I did a search and it appears this subject has not been discussed in a long time, so I thought I'd bring it up again.
Every long ride I've done this summer, in temps above 30°C, and in blazing sunshine, has resulted in a nasty case of heat rash. Usually it just covers my thighs from where they join the torso to the knee, but today, it's from where my legs join the torso all the way down to my toes. This heat rash is red, lumpy, and painful ... prickly.
I've had this before a few times in Winnipeg when I cycled on hot days, but not every time I went out, and not this badly. I had my worst case of it in Australia ... I cycled in hot temperatures there for about a month, but only had heat rash on one day in about the middle of that month. That time, everywhere my black lycra shorts touched my skin broke out in a horrible rash. For about a week after that, I rode in regular underwear and beach shorts.
So I've got two questions:
1) What causes heat rash ... and therefore what could I do differently on my rides to prevent it?
2) What can I do to relieve the heat rash when it occurs? One thing that used to work for me in Winnipeg was to put ice packs on my thighs ... a few hours later the rash was gone. I've been doing that, but it doesn't seem quite as effective as it did back then.