Hi Helen: I live in the Eastern part of the U.S. and have had asthma about five years. I am 54. I visited Phoenix Arizona last June 00, and didn't find it a problem. The heat is very dry. The only way I think you would have a problem is if you treked down the canyon. The real problem would be getting up the trail in the heat. Do you have exercise induced asthma? To be quite honest I would make it my mission to do what I want to do and ignor the consequences. I makes me mad to think I am limited because of asthma. You go girl and enjoy yourself. The place is spectacular. Enjoy, Ilene Seidel Baltimore, Md. U.S.A.