When I took these, it was about 109 with 37% humidity (which is EXTREMELY humid for the desert) and I had been walking around for a couple hours when my friends noticed that I was bright red and not sweating. I felt fine, until I sat down in an air conditioned car and realized that I was dizzy and shaky and felt like I was going to throw up.
If you are going to be exposed to dangerously high temperatures, set a timer for yourself once an hour to check in. Look in a mirror, take your pulse, feel your forehead, MAKE SURE YOU'RE STILL SWEATING (if you're not sweating in temps like this you're potentially dangerously overheated or dangerously dehydrated) - basically pause for a few minutes to verify that you still actually feel okay because when the heat index is super high you *already* feel so miserable that you might miss the fact that you are showing symptoms of heat exhaustion.
Some things you can do to reduce your chances of experiencing heat exhaustion in a heat dome event are:
Keep electrolyte powders, chews, or capsules handy to supplement lost electrolytes. Make sure that whatever electrolyte supplement you're using includes sodium and does NOT include B or D vitamins (you're going to have to take multiple doses if you're outside for hours and you don't want to have to worry about limiting your sodium based on how much Vitamin D you've taken)
Plan downtime - give yourself a few minutes to recover and rest as best you can every hour you're in the heat
Wear lightweight, loose, breathable clothing
Spend time in the shade, carry an umbrella to make shade if there is no available shade where you're going to be
Avoid activity during the hottest parts of the day
Make use of a cooling towel; these work by soaking up water that cools by slowly evaporating, they are less effective in high humidity.
Use high SPF sunblock - this won't prevent heat exhaustion but a bad sunburn can make it harder to recover from heat exhaustion
Make it a point to check in on people around you as well as checking in on yourself.
If you interact with your local unhoused population, make sure that you know where your local cooling shelters are. If you are able to, get a case of water bottles and stick them in the freezer now so that you have bottles frozen and ready to hand out to people who don't have a place to cool down.