Disinfection and sterilization with UV light

UV light eliminates various types of microorganisms – it is a good solution if you want to inactivate viruses and kill bacteria or destroy fungal spores. Still, it is worth learning how to use ultraviolet radiation for disinfection to be truly effective.

The market offers a number of UV sterilizers. They are commonly used in medical centres, treatment rooms, beauty salons etc. The most commonly used UV lamps are designed for disinfecting rooms. You can mount such a lamp on a special mobile stand and freely move it. UV sterilizers for tools and other small objects resemble large boxes or microwave ovens. They can be used to sterilize items such as scissors, tweezers, etc.

Radiation reaching the surface is counted in joules per square meter (J/m2). We know from school that one joule is the amount work completed in one second by the power of 1 W, so 1 J = 1 W × 1 s. This means that if the power of radiation is constant, the energy that reaches the surface of an object is dependent on the time of exposure. The longer the time, the more of the virus that stays on that surface gets inactivated.

For disinfection and sterilization, ultraviolet light with a wavelength of 270 to 290 nm is used, while the whole UV spectrum includes wavelengths from 100 to 400 nm. The resistance of microorganisms to UV radiation varies. For instance, bacteria die most quickly under UV light, while their spores, viruses and fungi spores are more resistant. Most of the bacteria, viruses and spores that we know are effectively eliminated after they are exposed to 8 mJ/cm2 of UV. Fungi spores are the most resistant to UV radiation, but pathogenic fungi are less resistant to UV radiation than bacterial spores.

The dose of radiation is the product of radiation intensity and exposure time. This means that in order to find out the necessary irradiation time, we need to know the intensity of the UV light source. For instance, if we want to use a UV disinfecting device with the radiation intensity of 70 µW/cm2, and the distance from the contaminated surface is short, we can calculate the radiation dose corresponding to 100 µJ/cm2 using the following formula:
Formula
formula

It should be remembered that UV sterilizers may be harmful to health and have a negative effect on eyes and skin, so it is important to keep every UV light source shielded and turned off when the lid of the disinfection chamber is open. If only one side of an object is irradiated, remember to turn the object after the recommend time in order to disinfect the other side as well. When a UV light source is used in a room, it is best to wait until the room is empty before switching it on. When someone is in the same room, the UV light must be turned off after no longer than 30 minutes, and PPE must be used (skin cover, UV-blocking glasses). Water or other liquids can be disinfected with the use of a radiation source that is immersed in the liquid or with an external source. If an internal UV light source is used, it should have a quartz glass cover. Whichever method you choose, remember that the water layer to be disinfected should not exceed 2 cm.

There are no official research publications that would indicate the amount of energy required to inactivate the virus that causes COVID-19. However, its structure is comparable to that of the hepatitis A virus (HAV), which is more resistant to UV-C radiation, and which can be inactivated by an irradiation dose of less than 7 mJ/cm2. Based on the data above, it is estimated that in order to inactivate the coronavirus causing the COVID-19 disease, a dose of approximately 4 mJ/cm2 is needed. This means that a UV lamp that emits 1 W/cm2 of UV-C radiation, placed 1 m away from the surface, needs ca. 7 minutes (400 seconds) to disinfect the surface in 90%.

Smartphone disinfection chamber

LEDs generating UV radiation have been available for some time now, but at first they were mainly used to excite the luminophores that emit white light. At present, LEDs that emit UV light of different intensities are manufactured by a number of companies for a variety of applications. The luminous power of this type of UV light sources has dramatically increased and the advancement of semiconductor technology has allowed to extend the scope of applications of UV LEDs.

UV diodes are a very good solution wherever traditional UV light sources (such as a mercury lamp) cannot be used for some reason. For instance, if we install a small UVC LED lamp inside the tank of a coffee machine, the growth of microorganisms in water will be inhibited, as the exposure to radiation continues. The risk of electric shock is reduced thanks to a very convenient, low-voltage power supply for the LED, which makes it a good solution for all those situations where increased humidity cannot be avoided. Moreover, the long life of LED lamps reduces the need for costly maintenance.

The same characteristics of UV LEDs make them a very useful solution for compact air purifiers or deodorizers, in which UV-A light can irradiate the titanium-dioxide-coated catalyst to produce free radicals and in this way break down large organic molecules. Such purifiers can be installed in refrigerators and air conditioning systems in order to neutralize unwanted smells. When they are combined with germicidal UV-C light, they will help keep any air conditioning system fresh and free from airborne pathogens, with far less frequent filter cleaning and replacement.

Liteon is one of the leaders among LED manufacturers. Liteon products are perfect components if you want to build a disinfection chamber. In our example, LTPL-G35UVC275GZ diodes in a ceramic case have been used. They offer a maximum power of 3W, which ensures fast and efficient disinfection. The viewing angle that can be used in the constructed chamber is 120°. The delivery current of the LED is relatively high, i.e. 0.35A, and the rated voltage is typically 6.2V. In our application, the power of the supply current of the diode will be 2.17W.