Answer to Question #12151 Submitted to "Ask the Experts"

Category: Lasers, Infrared, and Ultraviolet Radiation

The following question was answered by an expert in the appropriate field:

Q

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 US 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?

A

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

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 29 September 2017. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.