally would in this weather.
"We advise medical staff and athletes to be aware of early signs of heat-related illness such as fatigue and muscle cramping," she said.
Muscle cramps are typically attributed to strains or sprains, but in the heat, they're often due to dehydration, Alleyne said. More advanced cases may also notice a loss of focus, she added.
Hydrating with water and electrolyte-replacement drinks is key, as is cooling down with wet towels, she said.
Organizers have increased the size of their medical team in response to the temperature, which was expected to peak around 32 degrees with the humidex hitting the 40-degree range. Alleyne said they are also providing water, towels and cooling areas to athletes at the venues.
Athletes aren't the only ones being urged to stay cool. Organizers said spectators, too, have to take precautions in order to avoid heat-related illness.
They recommend wearing hates, drinking lots of water and applying sunscreen frequently.
— With files from Dhiren MahibanSuggest a correction