Cotton or microfiber material works the best. Loosely woven material is not going to protect you as much, though it could offer some.. Cloud shadows are one, but a more surprising one was the number of people walking around with umbrellas open.

I grew up in a heavily wooded area, so we didn't have a lot of direct sunlight all of the time. They're using the umbrellas to shield themselves from the sun. There are a number of things we do or assume about safety, and not all of it is accurate. When I moved to SoCal, I discovered a lot of things that I never noticed where I grew up. This begs the question, does it work.

Yes, clothing can protect you, but there are some caveats to that.

One is, of course, that it has to cover the skin. Long pants and a long sleeved collared shirt made of tightly woven material is ideal. I guess that depends on what you're expecting it to do. Here is how to protect you and your family. A pair of shorts may be comfortable, but they leave the rest of your legs exposed.

Yes, that would be expected if it was raining, but that wasn't and isn't the case.