UV Air Cleaners & their Efficacy

February 03, 2021 6 min read

UV-C Warning label

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In laboratory environments, UV light - at a certain wavelength, distance and exposure time - has been shown to inactivate microorganisms.  But it is not obvious that a UV light in a standalone air cleaner is a reliable way to sterilize the air and provides any actual benefits to the user.  UV and UVC lamps in standalone air purifiers are often too weak to do what some companies would like the customer to believe. If a UVC light in an air cleaner is at the right wavelength to in be strong enough, it would create a serious health risk. The size of the air purifier and operating cost would also make it unsuitable for a most environments. Furthermore, there seems to be no scientific evidence that shows that UV light provides an actual benefit if used within a proper standalone HEPA air purifier. The US Center for Disease Control and Prevention (CDC) writes:

"The use of UV lamps and HEPA filtration in a single unit would not be expected to have any infection-control benefits not provided by use of the HEPA filter alone." 1

Can a UV light air cleaner sterilise the air that goes through it?

An air cleaner needs to move enough air to achieve several air changes per hour in a given room to effectively clean it. If the airflow rate is high, however, contamination in the air - such as viruses and bacteria - travel through the air purifier so fast, that the exposure time of the UV light on the contamination is only a fraction of a second, and thus too short to actually destroy it.  This is supported by a recent study that evaluated UV light and its effectiveness at a distance of a few cm from the targeted microorganism.  The study states:  

at "3 cm distance...partial inactivation at 1 min, with increased efficiency up to 6 min" and "15 min completely inactivated"." 2

Some manufacturers try to extend the exposure time by using multiple UV lamps, or even banks of UV lamps, and try to organise them in a variety of different configurations to maximise the exposure time of contamination to the UV light. 2  Nevertheless, even if we construct an UV air cleaner with a very generous 40 cm inner diameter, a moderate 300 m3/h airflow rate, and a UV-chamber height of 66 cm, exposure time would only be 1 second - far from the 1 minute required for 'partial inactivation'. If the numbers are changed to a more realistic and workable 20 cm diameter, a 500 m3/h airflow, and a 66 cm long UV-chamber, the exposure time decreases to 0.13 seconds.

Do you need a UV light to destroy microorganisms in the air purifier?

Some manufacturers state that their UV air cleaner is not able to sterilise the air that goes through it, but argue that the UV light is needed to destroy microorganisms in the filters that collect them.  

Proper leakage-free HEPA filtration, however, traps and retains viruses effectively, as an EN1822 classification of H12 or H13 shows. Furthermore, a virus such as SARS-CoV-2 is thought to only stay viable for a couple of days, and the filter life in a high-quality air purifier should be at least 12 months on average usage. 

"Ultraviolet (UV) lamps are not recommended in BSCs (biosafety cabinets) nor are they necessary.“

US Center for Disease Control and Prevention (CDC)

It is also important to point out that it is difficult to see how a UV light in an air cleaner could shine on all parts of a proper pleated HEPA filter. Any area that is not directly exposed to the UV light - which could be a significant amount in a pleated HEPA filter - will be unaffected and thus viral contamination not inactivated.  An additional complication is that the UV lamp itself needs to be protected from contamination, so that dust does not build upon the lamps and cover it.  The UV lamp will also have to be protected from the airflow itself, as it could cool down the UV lamps and negatively impact their effectiveness. 

Are there any health concerns regarding UV air cleaners? 

UV air cleaners have the potential to pose a health-risk, as UV light can produce ozone as an unwanted by-product. Exposure to UV light that is strong enough to be considered of ‘germicidal wavelength’ can cause cancer and rapid sunburns. Temporary and permanent damage to the eyesight and painful inflammation of the cornea is a risk, as exposure of such UV light to the eyesight will damage the retina. Ozone can also break down natural tissue, such as lung tissue, posing a particular risk to people with respiratory issues.

The Association for Aerosol Research writes on February 2021 in "On understanding the role of aerosol particles in SARS-CoV-2 infection":

"While viruses deposited on filters with the aid of UV radiation can thus be efficiently inactivated, it is currently unclear whether the findings can be transferred to airborne viruses. The method also harbours potential risks: UV rays cause damage to human skin when irradiated directly. In addition, UV radiation can lead to the formation of ozone in the room air. Accordingly, such methods should not be used if there are people in the room who could be exposed to UV radiation or ozone."

The University of Michigan department of Occupational Safety and Environmental Health (OSEH) writes:

„(OSEH) has adopted the position of the NSF, NIH, CDC and the American Biological Safety Association (ABSA) in regard to the use of UV lamps ... UV lamps are neither recommended nor required in biological safety cabinets. The use of Ultraviolet germicidal irradiation (UVGI) ... lends little to product sterility or personal safety in research settings, and has caused numerous hazardous UV exposures to employees while creating a hazardous waste disposal problem from the mercury in the bulbs. ... UV bulbs typically start to degrade and fail 6 months after installation. Without measuring the UV output there is no indication that the bulb is failing.“

UV light can also change in wavelength over time and produce ozone as an unwanted by-product at a later point. 

“The US Environmental Protection Agency designated 0.05 parts per million (ppm) of ozone to be a safe level. Lamps designed to release UVC and higher frequencies are doped so that any UV light below 254 nm wavelengths will not be released, to minimise ozone production. A full-spectrum lamp will release all UV wavelengths and produce ozone when UV-C hits oxygen (O2) molecules.”1

US Center for Disease Control and Prevention (CDC)

The disposal of UV lights can also be a concern.

Why have a UV light in an air purifier if it is most likely ineffective?

The likely reason why some manufacturers include UV lamps in their HEPA air cleaners is that they are more concerned with creating a compelling marketing and sales message, rather than actually creating a better air cleaner. 7  Integrating different filtration technologies into an air cleaner is then done to ‘tick all the boxes’ and knowingly or unknowingly mislead the end-customer into believing that such an air purifier will end up being superior in performance. The reality is that such additional technologies are mostly ineffective. 

Some of the manufacturers we have spoken with did not actually believe that their UC air cleaner can increase the air purifier’s overall efficiency by including a UV light.  Nor were they able to guarantee that the viruses will actually be killed reliably by the use of the UV lamps in their air cleaner. The real reason seems to be that the UV lamps are included in the air cleaner is because it allows them to earn more money with the replacement of UV lamps.  For safety reasons, UV lights are suggested to be changed very regularly.  As most UV lamps are relatively inexpensive to manufacture, they can provide very significant profit margins. 

What do government organisations say about UV lights for air purification?

Health organisations such as the National Institutes of Health, Bethesda, Maryland, recommend that health-care institutions should not use UV light for germicidal purposes due to their unreliability and because of the false sense of security which UV lamps can create.

„We conclude that although UVGI is microbiocidal, it is not „ready for prime time“ as a primary intervention to kill or inactivate infectious microorganisms.“

National Institutes of Health, Bethesda, Maryland, USA (Infection Prevention and Control Services)

Ref.:

1) Enforcement Procedures and Scheduling for Occupational Exposure to Tuberculosis, OSHA Instruction, 06/30/2015

2) Darnell, ME, Subbarao K, Feinstone SM et al. (2004). Inactivation of the coronavirus that induces severe acute respiratory syndrome SARS-CoV J. Virol. Methods,121: pp. 85-91. At: https://www.researchgate.net/publication/8362562_Inactivation_of_the_coronavirus_that_induces_se vere_acute_respiratory_syndrome_SARS-CoV

3) https://en.wikipedia.org/wiki/Ultraviolet_germicidal_irradiation

4) http://www.cdc.gov/mmwr/preview/mmwrhtml/00035909.htm
http://www.cdc.gov/biosafety/publications/bmbl5/bmbl5_appendixa.pdf

5) http://www.ncbi.nlm.nih.gov/pubmed/20569852

6) http://www.oseh.umich.edu/pdf/Uvbulb.pdf

7) Frank Hammes, 


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