Wondering about the “horse heat index” today for instance in Pa it’s 70 degrees but 90% humidity giving a horse heat index of 160 …according to some of those charts floating around that’s in the code orange and says use caution …my one horse in question is a Friesian and has Anhidrosis so I don’t think i should risk it with him, and my other horse has mild asthma so probably not a great idea for her as well …I see so many people still continuing to ride though despite the conditions…am I just being wimpy and overly cautious??
I personally feel it is safe to ride most anywhere at a 70 degree temperature. You know your horse best. I’d just use discretion while working the horse and stop if you feel it is too much.
I am in South Georgia where it is 96 today and 60% humidity. LOL
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Yeah I hear you …and often wonder about all the people who are in the southern states …they would never hardly ride if that were the case
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I think you just need to be aware of what your horse is accustomed to. Others may have worked up the heat tolerance of their horses more than you have. When I bought my horse as a three year old, he made it clear to me that he doesn’t work in the heat. The few times I tried to work him in the heat, his pace was glacial. So his heat index is much lower than other horses around. Also, his rider is a pushover, so he sets a lot of the working condition rules 
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I think your issue is that both are somewhat health compromised and that the extreme humidity/ air quality can aggravate either condition and that you are wise to take it easier with them. Maybe just leisurely walking trail ride or arena ?
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Man, if it was 70/90 today I’d probably work on interval training since I like to schedule those on cooler summer days!! 
In other words, it’s what your horse (and you) are used to. If you’ve been riding at 70/60 and he struggles… There’s your sign. Today’s not a day to do a lot. Maybe some, and pay attention to his vitals as you do. But if you had a full workout at 80/80 and he recovered well, then I wouldn’t think twice about a full work at 70/90.
If you are unsure about your horse and heat you would do well to track his vitals at recovery. I did it as soon as I got off, at 10 minutes and 30 if he’s not quite where I think he should be in 10.
This means I know a lot about how he recovers. I know he comes in like a furnace, high temp, high respiration and probably a pretty good heart rate. Within 3 minutes his respiration and heart rate are in full recovery mode. If they aren’t, I make note of that unless the work is a step up in fitness and I expect recovery to take longer. Still, it’s almost never an issue after 6 minutes. Temp is always delayed because he’s a heavily muscled breed so it takes a while to dissipate that heat, but I know what the temp drop should be in 10 minutes and if it’s not, I keep cooling until we get there.
But what all of the above tells you is that I am paying attention to my horse’s physiological response to heat. Initially I kept a log of his vitals along with time of day and WBGT (I started with heat index but wbgt is a much more advanced value and easily available for me). Now I don’t bother because I know the pattern and when he’s got an outlier day. I do it because I compete in a sport that demands fitness. But you should do it because you have physically compromised individuals!
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I have seen those charts, and they always confused me because it doesn’t necessarily compute - and that’s now how a true “heat index” is calculated, although I realize it’s sometimes called a “horse heat index”. At 70 with 90% humidity, the true heat index is 71. Using the chart’s calculation of heat index, 60 degrees + 100% humidity would be in the “caution” zone at 160 horse heat index. But 100 with 30% humidity (close to my current weather) is totally OK? No, that makes no sense.
I would not hesitate to work a normal, fit horse in those temps assuming it was accustomed to the weather. Given your horses’ issues, it makes sense to be cautious, but I would just keep a close eye on them and work as normal unless they seem to be struggling.
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Humidity affects their ability to cool their body by sweating. I used around 150 -160 total. If the humidity was really high but temperature moderate I would usually ride, but at a leasureily pace. You can also ride in more heat if the humidity is low. If he is sweating too much, check his body temperature… Start cold hosing him asap. Keep at it until his body temperature drops to near normal. Don’t scrape the water off. That exposes his skin directly to the heat.
I saw an article recently that said use 85 degrees and 85% humidity as starting points. It is total of 170. If it is 90 degrees but humidty is in the 70s you should be okay. If humdity is above 85% you need a lower temperature.
The “feels like” temperature uses the dew point. That is the termperture where the air can’t hold anymore moisture so fog forms. As a lifelong yachtie that is what I paid attention to. If the dew point is 60, fog begins to form when the temperature drops to 60. That explains a lot of early morning fog . Overnight humidity burns off when the sun starts to rise. Dew points are more uncomfortable the closer they get to 70. Above 70 and it gets really oppressive. My asthma kicks up when it starts to get too humid. I can’t get a bright red inhaler anymore. It was easy to find if you dropped it in sandy footing.
This works here in Maine. Probably not so much in the desert. 
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Thank you everyone for all your responses…normally I wouldn’t think twice about riding in 70 with 90% humidity…most of my horses are Tb’s who don’t ever seem to struggle …my new Friesian with anhydrosis definitely struggles more than my Wb mare with asthma …not too long ago I rode the Friesian on what seemed like a comfortable nice summer day …temp was maybe 80’s and normal humidity…we did a brief ride in the outdoor ring and when we were finished he was blowing and flaring his nostrils with rapid respiration…I was really worried…in all my years I never had a horse get that overheated with such minimal work on not that hot of a day -he also didn’t sweat at all …since then I put him on a Anhidrosis formula oil …didn’t seem to do much …when that finished I switched him to KER EO3 oil …he seems to be sweating a tiny bit and also seems a tiny bit more heat tolerant but I still am cautious when it’s a humid day.
