This paper introduces various low-pressure gas discharge UVC lamps that produce germicidal UVC (253.7nm) and the basic physical characteristics of the UVC radiation and analyzes the germicidal effect of these lamps, the range of use, the various factors that affect their germicidal effect, and the precautions to be taken when using them.

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1 Various low-pressure mercury lamps that produce germicidal UVC

Low-pressure mercury vapor discharge UV disinfection lamps are made of quartz glass tubes or short-wave UV tubes, filled with low-pressure inert gas and a small number of mercury elements, metal cold electrodes, or hot filament electrodes at both ends, by adding high voltage to both poles or after triggering the high voltage to start the lower power supply voltage to maintain the discharge, thereby producing UV light mainly at 253.7 nm. The germicidal effect of this UVC is very strong and can destroy bacteria or viruses that are harmful to humans, thus providing disinfection. The main varieties of low-pressure mercury vapor discharge UVC disinfection lamps are as follows:

1) Hot-cathode low-pressure mercury germicidal UVC lamps

The electrode of this discharge lamp is wound with a tungsten filament into a double spiral filament, coated with a mixture of barium carbonate, strontium carbonate, calcium carbonate, when heated by electricity, the mixture is activated to decompose to form calcium, strontium, barium oxide. When the lamp is started, the filament is heated to emit electrons, which bombard the mercury vapor inside the lamp, causing the mercury atoms to be discharged.

The electrode of this discharge lamp is a double spiral filament wound with tungsten and coated with a mixture of barium carbonate, strontium carbonate, and calcium carbonate, which is activated and decomposed to form oxides of calcium, strontium, and barium when heated by electricity. When the lamp is started, the filament is heated to emit electrons that bombard the mercury vapor inside the lamp, causing the outer electrons of the mercury atoms to leap to high-energy orbital positions and become excited states. The electrons that jump to the high-energy orbitals only stay for 10-5 to 10-11 s and then fall back to the low-energy orbitals, releasing energy in the form of ultraviolet light. More than 95% of the ultraviolet rays emitted by this lamp have a wavelength of 253.7 nm, and there are also small amounts of wavelengths of 184.9 nm, 404 nm, 435 nm, 545 nm, 577 nm, and 579 nm.

The low-pressure mercury lamp discharge pressure of mercury vapor is about 0.006Torr, the general use of ultraviolet transmission rate of more than 80% of the quartz glass for the lamp. The radiation intensity of this lamp is maximum at a temperature of 40 ℃. Low-pressure mercury lamps include the following four types.

uvc lamps

①Straight UVC Germicidal Lamps

The straight tube UVC germicidal lamp is the most classic structure, the length, and diameter of the lamp can be calculated by the formula. UVC germicidal lamps prepared with quartz glass tubes, 30W lamp radiation intensity of 90μW cm² or more (1m at), required in use not less than 70μW cm². Lamp life of 3 000h, power 40W, 30W, 20W, 15W, 10W, 8W, 6W, 4W and so on.

②H-type hot cathode low-pressure mercury germicidal lamp

9W H-type hot cathode low-pressure mercury germicidal lamp, the radiation intensity at 3cm from the lamp is greater than or equal to 9 000μW cm². 30W H lamp at 100cm from the lamp radiation intensity is greater than or equal to 200μW cm². The UVC germicidal lamps produced by Shanghai Chen Chen Lighting Co., Ltd, especially the high-intensity energy-saving portable UVC germicidal lamps recently developed in cooperation with the Institute of Electric Light Source of Fudan University through industry-university research, have excellent performance compared to the general H-type UVC germicidal lamps in terms of UVC intensity, germicidal speed, energy consumption, ease of installation, and service life have obvious advantages.

Low O₃ UVC lamps

The method is to add 0.01%~0.05% titanium oxide and 0.07% alumina to the quartz glass, so that the UVC wavelength less than 200nm is absorbed and the O₃ produced is very small.

④High O₃ UV lamp

This lamp produces a large amount of 253.7nm UV light and also irradiates a strong 184.9nm UV light, thus producing a large amount of O₃, which improves the disinfection effect because of the synergistic effect of O₃ and UV light.

2) Cold cathode low-pressure mercury germicidal lamp

The lamp is made of nickel electrodes, filled with mercury and argon in the quartz tube, by the strong electric field, so that the cold cathode emits electrons, bombarding the mercury atom so that it excites the light. The lamp can be made into a variety of shapes, such as disc-shaped, U-shaped, straight tube, etc., radiation of ultraviolet radiation more than 60% of the wavelength of 253.7nm.

3) High-pressure mercury germicidal lamps

The pressure of the mercury vapor in the lamp can reach several atmospheres, the power can reach 500 ~ 1 000W or higher, only a small part of the radiation spectrum is germicidal UVC, but the total energy is large, so it is still a good source of disinfection UVC, generally used for water disinfection.

The physical properties of the 253.7 nm UVC produced by these mercury lamps are described below:

Linearly propagating UVC propagates in space as a transverse wave, and its propagation speed c is the speed of light, which is equal to the product of the frequency f and wavelength λ per second of vibration and can be expressed by the following equation:

fλ=c

The released ultraviolet energy is in the form of light quanta and can be calculated by the following equation:

E = hc λ

where, E — the energy of light quanta, unit: J ;
h — Planck’s constant, h = 6.625 × 10-17 J-s ;
c — the speed of light propagation, unit: cm s, c =2.99 × 1010 cm s.

Ordinary glass and Plexiglas can not transmit 253.7nm UV, but quartz glass can transmit more than 80%, and the general plastic transmission rate is also very low. Polyvinylidene fluoride resin film (0.1 mm thick) can transmit about 60 % of 253.7 nm UV, and ethylene copolymer film (0.1 mm thick) can transmit 40 %.

The 253.7nm UV light penetrates 2cm of water and is blocked by suspended particles in the air. When the 253.7nm UV radiation hits the surface of the material, it is generally absorbed and converted into the internal energy of the material but does not easily penetrate into the deep part of the item, so the UV radiation can only kill the microorganisms on the surface of the item.

UV radiation at the interface of the two media will be reflected and refracted when entering the second media. The reflection coefficient of various materials to 253.7nm UV is different, magnesium oxide is 80 % to 93 %; polished aluminum is 60 % to 90 %; white paint is 46 %; white lime is 40 %, white tile is 4.7 %. The reflective material of UV lamps can be selected accordingly.

2 UV disinfection lamps on the microbial killing effect

The strongest wavelength of the bactericidal effect is around 254nm. UV light can kill a variety of microorganisms, including bacteria, fungi, viruses, rickettsiae, etc. Each microorganism has its own specific UV death dose threshold. Each microorganism has its own specific UV death dose threshold. The bactericidal dose K is the product of the irradiation intensity I and the irradiation time t. It is expressed by the following equation:

K = It

The same effect can be obtained with high intensity for a short period of time or low intensity for a long period of time with the intensity of the UV source above 40 μW cm². The lethal dose (μW-s cm²) of ultraviolet radiation to microorganisms on the surface of the object is: Staphylococcus aureus, 2 180 to 4 950; Staphylococcus Albus, 3 300 to 4 200; Escherichia coli, 2 100 to 6 400; Pseudomonas aeruginosa, 6 500; Serratia marcescens, 5 500. The dose of microorganisms killed by UV light can be estimated by a mathematical model (Y = kill rate, X = dose in the following equation):

Bacillus cereus:

lg(100 -Y) = 10.714 2 -2.407 8lgX ;

Bacillus subtilis black variant bacilli:

lg(100 -Y)=9.101 3 -2.054 31lgX ;

Escherichia coli:

lg(100 -Y)=7.411 3 -2.169 21lgX ;

Staphylococcus Albicans:

lg(100-Y)=10.366 2-2.739 1lgX.

The doses required to kill 99.9% of microorganisms in the water are Bacillus subtilis, 40,000; Escherichia coli and Staphylococcus aureus, 12,000; Mycobacterium tuberculosis, 20,000; and influenza virus, >5,000. In general, Gram-negative bacteria are the most sensitive to UV light, followed by Gram-positive cocci; bacterial bacilli and fungal spores are the most resistant; viruses are also killed by UV light, and their resistance is between that of bacterial propagules and bacilli.

uvc lamps

Highly resistant to UV light are Bacillus subtilis bacilli, radiation-resistant Micrococcus spp. and Orange Octococcus; moderately resistant are Micrococcus, Salmonella typhimurium, Streptococcus lactis, Saccharomyces spp., and Protozoa; less resistant are cowpox virus, HIV, Escherichia coli, Staphylococcus aureus, Proteus mirabilis, Legionella, Brewer’s yeast and T3 E. coli phage. The black variant of Bacillus subtilis strain ATCC 9372 has been used as an indicator strain for UV disinfection.

3 Factors affecting the effect of UV disinfection

1) The influence of voltage

The radiation intensity of the UV source is obviously affected by the voltage, the same UV source, when the voltage is insufficient, the radiation intensity is much lower.

30W straight quartz UV lamp radiation intensity at different voltages can be calculated by the following formula:

Y = 1.287X -177.87

Where, Y — the theoretical value of UV intensity, unit: μW cm² ;
X — voltage, unit: V.

If the intensity of 100μW cm2 at 220V as the acceptance control point of the new lamp, then the control line of the radiation intensity of the new UV lamp measured at different voltages is Y = 1.287X -183.14 If the intensity of 70μW cm² at 220V as the control point of the old lamp replacement, then the control line of the radiation intensity measured at different voltages is

Y =1.287X -213.14

2) The influence of distance

Generally speaking, the radiation intensity of UV lamps is inversely proportional to the square of the distance. The radiation intensity Y of the 30W low-pressure mercury hot-cathode straight tube lamp decreases with the increase of distance x from the lamp, and the two are exponentially related. The exponential curve equation is

Y =10 (2.885 9-0.760 8 x)

According to this formula can be deduced from the UV lamp radiation intensity at different distances.

3) Influence of temperature

The effect of temperature on the disinfection effect of UVC is achieved by affecting the radiation intensity of the UV light source. Generally speaking, the strongest germicidal UVC radiation is at 40 ℃. The temperature decreases, the output of the UV lamp decreases; the temperature is higher than 42 ℃, the radiation of the UV back to absorb more, the output is also reduced. Therefore, both high and low temperatures are detrimental to UVC disinfection. However, some germicidal tests have shown that in the range of 5 to 37 ℃, the temperature has little effect on the germicidal effect of UVC. At low temperatures, microorganisms become more sensitive to UVC.

4) Effect of Relative Humidity

In the disinfection of air, relative humidity also has an effect on the disinfection effect; when RH is too high, small water droplets in the air increase, which can block UV, so it is required to disinfect the air with UVC, the relative humidity should be below 60 %. For surface disinfection, if the irradiated surface is far from the light source, the moisture particles in the air will also affect the disinfection effect.

5) The effect of irradiation time

UV disinfection effect is exponentially related to the irradiation dose, which can be expressed as

N/N0 = e -KIt

where, N0 — the number of bacteria before irradiation;
N — the number of bacteria after irradiation for a certain time;
t — irradiation time;
I — irradiation intensity ;
K — constant.

From the above formula, it can be seen that increasing the irradiation time t, or increasing the irradiation intensity I, can increase the disinfection effect, and the irradiation dose is the product of irradiation intensity I and irradiation time t. Therefore, in order to achieve a certain degree of killing rate, it is necessary to ensure sufficient irradiation dose. If the radiation intensity of the UV source meets the requirements (> 40 μW cm²), the dose can be adjusted by adjusting the irradiation time to achieve the required dose.

6) The effect of organic protection

When the microorganism is protected by organic matter, the irradiation dose should be increased, because the organic matter can affect the penetration of UV light into the microorganism, and will absorb UV light.

7) The influence of microbial species and the number

Different microorganisms have different levels of resistance to UV light, so the dose of irradiation should be determined according to the type of microorganism to be killed. The more contaminated microorganisms on the disinfected object, the worse the disinfection effect.

4 Application of UVC lamps in disinfection

1) Scope of use of UVC lamp

UV disinfection lamp can kill measles virus, tuberculosis bacillus, influenza virus, adenovirus, poliovirus, coxsackievirus, staphylococcus, meningococcus, pneumococcus, and other microorganisms in the air. UV light is a simple, convenient, inexpensive, and reliable way to disinfect the air. UV light can also be used to disinfect surfaces. At the same time, it has a good sterilization effect on microorganisms in water, no residual toxicity, and no harmful products are formed after disinfection, so it can be used for drinking water disinfection. UV light also has a good killing effect on microorganisms in sewage and can be used for sewage disinfection. Since the UV disinfection lamp has the function of air disinfection, surface disinfection, and water disinfection, it can be widely used in public places and public supplies in hospitals, cultural and entertainment venues, bath service units, hotels, restaurants, bars, teahouses, public and cab cabins, light rail and subway cars, aircraft and ship cabins, stores, and shopping places, community places, schools, libraries and bookstores, public secondary water supply tanks and storage containers, swimming pools, banks and currency, child care institutions, sports venues and public fitness equipment, beauty salons, air conditioning systems, etc., to prevent the spread of germs in public places or through public supplies.

uvc lamps

2) Various applications of UV disinfection lamps and precautions

① Disinfection of the surface of goods

When using irradiation to disinfect the surface of items, it is best to use portable UV disinfectors to irradiate at close range, and can also take the UV lamp hanging type irradiation. Small items, can be placed in the UV disinfection box irradiation. Since different types of microorganisms have different sensitivity to UV light, UV disinfection must be used at the dose required to kill the target microorganism. For general bacterial propagules, an irradiation dose of 10,000 μW-s/cm² should be used; for bacterial budding cells, 100,000 μW-s/cm² should be used; viruses are more resistant to UV than bacterial propagules and budding cells; fungal spores are more resistant than bacterial budding cells and sometimes require irradiation of 600,000 μW-s/cm²; when the target microorganism is not known, the irradiation dose is not as high as that of the target microorganism. When the target microorganism is unknown, the irradiation dose should not be less than 100 000 μW-s/ cm². The irradiation dose is the product of the irradiation intensity of the UV lamp used and the irradiation time at the surface of the irradiated object. Therefore, according to the irradiation intensity of the UV light source, the required irradiation time can be calculated. For example, if a UV surface disinfector with an irradiance of 70 μW/ cm² is used to irradiate the surface of an article at close range, and the selected irradiation dose is 100 000 μW-s/ cm², the irradiation time required is:

100 000μW-s/ cm² ÷ 70μW/ cm2 = 1 428.6s ≈ 24min

② Disinfection of indoor air

When disinfecting indoor air by indirect irradiation, the preferred method is a high-intensity UV air disinfector, which is not only reliable but also can be used when someone is active indoors, and can generally be disinfected for 30 min. Under unoccupied indoor conditions, direct irradiation can also be used to disinfect indoor air, which can take the UV lamp suspended or mobile direct irradiation. When indoor suspended UV disinfection is used, the number of indoor UV disinfection lamps (30W UV lamp, intensity greater than 70μW/cm² at 1.0m) should be no less than 1.5W per cubic meter on average, with an irradiation time of no less than 30min.

③ Disinfection of water and other liquids

The disinfection of water and other liquids can be done by in-water irradiation or out-of-water irradiation. When using the in-water irradiation method, the UV source should be equipped with a quartz glass protective cover. Regardless of the method, the thickness of the water layer should be less than 2 cm and the water flow rate should be determined according to the intensity of the UV source. Disinfected water must meet the national health standards.

3) UV disinfection lamp application precautions

First of all, in the process of use, the surface of the UV lamp should be kept clean, generally, every two weeks with alcohol cotton ball wipe, found on the surface of the lamp dust, oil, should be wiped at any time. Secondly, when disinfecting indoor air with UV lamps, the room should be kept clean and dry to reduce dust and water mist, and the irradiation time should be extended when the temperature is lower than 20 ℃ or higher than 40 ℃ and the relative humidity is greater than 60%. When disinfecting the surface of the object with UV light, the irradiated surface should be directly irradiated by UV light, and the irradiation dose should be sufficient. Finally, however, great care must be taken not to expose the lamp to UV radiation directly to the human eye or bare skin to avoid burns.

References

[1] A preliminary study on the application of ultraviolet germicidal devices in classroom disinfection [J]. Zhang Debao,Qin Bifang. China education technology equipment. 2020(15)[2]Preparation and luminescence characterization of UVC phosphor YPO4:Bi3+[J]. Xiao YQ, Chen P, Zhu YH, Zhang N, Zhuo NZ. Chinese Journal of Rare Earths. 2020(06)[3] Comparative analysis of physical techniques such as ionizing radiation and electromagnetic radiation for sterilization [J]. Lv Zeqi,Xie Yanzhao,Yang Hailiang. Strong laser and particle beam. 2020(05)[4]Testing and analysis of radiation irradiance of different models of UV germicidal lamps[J]. Li Linwei,Chen Yuyan,Li Kelong,Chen Jinpei,Zhang Zegui,Yang Yi. China lighting appliance. 2018(11)

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