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UV Questions from the Website

by Jim Bolton


Editor’s Note: From time to time, I receive questions that come in from the IUVA website. The following are some of the more interesting questions and answers.

Question: Tell me, why does glass have a good absorbance in the UV regions?

Answer: Glass is an impure composite of silicon dioxide. It is the impurities in glass that absorb UV, and, thus, ordinary window glass absorbs all UV wavelength below about 390 nm. If the impurities are largely removed, the material is called quartz; quartz is transparent to UV down to about 220 nm. If the silicon dioxide is ultrapure (made from zone-refined silicon), then it is transparent to UV down to about 160 nm.

Question: If wavelengths below ~390 nm cause the OH* production in TiO2, then are shorter wavelengths always better, monotonically? So when all other factors are held equal, 254 nm will produce more OH* from TiO2 than 365 nm, for example?

Answer: TiO2 is a semiconductor with a band-gap such that photons with wavelengths more than about 390 nm cannot create “holes” in the valence band. About 4 percent of these holes go on to produce OH radicals; that is, the quantum yield of OH production is only 4 percent [see Sun and Bolton, J. Phys. Chem., 100, 4127-4134 (1996)]. For photons with wavelengths less than 390 nm (e.g., 254 nm), the extra energy is rapidly lost in the crystal matrix, so the created “holes” have the same energy as for photons at 390 nm. Therefore, I would not expect that the quantum yield for OH production would change as the wavelength is decreased below 390 nm.

The following three questions are related.

Question 1: For a house with paintings/drawings/designs on the wall with gold paints, we would like to use a UVC germicidal lamp room sterilizer. The question is if the gold painting/carvings and drawings on the ceiling will be affected or will be faded if exposed to UV light? Would you still recommend them to expose UV light to this surface area? What will the effect be, and what is the recommended way to use it?

Answer to Question 1: As long as the UV dose applied is just sufficient for inactivation of microorganisms on the surface (UV dose about 40 mJ/cm2), there should be no effect on the surfaces exposed to the UV. That is, there would be no significant changes in the surfaces so exposed. Significant photochemical changes to the surfaces would require a UV dose at least 10 times that required for UV disinfection. If you can obtain a UV radiometer calibrated at 254 nm, you can measure the irradiance (mW/cm2) at the surface. The UV dose (mJ/cm2) is then the irradiance times the exposure time in seconds.

Question 2: I also would like to make an inquiry regarding plastics that will be exposed to UV light. Is it OK to expose any form of plastic to UV light, or are there restrictions? Because we had an incident where the UV light exposure to a baby bottle turned it yellowish even only after a couple of uses.

Answer to Question 2: As long as the UV dose is appropriate for UV disinfection (UV dose about 40 mJ/cm2), no plastic should be altered significantly by such UV exposure. However, if very high UV doses are used (>400 mJ/cm2), then some changes may occur. The degree of photochemical damage depends on the type of plastic used. It is clear that in the case of the baby bottle exposed to UV, the UV dose applied was far too high.

Question 3: We would like to ask if there will be any effect if makeup products are exposed to UV lights, or is it also safe to expose makeup products to UV light to sterilize them from any microorganism?

Answer to Question 3: Again, if the UV dose is not too high (about 40 mJ/cm2), then there should be no significant changes in the makeup materials. The important caveat is not to expose the makeup materials to very high UV doses (>400 mJ/cm2).