Insanely itchy skin during part of ride?

I ride on a multi-use path to Philadelphia every now and then, and towards the end there is a stretch of trail that is heavily wooded and right alongside the river. There is a stretch (maybe 4 miles long) that ALWAYS gives me an instant red, itchy rash on any exposed skin (face, calves, and worst of all my arms) that is almost unbearable. It goes away shortly after I am out of that area, but it has happened every single time I have ridden through that area. I'm assuming it is something from the trees or an allergen, but does anyone have a similar experience, and if you were able to pinpoint what it was, what did you determine?

It's driving me crazy taking guesses at what could be causing it.

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.

Wow, that's weird. And scary. If it's year-round I wouldn't think plants (or anything sprayed intermittently) would be the culprit. What's nearby where it happens? Farm, factory, water treatment plant, power plant, ???

It could also be a developing allergic reaction to sun exposure. Google it for more info.