I have 50% shade cloth on my greenhouse and from what I read UV light destroys the DNA of plants. I would think you will be Ok.

Not so simple, there are two types of UV light, UVa and UVb, one is more beneficial to plants and one is not(I cannot remember). I think it would be more important if you lived in the equator were the light is intense the same throughout the season. Here in continental US (Northern Hemisphere) I would argue that aside from Florida and the warmer sunnier, souther states, you shouldn't need to worry so much since the sun isn't that strong this time of the year anyway. And if anything you might find yourself trying to increase light if you are growing dends, catts, and vandas. I grow my orchids outside in summer, zone 5 (Chicagoland), and bring them into south facing windows with no shades for the winter. I drop the shades in late April or early May (and also in Sept/October if it is dry) when the sun is getting more intense and the possiblity of burning is high since all winter they were living with gray skies and sun a few days a week. If your orchids start "reaching" this winter (or summer, if you are in the Southern hemisphere), you may want to remove the shade cloth or get one with 20-30% transmission.

("reaching" means growing spinldy or elongated stems searching for more light.)

there are actually three types. UVC which is extremely harmful to plants becuause it is highly toxic.. UVB which causes plant colors to fade. and UVA which is neither harmful nor beneficial to plants growth. All are below 380nm. the start of the visibile light spectrum is 380 to 400nm. this is were the the process of chlorophyll absorptions begins. Your UV protected glass will not interfer with plant growth.

Actually UVC doesn't not reach the earth's surface due to the ozone layer so unless you are using a germicidal UV lamp, UVC is no problem.

L8r,
Don
http://www.justplainorchids.com

to go back to groucho7 original question the 99.9%uv light protection will not interfer with his growing of orchids or any other plant.