The density of the weave is the blocking mechanism for the UVA/UVB clothing, but as you say, I'm sure some manufacturers add suspicious chemicals to enhance the effect. An article interviewing some dermatologist I read said you could accomplish the same effect just by wearing multiple layers of clothing. I'm not allergic to all the things you are, my friend, but direct sunlight on my skin for more than a minute or two produces disastrous results!