Sweat Rashheat Rash on Forehead from Running

I didn’t used to get this. And I still don’t get it all the time. But sometimes during and after a long hard run, I get a rash on my forehead. I think it is caused by the profuse sweating and heat.
Last night I was running on my treadmill. I ran one easy mile at 10:00 minute/mile pace. Then I started on my fast running. I was scheduled to run 10 miles at 7:15 pace. I cheated by taking a couple minute breather between each mile. So I still ran 10 miles at 7:15 pace. I just kind of turned the tempo run into a long interval set. Hey, it has worked for me. Anyway, I was literally dripping sweat. I have a towel hanging off the side on my treamill, and I would be wiping my forehead. By the end of my workout, the towel was soaked. I was baking, and dripping sweat. It’s usually after workouts like these where I get a rash on my forehead. I am not sure if it is caused by the sweat, the heat, the combination of the heat and the sweat, or from wiping my forehead with a sweat soaked towel.
I did a search on “sweat rash” and “heat rash’, and some other terms. Some of the results mentioned a rash under sweathy clothing or a sweaty hat. But I don’t wear a hat on the treadmill. Some talked about blocked sweat glands, and that doesn’t seem like it either. So I am not sure what the cause of it is. It general doesn’t bother me too much. The skin is slightly sensitive, but not really painful. And it is red. Running my head under clean cold water usually helps it feel better.