I've used a UV light pen for water treatment, and typically scoop water
into the bottle. But will the pen kill any parasites, bacteria, and viruses
that are in the droplets around the rim of the bottle, or does it just kill
those in the water being stirred?
Submitted by - Ken S., Pembroke, NH
Everything inside the bottle dies, but, no, the droplets of water in the threads and on the outside of the bottle are not disinfected by a UV light device.
How much is living around the rim? Who knows? If you drink from the bottle, you should wipe the threads and rim dry first. You will soak up enough to almost always be safe. In fact, the chance of enough bad stuff being alive to make you sick after a good wiping is very, very small.
But the safest bet: scoop water into your water bottle with something other than the bottle you will drink from.