Greetings everyone, this is my first post. I was hoping that someone might be able to help me explain what I am experiencing. Before I get too far, I just want to say that I have already searched this forum for issues related to sweating / acne and haven't found what I was looking for. But I did read through the other exercise / sweating threads before posting this.
I live in Japan. I am in my early 30s. The derms here are usually pretty aware of different issues, but haven't been able to help me solve this problem.
At the beginning of last year I suddenly broke out with what I thought was a rash at the time. Literately woke up one morning and found my forehead covered in red marks, which turned into whiteheads shortly after. At first I just treated it like acne, washing my face a little more than usual. Saw a derm who gave me some creams to try. I kept using the creams for a few months but it never cleared up so I started seeing another derm who gave me stronger creams / gels that helped but caused irritation in my skin (actually damaged my skin). I even ordered some BP from this site and used it for a couple months. It worked great, but my skin turned red and I developed rosacea. I don't think it was just the BP that triggered the rosacea, but after my face would get inflamed / irritated from using BP there was no way to stop the redness. Even after stopping the use of BP. My face would just stay red for weeks. Acne would return, etc. I eventually found that IPL works for calming the redness and go every once in a while.
I eventually switched to azelaic acid which works great. It stops the big pustules from forming, but I still get red marks and small pimples. But while on the AA I was able figure out what causes my acne: sweating. Here in Japan we have pretty humid summers, so even walking to the train station can trigger the sweating. I exercise a lot, even back when the initial red spots occurred I was exercising pretty heavily, with the occasional dry sauna.
So now I exercise a lot less that I used to and always in front of the AC at full blast. I even stopped exercising for a month and my face completely cleared up. Does anyone know what this condition is called? Also, the acne isn't itchy like with Miliaria and the acne doesn't appear after flushing (only sweating) unlike acne rosacea. I am at a loss. The derms here in Japan that I have spoken to about the issue don't have any idea what it is. When I sweat I break out mostly on my forehead.
aSaaaaaaaaaThe most like reason is due to the mixture of sebum and sweat. I don't understand why some websites stated that working out can help prevent acne when most experience the other way round instead.
Usually what I will do is to rush to the toilet and take a shower right after working out.
No idea honestly, at first I was like oh this may be food allergies.
Just an idea, I'm allergic to pork so if I use a hair product that has pork fat in it(which is not rare at all) & I sweat the product tends to run a bit more on my skin & I break out further more. But I really don't think thats the issue your having. But eh food allergies may be something to check out.
Thanks for the replies. You guys are quick! 
Regarding the food allergies, I just had a allergy test two months ago, they took my blood and tested me against 33 allergens. Nothing. No allergies to anything they tested me against (soy, milk, nuts, etc.). Up until I started having this issue I was a strict vegetarian (over ten years). After I started breaking out I changed my diet to include meat, fish, and dairy to see if possibly there was something lacking from my diet. The only thing that changed was my skin turned oily. A couple months ago I switched back to the vegetarian diet and the oil problem stopped. I feel pretty good about my diet and don't think that it is triggering the acne. I have tried all sorts of exclusion diets since this started happening. When I thought I found some food that triggered it the acne would just pop back up.
Regarding the showers after exercise, I always shower right after my exercise. If I sweat during the exercise I get acne. It doesn't matter how quickly I shower afterwards. I think it has more to do with the fact that the process of producing sweat is occurring rather than the sweat itself. Like maybe the sweat glands are irritated and causing inflammation that leads to acne. But I am unable to find anything related to this online and the derms here in Japan have no suggestions. For now I am just going to stop exercising until this is completely cleared up. Fortunately where I live I have to walk a lot during the day so I know I get plenty of exercise just getting around.
...but I want to find out what this is. Why is it that when I sweat I break out? Maybe it isn't sweat, maybe it is flushing? But only doing strenuous things causes the type of flushing that triggers the acne?
I exercise a lot too, and often wondered if there was a link between my exercise and acne. Anyways, I had the thought for you to try using those oil blotter sheets periodically during your workout. They would get the oil and sweat off sooner than a shower. I think some of them are even medicated for acne. Not sure, never tried them, just a thought (I wear makeup, and don't dare go without as I am the instructor and everyone is looking at me!). Anyways, don't know if it will help but could be worth a try. Also, since your breakout is on your forehead, try switching your shampoo or hair gel to something different if you have not already. Maybe its as simple as that, wouldn't that be wonderful?! Good Luck to you! 
Thanks for the reply. The acne showed up last February, so since then I have questioned just about everything I can think of. I wash my face before exercising. I don't use any hairspray, wax, gel, etc in my hair. Stopped eating sweets / fatty foods. You name it. But it wasn't until I stopped exercising that the acne cleared up. It isn't just exercise, but anytime I sweat heavily. Even a walk in the humid heat will trigger the acne.
I really only have one question: what is it called when acne is triggered by sweating? I am not really looking for tips or suggestions for prevent the acne. I know what is causing my acne and I am looking for a name for it so that I can speak to my derm about it.
I have read the articles about how exercise helps acne, and sweating is good for the skin. And I believe that is true. But in my case something connected to sweating is causing my acne. I don't think this is common, but I have no doubts about what causes my acne. And I certainly don't need a doctor to tell me his / her opinion about what they think is causing it. I know that sweating triggers mine (or something related to sweating process).
Also, for anyone else out there who suspects that sweating / exercise is triggering your acne, please experiment and find out. Take a break. give yourself a couple months off. No gym, no sports, no running. Just rest. Use your typical acne medication and get some R&R. Stay indoors, chill out. See what happens.
Right, I guess I just need to know what causes acne-like symptoms after sweating. It is possible that this isn't acne in the sense of acne-vulgaris but more in the sense that white heads develop. I could word this differently: what is it called when sweating causes white heads? Or, what is it called when white heads are a symptom of some other non-acne problem related to sweating / heating up?
Hi!!! I am having the same problem as you... Sweating would cause acne on my forehead & cheeks. You are so right about whiteheads! Whiteheads then inflammation then cystic acne! Were you able to find out the name of this condition? or solution on to how to prevent acne caused by sweating (other than not to sweat because that would be nearly impossible) ? If yes, please share. THANK YOU!
I also have skin troubles that flare up when i get over heated and sweat. My derm has done a few small biopsies and diagnosed me as having two conditions: 1) Grovers disease, 2) Pityrosporum folliculitis. Try looking these up and see if they match what you are experiencing. Sweat and heat are triggers for both. The first condition is believed to be due to occluded sweat glands. It is almost like the sweat glands are partially blocked, the sweat collects in/under the skin, inflammation and small infections result. Hope this helps.