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9 December 2013

Germicidal Lamps: Optical Properties and Hazards

So about two weeks ago, I came across this pet shop which sells some inexpensive ultraviolet water sterilizer for the purpose of disinfecting water supply for display aquariums. For MYR 50, I got a short-tube 17 cm in length, rated 11 watts germicidal lamp (photograph below)

water sterilizer mercury-vapour shortwave ultraviolet lamp: quartz enclosure 

A product of China, the package briefly mentioned its hazard for "unseen" radiation emitting from these lamps when energized. It takes first hand experience to truly feel the intrinsic danger of using these lamps without proper protection. Before I go into that, lets take a look on the design of germicidal lamps. 

Germicidal lamps are basically mercury-vapour fluorescent lamps we can find in our house or offices without the white phosphor coating. Typically lighting up fluorescent lamp emits visible light but together with it, substantial amount of radiation are emitted in the ultraviolet (UV) range. Fortunately most of it especially the "harmful ones" are blocked by the phosphor powder and glass tubing itself. Germicidal lamps however utilize particularly these UV radiation emitted from energized mercury-vapour to kill microorganisms and so instead of glass, which absorbs or "block" UV radiation, the tube was constructed out of fused quartz to allow these UV radiation to escape. This light, in the range of the so called C-subtype ultraviolet radiation (UVC at 280-100 nm) or also known as shortwave UV can be easily absorbed by bio-molecules (especially DNA) within microorganisms or cells, breaking its molecular bonds and thus "disable" or perhaps destroy its structure and thus the organism. 

Emission spectra of germicidal lamp (black) and shielded light with 2 mm thick glass sheet (red)

Using a commercial USB input spectrometer, I have recorded here the emission spectrum of my germicidal lamp (image above). The two sets of data corresponds to emission spectra for non-shielded germicidal lamp light-source (black line) and spectra for light shielded with a sheet of 2 mm thick soda-lime glass. There are a few important points we can extract from the spectra above. 

For one, we know that the lamp emits strongly with wavelength at 253.7 nm. This precisely falls into the UVC range and it is characteristic to electrically excited mercury atoms. What was interesting is that it takes only a thin sheet of glass (I used a 2 mm thick glass from photograph frames) to blocks completely the 253.7 nm and most of the 313.35 nm emission lines. This suggest typical glass can be used as a shield against such shortwave UV radiation. Note that the glass sheet also blocks a bit of near infrared light (from 700 nm towards 900 nm) but it let visible light (400 to 700 nm) passes without any blockade.

UVA (365.49 nm line) cannot be blocked by ordinary glass. 

Figure above shows the emission line of the germicidal lamp in the ultraviolet regime. We can see other than 253.7 and 313 nm peaks, one relatively strong peak is with wavelength around 365 nm. These fall into the range of A-subtype (UVA) ultraviolet which the glass sheet did not block it as effectively as it does to UVC radiation. 

Now, moving on to the biological hazards of using this lamp. As we know, ultraviolet radiation is the kind of light that people normally say it is harmful to our unprotected skin and eyes. From the guidebook: Hazards of Optical Radiation by McKinlay et. al. (1988) it states germicidal lamps are very efficient source of UVC radiation with approximately 50 % of the 11 watt electrical energy being converted to light, where up to 95 % (depending on the pressure of mercury vapour inside the tube) is emitted in 253.7 nm line. The monochromaticity of these source of light and the biological efficacy when the light is shine on it made these lamps hazardous, if used carelessly. 

 Observing filament heating to provide thermionic electrons for impact ionization to start a mercury-vapour plasma

Failure to observe exposure safety using these light can result in unintended biological damages. The most significant damage will be on the eyes, which I have a personal story to tell. When I bought the lamp, I knew  germicidal lamps being an excellent UVC source, its 253.7 nm photons can actually breakdown the molecular bond of oxygen in our atmosphere, producing ozone gas. Now ozone gas has its distinctive "musty" smell, so I was quite curious to observe this effect. Plus, turning on the lamp the first time to observe the glowing cyan plasma (picture above)  is quite fascinating itself. I can't help but to look closer at the filaments (the ones similar to incandescent light bulbs) as it heats up providing thermal electrons to "knock off" electrons in mercury atoms creating cascade of "knock offs" in more and more mercury atoms until the formation of the cyan plasma.

During the process of observation, I wasn't wearing any protective glasses. It only takes a while, less than 10 seconds of direct exposure, the distinctive smell of ozone is obvious. I used a powerful rare-earth magnet to bend the shape of the plasma (which I will provide a detail account later) and that was the end of observation. The observation was in the late afternoon and there are no acute symptoms immediately after exposure.

Couple of hours later around 4 am, I must have entered REM sleep which the movement of my eyeball start to trigger immense pain and irritation within both my eyes. It felt like grains of sand trapped underneath the eyelids but there was of course no sand. The tears produced (epiphora) was very profuse and rubbing the eyes seems to make matters worse. It was quite a frightening experience especially when I have no idea it was due to exposure of UVC radiation. Sensation of pain gains progressively worse after waking up and the only way to relieve it from getting worse is to close the eye and prevent any eyeball motion. Immediately, I was rushed to the emergency where I was laid down and flushed to the eyes copious amount of saline. After thorough flush-out, they did not find any contaminants but the pain was slightly relieved and I am able to forcefully open my eyelids, which all point light sources now appears to be fuzzy diffused spots. 

It was 6 am in the morning. The emergency sent me to ophthalmology dept. for diagnosis. About 2 hours later, just before examination, the pain returns and I was given a temporary eye-drop anesthesia, which works in immediate effect. Ophthalmologist couldn't find the irritant (of course) but based on observation told me I had a mild eye-infection, following prescription of anti-inflammatory eye-drops.  
The healing was rapid. In fact, without application of the medication, the pain was significantly relieved in less than 48 hours after hospital diagnosis. After a second separate incident (of a much smaller scale) I come to realize the source to the incident was my germicidal lamp. 

Because the naked eye was not protected against the bright 253.7 nm light (which is invisible, mind you). Much of it was absorbed by the first layer of the exposed eye, i.e. cornea and the conjunctiva. The injury of the cornea from UVC radiation is photochemical: unlike our skin which react to protect UV exposure by secretion of melanin, (the pigment that turns us tan under the sun) the cornea have no cellular components for releasing chemicals to protect itself. As such, the damage was directly a result of light-induced cellular death to the epithelial cells of the cornea (photokeratitis) and if the damage extend to the epithelial cells of conjunctiva (photoconjunctivitis). Full recovery can be expected in about 2 days which is the normal life-time of the epithelial cells of the cornea. Experiments on rhesus monkeys by Zulich (1980), shows the threshold radiant exposure for corneal injuries range from a few J/m^2 at 260 nm to 10,000,000 J/m^2 at 380 nm. 

Because UVC radiation from germicidal lamps are so bright, I also want to add that I have observed minor skin-peeling on my face a few days after close direct exposure, especially on soft epidermis near the eyes. This the same mechanism exhibited in sun-burned skin and to make matters worse, there is a slightly increased risk of skin cancer in exposed part due to the interruption of DNA chain inside the skin from energetic UVC photons.

In short, I just want to emphasize and I can never express it enough with the safety on working with open-air germicidal lamps. Although the damage on human skin and eyes are very real and vivid to me, it should be noted that UVC radiation can be shielded and it must be under any circumstances. Germicidal lamp can be a very useful source of light for disinfection, sterilization even in mineral identification but proper precautionary steps must be taken to avoid unintended biological damages. 

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