What percentage of UV light is blocked out by glass?

(This is one of many questions posted on an interesting Web Site that should be great for elementary and secondary school students, not to mention adults as well!)

Normal glass (as used in windows) is transparent to UV radiation to a wavelength of about 330 nm (UV-A). The transparency is quite high so almost all UV-A light will pass through glass. Below 330 nm (UV-B and UV-C), almost 100% is block by normal glass.

I am a research student, and I am working on UV treatment for air pollutants. Would you please tell and guide me as to how to calculate the UV dose for air pollution treatment?

This is a very complex issue. First, you have to be able to calculate the fluence rate (irradiance) distribution within the UV reactor. This requires a sophisticated computer program. Then you must carry out a volume average of the fluence rate (irradiance) over the entire reactor. The residence time of the air in the reactor is given by: volume/(flow rate). The fluence (UV dose) is then the product of the average fluence rate and the residence time.

This calculated UV dose is a "theoretical maximum" because it assumes that the air is perfectly mixed (in a radial sense) as it passes through the reactor. This is usually not the case, so the actual fluence (UV dose) will be less than the theoretical maximum.

One can experimentally measure UV dose by using biodosimetry. Here a (non-pathogenic) microorganism is infected into the air upstream of the UV reactor and allowed to thoroughly mix in the air stream. Samples are taken of the upstream and downstream air (after mixing). These are then compared with a laboratory (determined using a collimated beam apparatus) fluence (UV dose)-response curve, where the fluences (UV doses) are accurately known.

What causes absorption in the UV region?

Most molecules have absorption bands in the UV region. The absorption properties of a molecule are described by the "absorbance" (A) defined as

A = log[Eo/El]

where Eo and El are the irradiances incident and transmitted through a cell of depth l cm. The absorbance is also related to the concentration of the substance (let's call it X) by the relation

A = ε l [X]

where e is the "molar absorption coefficient" (units M-1 cm-1) and [X] is the concentration of X in molar (M) units (molar means mole/L)

When a molecule absorbs light, it is raised to an excited electronic state. In this excited state, it can react (either by dissociation or by reacting with another molecule) - this is called "photochemistry"; it can also return to the ground state either by releasing the excess energy as heat or by emitting a photon of light (this is called fluorescence).

What is the difference between a blacklight and a bluelight?

A "blacklight" is a fluorescent light tube that emits at about 365 nm - this is just below the wavelengths that humans can see, but it is absorbed by most pigments in clothes so that they "fluorescence". This is the effect seen in many bars and discos.

I'm not sure what you mean by a "bluelight" - perhaps you means "germicidal" low-pressure mercury lamp. They do glow "blue", but most of their output is at 254 nm, so DO NOT look directly at such a lamp when it is operating. These lamps are used in air and water disinfection, since the 254 nm light is absorbed by DNA in bacteria and viruses causing their inactivation.

I am familiar with the Fenton's Reagent (hydrogen peroxide + iron salts at pH ~4 to destroy phenol in wastewaters. Why I do not see any data or publications on the use of Fenton's Reagent as an antimicrobial? The hydroxyl radical is so strong; it should kill microbes.

Yes, Fenton's reagent would be very effective as an antimicrobial treatment of wastewater. The antimicrobial effect would probably arise more from the low pH (Fenton's works best at a pH of about 3) than from the generation of ·OH radicals.

It is more a question of economics. Lowering the pH to 3-4 is very costly and increases the dissolved solids level of the water. I think that you would find that UV disinfection would be a much cheaper process that Fenton's treatment.

Many of the specifications I have read concerning UV dosage ratings are in microwatt seconds/cm2. For example, one such unit is rated at 16,000 microwatt seconds/cm². I notice a UV dosage in your calculations or 87 mW/cm2. How do you convert this number to microwatt seconds/cm²?

Most scientists and engineers in the UV business now use the units "mJ/cm²" (millijoule per square centimeter) or "J/m²" (joule per square meter) for UV dose (the correct term is "fluence"). The units "J/m²" are used in most parts of the world except for North America, where "mJ/cm²" are used (1 mJ/cm² = 10 J/m². The old term "mW-s/cm²" (milliwatt-second per square centimeter) is equivalent to "mJ/cm²", since a "W-s" is the same as a "J" (joule). Note that 1000 microwatt = 1 milliwatt.

Note that the term "dose" ("dosage" is a word that is redundant with "dose" and should be discouraged) is normally applied in situations where the radiation is totally absorbed (e.g., UV in sunlight absorbed by the skin to cause sun tanning or sun burning). Since less than 1% of the UV incident on a microorganism is absorbed, the term "dose" is not appropriate for this situation. This is why the term "fluence" (which is defined in terms of UV "incident" on a tiny sphere from all directions) is more appropriate.

The units "mW/cm²" (for fluence rate or irradiance) are often confused (as you have in your question) with the units "mJ/cm2" (for fluence or UV dose). The "fluence" (UV dose) is obtained by multiplying the "fluence rate" (or irradiance) (units "mW/cm²") by the exposure time in seconds.

Is it possible to use UV lamps to protect postal workers from the terrorist threat of Anthrax contamination of mail?

Very little is known about the inactivation of Anthrax spores by ultraviolet light in air. One would have to arrange that the UV irradiance would be high enough so that the spores would receive a sufficient UV dose. If one were to set up such an arrangement, there would have to be safeguards to avoid exposing the workers' eyes to UV and also the workers should wear latex gloves to block UV from exposing the skin of the workers.

When Ultraviolet (UV) radiation is used to treat water, does the water become radioactive?

You are justifiably confused about the word "radiation". I prefer to use the term "UV light" rather than "UV radiation" for the very reason that you are confused. Ultraviolet is "light" - you can't see it because our eyes are not sensitive to UV; however, it is a form of light with wavelengths beyond the "violet" end (hence the term "ultraviolet") of the rainbow spectrum.

Since UV is "light", it travels through air and water at the speed of light and when the UV source is turned off, the UV is gone. There are no "residuals" and the water that has been exposed to UV is the same as it was before exposure, and certainly the water is not "radioactive". It is like shining a bright light into a glass of water. I think you would agree that when you turn off the light, the water has not changed.

UV water disinfection units are designed to provide enough "UV dose" so that any pathogenic microorganisms in the water are rendered "inactive". What happens is that the UV is absorbed by the DNA in microorganisms; the DNA is damaged so that the microorganism cannot reproduce. Cells that cannot reproduce cannot cause disease. The beautiful thing about UV is that it does its job while the water is passing through the unit, but after the water has passed through, it has been "disinfected", but its "water quality" has not changed.

I understand one needs 1/100 of a watt-second per square cm for water purification using 254 nm UV. What is the dosage for air?

Most regulatory bodies now specify a fluence or UV dose of 40 mJ/cm² (note that 1 mWs = 1 mJ) to assure at least 4 logs inactivation of any pathogenic microorganisms. Since the fluence or UV dose applied is independent of the medium, this requirement would also apply to air. However, I am not aware of any regulations as yet regarding UV air treatment.

Forgive what might be an unusual question: I am considering using UVC germicidal bulbs for oxidizing the surface of wood. Some of my peers have used UVB bulbs for years to create a 'tanned' surface to freshly exposed wood, to give it a jump-start in its aging process, but I seem to feel the need for a more intense treatment, and ultimately would love to know which spectrum is the best for this process. I will not be present while the lights would be operating, and there shouldn't be a safety problem; just a need for the most intense oxidizing exposure possible, in the shortest amount of time. Which wavelength is for me? For the record, I'm a violin bow maker, and both violin makers and bow makers in the last centuries have simply hung their assembled yet unfinished instruments and bows under the eaves to expose them to direct sunlight for 6 months for the desired result, but needless to say this is not a cost-effective way of violin or bow production in the 21st century. The radical alternative is direct nitric acid, or nitric acid fuming, but that has a rather negative health impact, and I would avoid it if I could.

Well, this is an interesting question! he word "oxidation" can mean many things:

1. Reaction with oxygen to form more "oxidized" compounds (e.g., the oxidation of methanol to formaldehyde).
2. Increase of the oxidation number of an element [e.g., conversion of Fe(II) to Fe(III)]
3. Complete mineralization of organic compounds to form CO₂, H₂O and mineral acids for any Cl, N, P, S, etc. present in the compound.
4. Direct photolysis of a compound {e.g., photolysis of nitrosodimethylamine [(CH₃)₂N-N-O]}.

I presume that you are referring to the first definition, as regards the "tanning" of wood.

Wood is a very complex organic structure with many components that absorb UV. However, not all UV absorption leads to chemical reaction (photochemistry). Most photochemical reactions occur at wavelengths less than 300 nm (there are many exceptions, e.g., photosynthesis, which operates with visible light up to 700 nm). Hence, I would suspect that the most effective UV lamp for your purposes would be a "germicidal" UV lamp, such as a low pressure mercury lamp (emits principally at 254 nm) or a medium-pressure mercury lamp (emits over a broad range from 200-400 nm in the UV). Low pressure lamps are relatively low power (a 1.2 m long lamp has a power of about 40 W). Medium pressure lamps are much more powerful (e.g., a 10 cm lamp has a power of about 1 kW). The "UV effect" will be generated at a rate that is roughly proportional to the power of the lamp (given that the distance to the "target" is the same).

You may find that pre-treatment with hydrogen peroxide will enhance the "UV effect", since photolysis of H₂O₂ generates hydroxyl radicals (·OH), a very powerful oxidizing agent.

You mention "safety", and I cannot emphasize enough that you must totally enclose the irradiation chamber so that there is no possibility of skin or eye exposure to anyone. UV is totally unforgiving in this manner.

If I wanted to use a UV germicidal bulb to disinfect tools in a PVC container, what effect would the UV light have on the PVC? What would you recommend to build a cost effective container out of?

PVC (polyvinyl chloride) blocks (or totally absorbed) the 254 nm ultraviolet light from a "germicidal" UV lamp. Thus any tools inside a PVC container would not be disinfected at all by a germicidal UV lamp outside the PVC container. Eventually, the PVC will degrade due to the photochemical attack by the ultraviolet light.

Some types of clear polyethylene and Teflon are transparent to 254 nm, but only for a thin layer.

In disinfecting water, does the temperature of the water and hence the lamp affect the UV output intensity? Is there an optimum operating temperature for the UV lamp and maximums and minimums thresholds?

Yes, the water temperature does affect the UV output of low-pressure UV lamps (not very much for low-pressure high output or medium pressure UV lamps). The optimum operating temperature for a UV lamp operating in the open is about 40 C (104 F). At 20 C (68 F), the output drops to about 50% and to about 10% at 0 C (32 F). When encased in a quartz sleeve with water on the other side, the effects are not so large. The optimum water temperature is about 22 C (71 F) and the output drops to about 80% at 0 C (32 F).

I wish to find out if all UVC lamps contain mercury in order to be germicidal, and do all UVC lamps produce an ozone smell?

Most UV lamps used for UV disinfection contain mercury. In the case of low-pressure lamps, only a few milligrams of mercury are present. In the case of medium pressure lamps, which are much higher power, a few grams are present.

Low pressure and medium pressure lamps do generate ozone if they have an envelop made of very pure (synthetic) quartz, which allows the mercury 185 nm emission to enter the air. This UV light is absorbed by oxygen in the air to generate ozone. Most UV lamps are made from a form of quartz that contains impurities that absorb the 185 nm emission entirely, so that they produce no ozone.

We are an industrial laundry company and are very interested in new technologies to facilitate the treatment and reuse of our process wastewater. Our plants generate 60,000 - 100,000 gallons per day of wastewater with BOD of 1,000 - 2,000 mg/L, TSS of 1,000 - 2,000 mg/L, oil/grease of 300 - 1,000 mg/L, etc. Our current method of treatment is typically a Dissolved Air Flotation (DAF) system.

Note: technologies proposed in the question are in italics

• Removal of color and residual dyes from DAF effluent for wastewater reuse.
  Again UV can be very effective. This requires what is called an "Advanced Oxidation" process, usually involving the addition of hydrogen peroxide, which on activation by UV generates hydroxyl radicals that attack and decolorize the dyes. However, the cost will be higher (perhaps by a factor of 10) than that for UV disinfection. Ozone is a competing technology for color removal, either alone or in combination with hydrogen peroxide for generating hydroxyl radicals.
• Process wastewater treatment as an alternative to DAF.
  If the DAF is reducing the BOD and TSS levels significantly, you probably want to retain this process or replace with another more efficient process.
• Process wastewater treatment to reduce BOD levels.
  UV (advanced oxidation processes) can do this, but you will probably find that biological treatment is much cheaper.
• Process wastewater treatment to remove heavy metals (copper, lead, mercury).
  UV can't help much here — UV works best to disinfect and to reduce organic levels. Groundwater remediation at sites contaminated with organics.
• UV is a very good option here in combination with hydrogen peroxide for generating hydroxyl radicals for advanced oxidation of organic contaminants.
• Air pollution control for VOCs.
  UV can be used for this application but there are competing technologies (GAC, catalytic thermal oxidation, etc.)

Our initial concerns with UV technology are residence time, contact chamber design for our flow rates (50-150 gpm), etc.

I suggest that you look at the UV Buyers' Guide (http://www.iuva.org/buyersguide) on the IUVA Web Site. One of the UV Consultants should be able to make a detailed assessment for you. Also look at companies in the Sections on UV Treatment of Wastewater and Advanced Oxidation. One of the companies listed in both sections should be able to give you an assessment as well.

Is it possible for the sun's rays to be a danger to the skin even in cloudy weather? smell? (submitted & answered by Joe Murgo)

The sun radiates a broad stream of energy to our planet. Most people think that the bright sunshine we see is the harmful stuff, but that's not quite the truth.

The energy that is most harmful to our skin is ultraviolet and is not visible to our eyes. It is this radiation that can damage our eyes and skin, increasing the chance for skin cancer and wrinkles. Like the visible light, clouds do absorb and scatter some of this ultraviolet radiation. But, even when it's cloudy, enough of this energy comes down to your skin to cause harm.

If you spend a lot of time in the outdoors, you should protect yourself -- even if it's cloudy.

Although it's not the visible light that does the damage, you can use it to determine your danger from ultraviolet radiation. A good rule of thumb is that if you can see your shadow and its length is shorter than your height, you should avoid exposure to the sun or wear sunscreen with an SPF of at least 30.

If your shadow is not visible, then the odds are that the clouds are thick enough to significantly reduce the amount of UV energy reaching the ground. Short-term exposure won't be harmful in this case, but when outside for a long period, sunscreen is still recommended.

They say that UV-C radiation is blocked by the atmosphere. Would the atmosphere become transparent to UV-C without ozone layer?

The sun's rays with wavelengths less than 300 nm are blocked by absorption by the ozone in the stratosphere. You can download the extraterrestrial (AM 0) and the standard terrestrial (AM 1.5) spectra from the National Renewable Energy Laboratory Web Site (http://www.nrel.gov/rredc/). This is why there is so much concern about the reduction in ozone levels caused by ozone depleting compounds (e.g., chlorofluoro hydrocarbons from Freon gases) escaping into the atmosphere.

We keep seeing the terms ‘watts’ and ‘joules’ in descriptions of how much UV is required to disinfect something. What do these terms mean? Are they the same? Why are they important?

Most people seem to be familiar with the term ‘watts' (from light bulbs and electric bills); but probably not the term ‘joules' (a metric measurement term). In short, both are used in measuring energy in any form (e.g., electricity as well as light):

• A Watt is a measure of the rate of energy delivery (analogous to gallons-per-minute flow rate for water delivery).
• A Joule is a cumulative measure of the total amount of energy delivered (analogous to total gallons of water delivered).

It usually is associated with how much time was needed to deliver the energy.

The way the units work is 1 Joule (J) of energy delivered = delivering 1 Watt (W) of energy for 1 second. In the UV world, we usually measure things in small increments, i.e., thousandths of a Joule or Watt. These are shown as ‘milli-Joules' (i.e., ‘mJ’ or 1/1,000 of a Joule), and milli-Watts (i.e., ‘mW’ or 1/1,000 of a Watt).

Example: 40mJ (cumulative energy) = 10mW delivered for 4 seconds