The problem is the lungs are an incredible network of tunnels. If you spread out the surface area of your lungs, they would cover a tennis court (something I heard a long time ago, likely not exactly correct but generally right).
You could get UV light to the large bronchi but you couldn't get UV light into all the million little aveoli.
You could get UV light to the large bronchi but you couldn't get UV light into all the million little aveoli.