What makes a light source safe?

Working in a science comes with an occupational hazard of  head-slapping. It doesn't matter how tolerant, how understanding you are of the fact that not everyone is a scientist and therefore lacks some of the insight that we take for granted. On a semi-regular basis, you find yourself face-palming, banging your head against a wall and generally weeping for the scientific literacy of humanity. Whether it's  a friend from college who has gone all homeopathic, a movie with impossible physics or a local news item that gets you to scream at the television, it's part of the territory.

Which is why I couldn't be all that surprised when, upon complimenting a coworker's manicure, I learned that her mother had gotten a free UV nail lamp because the cosmetology board had decided to replace all their old bulb UV lamps with LED light sources because they were 'safer'. My coworker laughed as she told me this, because we both know that it doesn't matter if your UV light is naturally emitted from unicorn horns fed only the most organic of herbage, it's still UV light and it can still give you cancer.

So, what makes a light source 'safe'? It depends, in part, on what you are using the light source for.

For example, in your house, you want a light bulb that isn't going to set your house on fire, explode, or release toxic gases. You aren't really worried about whether they can give you cancer, because the light they output is in the visible range, and sometimes into the infrared, all of which is non-ionizing. There are three options widely available to average person these days. There is old school incandescent bulbs, halogen bulbs and LED bulbs.

 Incandescent are familiar, and what most people alive today grew up with. They have a filament that glows white hot when you pass a current through them. They do not have any toxic gases and most people seem to think of them as the non-toxic bulb (though the tungsten in the filament is highly toxic, it's sitting there and who is going to lick it?) But it's incredibly energy inefficient. Most of the energy it uses goes to heat (infrared), not visible light. You can burn yourself by touching one that's been on a little while, and they can explode from thermal shock if you accidentally sneeze on one that has been on long enough to get hot (yes, I have done this).

 Halogens are becoming more familiar. They work just like any good old fluorescent bulb, with less flickering, by exciting electrons in a diffuse gas until they give off light, which then excites a coating on the bulb into giving white light. They are more energy efficient since heat is a byproduct and not the means of producing light, but they aren't hugely more efficient and they contain small amounts of mercury. They have to be disposed of properly at hardware stores or recycling centers, and it's not clear what you should do if one breaks.

 LEDs are the newest contenders. They use light emitting diodes to create light, which means they are semiconductor based. Semiconductors aren't the nicest things in the world to make, but they are not toxic if they break. They are very efficient (some of the better ones barely get warm) and very pricey. They have by far the longest lifespan, and are probably the nicest looking.

So for safety, in my house, I am switching over to LED bulbs as each of the other style bulbs give up the ghost. Lower fire risk and no risk of mercury poisoning. This is what a safe bulb in my house means.

But the safety question when it comes to things like tanning beds and nail polish curing is very different. The customer is unlikely to have to deal with broken bulbs and they aren't immediately concerned with the energy efficiency or fire risk. The questionable safety of such devices arises from the specific wavelengths of light used, namely ultraviolet or UV light.

UV light exposure is concerning over long periods because the wavelength of UV is small enough to interact with DNA molecules and energetic enough to damage them. When DNA gets damaged, it leads to mutations, some of which are harmless and others that can be very harmful  indeed.

 Now, we can put this to good use in sterilizers using UV-C, because it doesn't involve chemicals that might be dangerous to us or that bacteria might grow resistant to. The light destroys the  bacteria from the inside, like someone smashing your hard drive and motherboard would effectively destroy your computer. This is a good use of UV light.

UV light can be produced by any number of bulb types, including fluorescent bulbs, lasers of various types and LEDs. Older models of nail lamps used fluorescent bulbs, which are relatively cheap and give even light coverage.

So does switching over to a different type of bulb make it any safer for people who want to use these lamps? Nope. So long as they are using the same polymers that require the same wavelength of UV light to cure, the LED bulbs will be giving off the same  UV radiation as the old bulbs.

How safe are any of these nail lamps? Depends on who you ask. It is difficult to predict cancer risks in any population. This letter to the Journal of the American Academy of Dermatology suggests that, at least for the two national brand models they tested, in three minutes your hands are getting the equivalent of 4-6 hours of allowable UV exposure for construction workers. Each lamp puts out over 4 times the amount of UV energy than the sun. So while using them on occasion won't bring any more risk than staying outside in the sun all day, you may not want to use them on a regular basis.

Me, I'd rather get my skin cancer risk from taking a walk on a nice day.