In our Cardio Rehab class (at Toronto Rehab), they gave us a chart of temperature vs. humidity, with maybe 4 bands on it, labeled like "OK/Green", "Caution/Yellow", "Extreme Caution/Orange", and "Forget It/Red". When I asked whether that chart isn't just a variation on the (Canadian) Humidex [index], I got an unsatisfactory answer, IMHO. ("The two indeces were designed for different purposes, so they're not directly comparable," type thing.)
In fact, I think the two charts/indeces, AND the similar U.S. heat index, are all designed for virtually identical purposes -- to integrate the additional stress that increasing temperature and increasing humidity add to an exercising body (and heart, lungs, etc.).
I briefly compared the three of them, and they all seem quite similar, though the lines (e.g., between "You can go outside but don't exercise" and "Don't even go outside") often fall at slightly different places. As I recall, the US & Canadian indeces -- neither of them specifically tailored to OHS patients -- were more different from each other than the supposedly heart-specific one was to either.
Bottom line is that, according to my experts and (I think) many others, there comes a level of combined heat and humidity at which one -- we all -- should stop doing strenuous exercise outside, and a higher level of combined heat and humidity at which one -- we all -- should stay indoors with A/C or with our wrists in a bucket of cold water, etc.
If people are recovered enough and healthy and fit enough, they can presumably withstand a higher level of (combined) heat, humidity, and exertion. But one take-home message is that it's the "sum" of those three -- heat, humidity, and exertion -- that determines how hard, how stressful, the exercise is. So most of us (MAYBE with the exception of ShezaGirlie!) should back off on exertion as heat and humidity increase.
BTW, my Cardio Rehab clinic has made maps of many of Toronto's indoor shopping malls, mapping out walking paths that are 1 mile (or 1/4, 1/2. . .) long, so patients can drive or subway to those malls then do their prescribed exercises in the air-conditioned malls. They've asked us to suggest unmapped shopping malls that we'd like mapped, and they'll try to get them covered. At the Rehab clinic, they have an outdoor track (half of it in the sun!) and in indoor track; when the various indeces are high (or the Air Quality Index is bad), they get us to do our walking or walking/jogging on the indoor track.
I hope this helps somebody. I don't think they've put their own index online, I just got a paper printout.
