Most standard window screens do not block all UV rays, but they do provide some level of protection against them. The amount of UV protection provided by a window screen depends on several factors, including the type of screen, the material it is made of, and its mesh size.

Typically, window screens are made from materials such as fiberglass or polyester, which are not UV-blocking on their own. However, the weaving pattern and thickness of the screen can affect its ability to block UV rays. For example, screens with a tighter mesh weave and thicker strands will generally provide better UV protection than screens with a looser weave and thinner strands.

Some window screens are specially designed to provide increased UV protection. These screens may be treated with coatings or other materials that block more UV rays, or they may be made from materials that inherently block more UV radiation.

It's important to note that while window screens can provide some level of protection against UV rays, they are not a substitute for other sun protection measures such as wearing sunscreen and protective clothing, seeking shade, and avoiding prolonged sun exposure during peak hours.