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I've noticed newer air hand dryers sometimes have blueish LEDs on the bottom that shine light on your hands when you're drying them. One theory I've heard is that the UV light emitted by these LEDs helps to kill bacteria on your hands as you're drying. Example source:

The UVC Hand dryer has just the answer; as with many hospital operating theatres, it uses a powerful UVC light source to kill bacteria and viruses from the air that dries the hands, preventing their spread.

Is there any truth to that theory? It seems to me that neither the intensity nor the duration of exposure would be enough to make a real difference.

Blue light shining on hand below a dryer

closed as unclear what you're asking by Oddthinking Jul 13 '17 at 3:59

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  • UV radiation in itself is effective against waterborne bacteria (doai.io/10.1016/j.watres.2005.10.030), but I don't know how well that works with hands. – JAB Jul 10 '17 at 21:30
  • I guess the question remains whether the blue LED actually emits enough UV to be relevant. – Greg Hewgill Jul 11 '17 at 3:46
  • @Greg Hewgill: Yes, AFAIK all LEDs are pretty much monochromatic (white LEDs use an ultraviolet LED to excite phosphors), so actual UV LEDs in the drier wouldn't produce blue light. OTOH, it would be simple to add a few blue LEDs as a marketing tool... – jamesqf Jul 12 '17 at 18:08
  • @jamesqf: Thanks, that's more to the point of my original question. I had originally specifically asked about blue LEDs, but somebody changed my question to ask about UV LEDs instead. – Greg Hewgill Jul 12 '17 at 18:24
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    @Greg: To recap: You post claiming that hand dryers have blue LEDs, but give no source. You claim that people believe that this is because they emit UV and that helps eliminate bacteria, but you give no source. You include a photo that doesn't address these questions, with no source. Two people jump in to help fix it. Ultimately that doesn't help because the question was so unclear, but rather than clarifying, you complain. Please fix the question with notability references so we know what the question actually is about. – Oddthinking Jul 14 '17 at 17:53
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In 2005, a study was done comparing a number of hand drying methods:

The abstract explains the study supports the use of UV light on hand dryers:

Log colony-forming units (CFU) on palms and fingers increased significantly when hands were dried with warm air while being rubbed for 15 seconds (P < .001), and many bacteria remained at 30 seconds without ultraviolet light (P < .001) [...] Few CFU were detected on palms and fingers dried with ultraviolet light. [...] Ultraviolet light reinforced the removal of bacteria during warm air drying.

However, there was a 2008 systematic review:

It looked at some of the other conclusions that this study drew, and compared it to a larger study that drew different conclusions. The review authors concluded that

the evidence for whether any particular method of hand drying is more effective than another, is conflicting/level 4.

So, the first paper provides some evidence that UV lights on dryers help, but there are reasons to wait for more conclusive evidence.

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    Sure, I understand that UV light helps to kill microorganisms (in general, with sufficient exposure under the right conditions). But my question asked specifically about whether the blue LEDs on the bottom of these hand dryers emit enough UV to be effective at doing anything other than make my hand look blue. – Greg Hewgill Jul 12 '17 at 18:29
  • @GregHewgill: I do not understand. Are you alleging that the LEDs on some models of hand-dryers are only blue and not ultra-violet? I thought you were just bandying the different terms indiscriminantly - hence my correction. – Oddthinking Jul 13 '17 at 3:52
  • For the record, this is the unit that was tested: Clear Lady CP-8000 II [Japanese text] which is based on this non-LED UV light source – Oddthinking Jul 13 '17 at 3:53
  • @Oddthinking: I think it's clear that the blue LEDs on the unit in the picture wouldn't kill bacteria. So we'd need to ask whether some hand driers have UV LEDs (your example doesn't, but there may be others), and if so, whether they produce sufficient UV to be effective. And we might also ask whether they're more effective than soap & water :-) – jamesqf Jul 13 '17 at 17:28

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