"Heavy, dark, opaque fabric blocks more UV rays than thin, sheer fabric, and the larger the umbrella, the more rays it can block," says Wu. Indeed, a study by dermatologists at Emory University in Atlanta found that standard rain umbrellas can block at least 77 percent of UV light [Source: JAMA Dermatology].

Can UV rays penetrate an umbrella?

According to the study, umbrellas intercept the full direct flow that comes from the Sun, but not the diffused radiation that penetrates through from the sides. A team of researchers from the University of Valencia (UV) has found that 34% of ultraviolet radiation filters through under beach umbrellas.

Can you get sunburn sitting under an umbrella?

That means as much as one-third of UV gets through — though other studies find as much as 84 percent does. You can get a noticeable sunburn from sitting under an umbrella in the grass in the middle of the day (sunny or cloudy) after just 35 minutes.