I'm sure you are being exposed to some type of airborn mold/poison shrub/etc. I would say to stop and take a gander at what is different about that section and see if you can narrow down if it is some type of seasonal growth. Could the park have sprayed some type of weed killer/ plant spray? Sounds bad. It stinks to have to avoid that area, but maybe a little prevention might help. I break out with a terrible rash/blister thing on my chest. When I sweat my chest turns bright red and I get small blisters that sting. It's basically heat rash. I have found that clear caladryl gel on my chest prevents the rash. Maybe a small amount of some type of benedryl gel or caladryl lotion could help. Good luck.