UV filters cumulative?

[slag magnet]

I've often wondered and have searched but cannot really find an answer.

If your hood filters out 99% or UV rays, and you wear safety glasses under the hood, do the safety glasses filter out 99% of the already 99% filtered light? And if you wore prescription glasses/contact under the safety glasses, would they filter 99% of the 99% of the 99% light?

Seems to me you could practically filter 100% of UV entering your eyes.

Thoughts?

[rlitman]

Yes, it is additive, but a welding filter is darker than 99% of 99%, etc.
Oh, and UV has a wide range of wavelengths. Welding filters must meet a minimum amount of filtration across all of them, safety glasses don't necessarily.