Answer to Question #12151 Submitted to "Ask the Experts"
Category: Ultraviolet Radiation
The following question was answered by an expert in the appropriate field:
I am vitamin D deficient, so I am looking to raise my levels with a sunlamp specifically for this purpose. I live in a northern latitude and have dark skin, so for much of the year, natural sunlight is insufficient. I don't want to take oral supplements as it's unnatural, evolutionarily speaking. I'm considering a sunlamp (compliant with U.S. Food and Drug Administration regulations) whose glass filters out most of the ultraviolet-C (UV-C), but a very small amount remains. I would like to compare this with the amount of UV-C light that reaches earth from the sun, as one of the factors in my safety assessment. I understand that the amount of solar UV-C reaching the surface of the earth is negligible but not zero (I learned this from the World Health Organization [WHO] website and a few other reputable sources). I've been unable to find a number, though I've searched the internet. Can you help me or send me a reference?
The short answer to your question regarding the amount of UV-C that reaches the earth's surface is that most technical sources indicate that all the solar UV-C is absorbed by the atmosphere. Typically, scientific sources add caveats to their findings since there is always a small chance they could be incorrect. For example (and as you stated in your question), the WHO says that UV-C is totally absorbed by the earth's ozone layer, thus having minimal penetration to the earth's surface. However, the WHO also states that UV-C "is completely filtered by the atmosphere and does not reach the earth's surface."
Paul A. Charp, PhD