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Jim Bolton

Editorís 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: Kindly inform about the UV protection by various types of materials, e.g., latex, nitrile and vinyl, etc. Which material can give complete protection against UV radiation?

Answer: Most plastics absorb UV quite strongly. Exceptions are polyethylene and polytetrafluoroethylene. You should be able to obtain the transmittance properties of a given plastic from the manufacturer.

Question: Hello, Iím working in a lab for forensic document examiners, where we examine identity documents. Different UV lamps are essential to this work. One of my tasks is also to recommend equipment to police officers and people who work with document examination. The most common source of UV is ~365 nm torches – some more powerful than others. We are not exposed to the rays over long periods of time, but we use them quite often. Sometimes we are recommended to use protective glasses by the producers, but to be honest, we donít.

Now, my question is, how harmful is it to use 365 nm torches without protective glasses? Have in mind that we donít expose our eyes directly to the UV light but indirectly as the light is reflected from the surfaces we examine. (Although, our skin is directly exposed when we hold the documents). Also keep in mind that Iím asking this question in a health and safety context, so itís something that affects our work.

Answer: There is an article by Wieringa in IUVA News (http://www.iuva.org/sites/default/files/member/news/IUVA_news/Vol08/Issue2/WieringaArticle.pdf) that gives the acceptable safe limit for exposure to 365 nm UV. The limit value is 27,000 mJ/cm2. This is a very high limit; for example, the safe limit at 254 nm is 6 mJ/cm2. If the irradiance at your eyes is 1 mW/cm2 (a fairly high value), it would take an exposure of 27,000 seconds, or 450 min, to come to the limiting UV dose.

Question: I want to make a sunlight box for winter tanning without exposure to cold wind. What kind of translucent material should I buy that will assure ultraviolet light transmission?

Answer: The best material would be quartz; however, there are some grades of polyethylene that transmit down to 300 nm (the limit of the solar spectrum at the earthís surface.

One thing that you should be aware of is that the ultraviolet fraction in sunlight decreases sharply with the angle of the sun away from the zenith. Hence, in the winter, there is not much ultraviolet in sunlight. So you are not likely to get much tanning in a winter exposure.

Question: We are producing cheese products and packaging into transparent glass jars. Those glass jars are cleaned by compressed air, but we want to make a second step in cleaning. We are discussing with our HACCP consultant about installing UV lamps in the storage area, as well as the conveyor belt where the jars are moving into filling machines. Would that help with sterilization? Which wavelength is most appropriate?

Answer: First of all, ordinary glass does not transmit the UV wavelengths that are necessary for UV disinfection. Only quartz glass is transparent to these wavelengths. Secondly, the most effective wavelengths for UV disinfection lie in the range of 220 to 280 nm. Many UV lamps are based on emission from a low-pressure mercury lamp, in which the emission is at 254 nm. Thirdly, UV disinfection is ďline of sight.Ē This means that only the surfaces that receive UV directly from the UV lamp will be disinfected. For example, the insides of the glass bottles and the rear of the bottles will not be disinfected.

Question: In a large drinking water utility using 100 million gallons of water per day with a UV disinfection system, how far does the UV light/wave penetrate through the glass sleeve and into the water to provide effective microbial disinfection?

Answer: The penetration depth of UV into water depends on the UV transmittance (UVT) of the water. For drinking water, where the UVT is >85%, the UV penetrates 10 to 15 cm. However, for wastewater, where the UVT is 40 to 60%, the UV penetrates only 1 to 2 cm. This means that the spacing of UV lamps should be much further apart for drinking water UV reactors than for wastewater UV reactors.

Question: I have units of mWs/cmm2 and mWs/mm2. How do I compare those?

Answer: There certainly is confusion in the literature in regards to units and names. Here is a brief summary:

Intensity. Not well defined. Itís better to use irradiance or fluence rate. Intensity can be used qualitatively as in ďsunlight has a higher intensity than moonlight.Ē

Irradiance. The total UV power incident from all upward directions on a small element of surface dA divided by the area dA of that surface. The units of irradiance are W/m2, but often mW/cm2 are used; 1 mW/cm2 = 10 W/m2.

Fluence rate. The total UV power incident on a tiny sphere of cross-sectional area dS divided by the area dS. The units of fluence rate are the same as that of irradiance.

Note that when the UV beams are parallel, the irradiance and the fluence rate become the same.

Radiant exposure. The time integral of irradiance – units are J/mm2 or mJ/cmm2; 1 mJ/cmm2 = 10 J/mm2.

Fluence (or UV dose). The time integral of fluence rate – units are the same as that for radiant exposure.

Other units are used, such as mWs/cmm2. Since Ws = J, this is the same as mJ/cmm2; microwatts/cmm2 = 1000 x mJ/cmm2.