UV from sunlight compared with other sources

Quartz halogen lamps, Xe arc lamps, and sunbeds ....

I got a nice message from one of my regular readers - I don’t know his surname, but his first name is Steven. Here’s the substance of what he sent.

I'm an avid reader of your news letter, it's very much appreciated. I have a question that I hope you can help me with. I am trying to figure out the UVA exposure in my very bright home with a lot of windows. Essentially I would like to know whether the amount of exposure in my home is harmful or not. Do you have any advice?
(Perhaps even a topic that might interest other news letter readers as well?)

How could I refuse? Here’s the guts of my response ….

I’m pretty sure the amount of UVA indoors will be very small indoors. I know that the amount of UVA 0.5 m from a 1000 W quartz halogen lamp (that we use for calibrations) is a tiny fraction of that in sunlight.
…
Any UVA from sunlight coming through the windows will be less than you get outside because the glass blocks some of the (most damaging) shorter wavelength UVA. And the walls and roof block all of it. So overall there’ll be much less than outdoors.

If you want to verify UVA levels for yourself, you could purchase one of these hand-held meters from Solar Light. It looks like their model 4.0 (the green one pictured) would be best for comparing UVA sunlight and from artificial lamps. It would probably cost about $300 dollars.

Hope this helps.

I included the plot below (note the log scale for the y-axis). As you can see, in the UVB region, sunlight is at least 20 times more intense than the output from this bright 1000W calibration lamp at a distance of 0.5 m. And the energy you receive from a normal 100W ceiling lamp would be smaller by at least another factor of 10. Sorry, I haven’t yet measured the UV output from modern LED lamps. My guess is they’ll be comparably safe …

What I didn’t say in my response was that there are some sources which do emit far too much UV for safety. In particular, lamps used in sunbeds and solaria. The red line in the plot below is from a typical solarium that we used in studies to find the relationship between UV exposure and vitamin D production (results for those studies can be found here and here). The plot above is a simplified version of the ones below, which I drew way back in 2009. The upper version uses a linear y-axis that you’re more used to.

As you an see, the amount UVA (that causes skin-wrinkling) far exceeds that from sunlight, shown by the blue curve. In this case, UVA the sunbed exceeds that from sunlight by a factor of 5. If you look closely, you can just see the spectrum of light from the 1000 W quartz-halogen lamp (black line).

The lower plot shows the same three curves this time plotted on a logarithmic y-axis, and with the x-axis restricted to a shorter range of wavelengths. It also shows the spectrum from a powerful 1000 W Xenon arc lamp (yellow). The UVA output from the xenon arc lamp is far less than sunlight, but its UVB output far exceeds that from sunlight. It is dangerous. As is the UV radiation emitted by arc welders.

Bottom line. Don’t use sunbeds without the advice of a skin specialists. They’re dangerous. And be careful to protect your eyes and skin whenever you’re near arc welders.

If you have any questions about UV, please pass them on. I’ll do my best to answer them.