Your reaction to heat is highly dependent on acclimation. I live in Texas and have to re-acclimate every year. Exercising on the first 85º+ day of the year is miserable, but a month later 85º feels quite tolerable, and 95º is doable, though performance suffers.
I grew up playing baseball and tennis in 95-100º weather with high humidity routinely. It wasn't pleasant, but nobody was getting heatstroke, nobody was cancelling games or practices. But on a visit to Montana a few summers ago, I saw that kids' baseball games had been cancelled because the temperatures had reach a dangerous level: 90º (in dry mountain air.) Same human beings, different levels of acclimation, very different safety thresholds.
I've never been in the temperatures described in this article, though, and I don't know what the physical limits of acclimation are.
If the pool is heated, swimming in 15C weather would be fine. If not, I wouldn’t swim in 15C water without spending time in a sauna or hot tub before jumping in; surfers wear wetsuits when the water is that temperature.
The "trick" I used in GA (not quite as hot as TX, but as humid in that part of GA) was to exercise outside year round. It was rarely so cold that winter running was a problem, and as the temperatures warm up or cool down you naturally get acclimated to the new season. It's worked here in CO as well (though due to an injury I didn't this past winter) with getting used to very cold temperatures (I wouldn't exercise outdoors on our very coldest days, but that's about 1-2 weeks out of the entire winter that I'd stick to indoor only training).
I occasionally ride my bicycle to work in Austin and some days when I get home I have a hard time cooling down even after a long cold shower. I've since set a rule for myself that I won't ride on days where the temperature is 98 or higher.
I just turned 55 and have been thinking about this a little, wondering if maybe I should back off on the biking?
Maybe do it more consistently! Heat acclimation takes a week with daily exposure or 2-3 weeks if you get exposure every other day[0]. If you work out in the heat once a week, you might be living that miserable first hot day of the year over and over again all summer.
[0] I don't know how well nailed down this is, but I didn't find any wildly differing opinions in my internet research: you achieve roughly 80+% of the heat adaptation you're capable of in this time period, assuming 90 minute sessions with physical activity.
Not a medical expert by any means, but 98 degree Fahrenheit sounds like a dangerous high temperature for exercise (at high humidity, I'm sure it is). I'd go earlier / later in the day, when temperatures are more reasonable. There is a reason for Siesta.
The first exercise at 105⁰ seems insane. A week later bearable.
I recall with some amusement thinking I was coming down with heat stroke one summerbecause the light wind felt chilly on my skin. But then I realized it was only 95 degrees
I grew up playing baseball and tennis in 95-100º weather with high humidity routinely. It wasn't pleasant, but nobody was getting heatstroke, nobody was cancelling games or practices. But on a visit to Montana a few summers ago, I saw that kids' baseball games had been cancelled because the temperatures had reach a dangerous level: 90º (in dry mountain air.) Same human beings, different levels of acclimation, very different safety thresholds.
I've never been in the temperatures described in this article, though, and I don't know what the physical limits of acclimation are.