In me, it's because of the immunosuppressive effects of the stress hormones that flow during exertion. If I could only ride my bike nonstop from mid-July through the end of September, I wouldn't have to worry about the ragweed pollen 
. I remember vividly one day, years ago when I was racing, when I was sneezing uncontrollably on the sidelines, but as soon as my race started, I was breathing perfectly clearly.
Air flow actually brings more pollen into the nasal passages - as any motorcyclist with allergies will tell you. Again during the ragweed season, if I want to ride my motorcycle at all, I have to jam an OSHA-approved respirator into my helmet, or I'll be too groggy from allergies to ride.
				
			 
			
		 
			
				
			
			
				Speed comes from what you put behind you. - Judi Ketteler