It works in the area where the light reaches.
I bought a pair of UV (Yellow) glasses and I actually could see the difference within two days of using the germguarian air sanitizer.  Spots that were on my carpet and along the wall that you could not see with the naked eye (in the area of the UV C sanitizer) disappeared after two days.  I bought two more to place in other areas of my basement.
Rating: 5 stars