Ultraviolet light can make indoor spaces safer during the pandemic – if it’s used the right way

A nice article by Prof. Karl Linden at U Colorado, republished from “The Conversation” under CC license. Prof. Linden is a well-known fellow member of the UV research community and IUVA organization. I couldn’t say it any better than him!

Institutions like hospitals and transit systems have been using UV disinfection for years. Sergei Bobylev\TASS via Getty Images

Karl Linden, University of Colorado Boulder

Ultraviolet light has a long history as a disinfectant and the SARS-CoV-2 virus, which causes COVID-19, is readily rendered harmless by UV light. The question is how best to harness UV light to fight the spread of the virus and protect human health as people work, study, and shop indoors.

The virus spreads in several ways. The main route of transmission is through person-to-person contact via aerosols and droplets emitted when an infected person breathes, talks, sings or coughs. The virus can also be transmitted when people touch their faces shortly after touching surfaces that have been contaminated by infected individuals. This is of particular concern in health-care settings, retail spaces where people frequently touch counters and merchandise, and in buses, trains and planes.

As an environmental engineer who studies UV light, I’ve observed that UV can be used to reduce the risk of transmission through both routes. UV lights can be components of mobile machines, whether robotic or human-controlled, that disinfect surfaces. They can also be incorporated in heating, ventilating, and air-conditioning systems or otherwise positioned within airflows to disinfect indoor air. However, UV portals that are meant to disinfect people as they enter indoor spaces are likely ineffective and potentially hazardous.

What is ultraviolet light?

Electromagnetic radiation, which includes radio waves, visible light and X-rays, is measured in nanometers, or millionths of a millimeter. UV irradiation consists of wavelengths between 100 and 400 nanometers, which lies just beyond the violet portion of the visible light spectrum and are invisible to the human eye. UV is divided into the UV-A, UV-B and UV-C regions, which are 315-400 nanometers, 280-315 nanometers and 200-280 nanometers, respectively.

The ozone layer in the atmosphere filters out UV wavelengths below 300 nanometers, which blocks UV-C from the sun before it reaches Earth’s surface. I think of UV-A as the suntanning range and UV-B as the sun-burning range. High enough doses of UV-B can cause skin lesions and skin cancer.

UV-C contains the most effective wavelengths for killing pathogens. UV-C is also hazardous to the eyes and skin. Artificial UV light sources designed for disinfection emit light within the UV-C range or a broad spectrum that includes UV-C.

How UV kills pathogens

UV photons between 200 and 300 nanometers are absorbed fairly efficiently by the nucleic acids that make up DNA and RNA, and photons below 240 nanometers are also well absorbed by proteins. These essential biomolecules are damaged by the absorbed energy, rendering the genetic material inside a virus particle or microorganism unable to replicate or cause an infection, inactivating the pathogen.

It typically takes a very low dose of UV light in this germicidal range to inactivate a pathogen. The UV dose is determined by the intensity of the light source and duration of exposure. For a given required dose, higher intensity sources require shorter exposure times, while lower intensity sources require longer exposure times.

Putting UV to work

a robot emitting ultraviolet light in an empty hospital room
UV disinfection, which can be performed by robots like this, reduces hospital-acquired infections. Marcy Sanchez/William Beaumont Army Medical Center Public Affairs Office

There is an established market for UV disinfection devices. Hospitals have been using robots that emit UV-C light for years to disinfect patient rooms, operating rooms and other areas where bacterial infection can spread. These robots, which include Tru-D and Xenex, enter empty rooms between patients and roam around remotely emitting high-power UV irradiation to disinfect surfaces. UV light is also used to disinfect medical instruments in special UV exposure boxes.

UV is being used or tested for disinfecting buses, trains and planes. After use, UV robots or human-controlled machines designed to fit in vehicles or planes move through and disinfect surfaces that the light can reach. Businesses are also considering the technology for disinfecting warehouses and retail spaces.

ultraviolet light filling the interior of an empty New York City subway car
The New York City Metropolitan Transit Authority (MTA) is testing the use of ultraviolet light to disinfect out-of-service subway cars. MTA, CC BY-SA

It’s also possible to use UV to disinfect air. Indoor spaces like schools, restaurants and shops that have some air flow can install UV-C lamps overhead and aimed at the ceiling to disinfect the air as it circulates. Similarly, HVAC systems can contain UV light sources to disinfect air as it travels through duct work. Airlines could also use UV technology for disinfecting air in planes, or use UV lights in bathrooms between uses.

Far UV-C – safe for humans?

Imagine if everyone could walk around continuously surrounded by UV-C light. It would kill any aerosolized virus that entered the UV zone around you or that exited your nose or mouth if you were infected and shedding the virus. The light would also disinfect your skin before your hand touched your face. This scenario might be possible technologically some day soon, but the health risks are a significant concern.

As UV wavelength decreases, the ability of the photons to penetrate into the skin decreases. These shorter-wavelength photons get absorbed in the top skin layer, which minimizes DNA damage to the actively dividing skin cells below. At wavelengths below 225 nanometers – the Far UV-C region – UV appears to be safe for skin exposure at doses below the exposure levels defined by the International Committee on non-Ionizing Radiation Protection.

Research is confirming these numbers using mouse models. However, less is known about exposure to eyes and injured skin at these Far UV-C wavelengths and people should avoid direct exposure above safe limits. https://www.youtube.com/embed/YATYsgi3e5A?wmode=transparent&start=0 Research suggests that far UV-C light might be able to kill pathogens without harming human health.

The promise of Far UV-C for safely disinfecting pathogens opens up many possibilities for UV applications. It’s also led to some premature and potentially risky uses.

Some businesses are installing UV portals that irradiate people as they walk through. While this device may not cause much harm or skin damage in the few seconds walking through the portal, the low dose delivered and potential to disinfect clothing would also likely not be effective for stemming any virus transmission.

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Most importantly, eye safety and long-term exposure have not been well studied, and these types of devices need to be regulated and validated for effectiveness before being used in public settings. The impact of continuous germicidal irradiation exposure on the overall environmental microbiome also needs to be understood.

As more studies on Far UV-C bear out that exposure to human skin is not dangerous and if studies on eye exposure show no harm, it is possible that validated Far UV-C light systems installed in public places could support attempts at controlling virus transmission for SARS-CoV-2 and other potential airborne viral pathogens, today and into the future.

Karl Linden, Professor of Environmental Engineering and the Mortenson Professor in Sustainable Development, University of Colorado Boulder

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Happy Birthday Maud Menten!

Today (March 20) is the birthday of Canadian Dr. Maud Menten, born in 1879. Who’s that? Anyone who has learned about enzymes and biochemistry (including chemical engineering students) has likely come across the “Michaelis-Menten” equation. This is a way of characterizing how some enzymes work, and a mathematical equation that we can use to measure or predict enzyme kinetics (how fast an enzyme-mediated reaction will occur), which Michaelis and Menten published in 1913.

According to Wikipedia, Maud Menten was born in Port Lambton, Ontario, Canada, which is about 40 km south of Sarnia, on the St. Clair river border between Canada and the U.S. She was one of the first Canadian women to earn a medical doctor degree (at University of Toronto) in 1911. She went to Berlin, Germany, to work with Leonor Michaelis around 1913, who’s team was doing some ground-breaking medical research work in pH, buffers, and enzymes. This collaboration led to the famous publication and Michaelis-Menten equation which is mentioned by students and researchers a myriad of times since.

After some time in Germany, Menten returned to the University of Chicago where she completed a Ph.D. in biochemistry in 1916, studying the effects of adrenalin on hemoglobin. She went on to establish a career on the faculty of the University of Pittsburgh where she continued making significant discoveries in biochemistry and medicine, including early work on electrophoretic separation of proteins (a key biochemistry technique used to this day). After retiring from Pittsburgh, she worked in British Columbia for a few years on cancer research, then returned to Ontario where she passed away in Leamington in 1960.

I frequently ask students if they know who “Menten” was, in the Michaelis-Menten equation, and usually they don’t know. That’s a shame for Canadian students, since she maintained her Canadian citizenship throughout her life, and was a remarkable female scientist at a time when there weren’t very many women accepted, encouraged or active in science.

For more information, Wikipedia is OK, or this article from the Biochemical Society is good.

Pandemic Design – Future Impacts

Source: Pandemic Design –

One interesting topic I come across is “how will our pandemic experience influence technology and design in the coming years“, even after the coronavirus is long gone (preferably) or at least under control? There is a growing awareness that there are things we could be doing better to minimize infection transmission in various commercial and institutional settings, in addition to hospitals where this has been an obvious concern. Even if the coronavirus is completely defeated, reducing the spread of more routine “germs” like colds and influenza or gastrointestinal “bugs” would make good business sense overall, as those account for lost productivity and suffering too. Maybe it’s time we pay more attention to infection prevention in general, beyond just hand washing.

With this interest in mind, I recently agreed to participate on an Advisory Board with a local firm, fabrik architects inc., to provide input on design, materials, and devices that can be used in projects to address the current pandemic and possibly other infection transmission concerns. The Advisory Board members include architects, engineers, and epidemiologists. I look forward to contributing whatever expertise and ideas I have on things like UV disinfection and antimicrobial materials, in what is sometimes called “engineered infection prevention“. It is one way that academics can help to translate current research into new best practices.

UV Disinfection Confusion

The pandemic situation has generated a lot of interest and activity in UV disinfection, which has been keeping me busy. Whether it’s for masks, air, surfaces or whatever, there are lots of things getting posted and promoted for using UV. There seem to be an overwhelming number of devices and designs being suggested or sold online. Unfortunately there are also a lot of misconceptions, errors and possibly fraudulent claims being promoted. I’m not going to try and address each and every device (there are too many!), but I can provide some basic ideas that one should know or ask about when considering UV devices. If the supplier can’t readily provide answers or details, then something is possibly wrong. Here are a few key confusing points:

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Classroom Air Cleaners?

Schools of all sorts are looking for ways to re-open while minimizing coronavirus transmission risks. Harvard University’s School of Public Health recently issued a downloadable document on “Schools for Health”. In it they suggest a number of administrative and engineering approaches for reducing virus transmission in a classroom and school setting. It’s interesting and worth a look.

Photo by Pixabay on Pexels.com

Since I teach and do research in some aspects of HVAC (Heating Ventilation and Air Conditioning) and indoor air quality, those parts of the report caught my attention. They are suggesting that people consider using portable air cleaners in the classroom, especially in situations where the HVAC is non-existent or poor. They don’t give a lot of numerical detail behind that recommendation, but it’s fairly easy to work it out. So I’ve done some quick calculations to see where air cleaners might be useful from a more quantitative perspective.

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Coronavirus drifts through the air in microscopic droplets – here’s the science of infectious aerosols

Here’s a nice summary article about aerosol viral transmission, by a mechanical engineering professor.  The physics of aerosols is a foundational concept in air pollution control.

Aerosols are the tiny particles of liquid and material that float around in our environment. When they come from an infected person, they may be a significant source of coronavirus transmission.

Source: Coronavirus drifts through the air in microscopic droplets – here’s the science of infectious aerosols

N95 Masks and Re-Use

Recent pandemic developments have strained the supply of N95 filtering facepiece respirators (FFRs), which protect users from particles and aerosols in the air that they breathe. Technically, they must filter out at least 95% of 0.3 micrometre particles.

Normally these are meant to be single-use devices, and are removed and disposed of in a secure way to prevent infection transmission. However, with supply shortages people are considering or resorting to re-using these FFRs, possibly with some sort of chemical or physical disinfection process. Disinfection processes are never 100% effective, so this is not a great option, but I guess it’s better than having no protection.

One disinfection method that I’m very familiar with is UV-C disinfection, having done research in the area of photochemical processes for several decades. There is published literature available demonstrating reasonable disinfection success for UV when applied to N95 FFRs, so this may be an approach to consider if necessary.

I’m working on an overview of this literature (draft version now available at this link), but I’m happy to consult (pro bono) with health care institutions that are considering UV applications to deal with their situations (wanderson@uwaterloo.ca).


With the recent development of a viral pandemic, people are being reminded about the importance of handwashing for infection prevention. Coincidentally, in 2019 my colleague Prof. Marc Aucoin and I supervised a research study on handwashing for the CSA Group, a product standards organization. Specifically, our study aimed to determine if the faucet water flow rate had a significant effect on the ability of handwashing to remove bacteria from the skin.

You can access and read the full report on their website. The bottom line is that no, the water flow rate from the faucet didn’t have a significant effect over the range we tested, from 0.5 to 2.2 gallons per minute (about 2 to 8 litres per minute). Under all of those flow rates, on average about 99.3% of E. coli bacteria would be removed from the hands, which is good to know.

To do this study, we had to control all the other variables as much as possible, including the water temperature, and the amount and type of hand soap used by each person. The other big factor is the way that the hands were washed, including the length of time. For this study, we used a certain protocol from Public Health, and everyone involved in the study learned how to properly wash their hands. This was a good learning opportunity for people, including me, and so I reproduce the protocol that we used below. It’s a useful skill to know how to thoroughly wash your hands these days.

Recommended handwashing technique for infection prevention.

The Recycling Economy

Resource Recovery Partnership Conference

For the sixth year, I’ve been helping organize the “Resource Recovery Partnership” conference in collaboration with industry, government, and academic colleagues. This year’s event is on Thursday September 19, 2019, and registration is free for either in-person or webcast attendance. The final agenda is available, and anyone interested in the ideas behind sustainable materials, recycling, circular economy, zero waste, or materials and energy recovery might want to attend some of the webcast sessions. There are a range of speakers and panelists covering various aspects of policy development, technologies, and current statistics and trends. The talks are not highly technical, and anyone could benefit from some of the insights available here.

As our landfills (and oceans) fill with wastes, it has become clear to most people that solutions are needed to reduce wastes and to recover some value from the remaining waste materials. This is easier said than done, and requires a comprehensive approach incorporating technology, smart policies, economic drivers, and societal buy-in. These conferences have tried to bring together people from a wide range of backgrounds and interests, to try to advance progress in waste reduction. It’s a long and slow progress, but momentum seems to be building around the globe.

Autism and Air Quality

Autism, or more accurately Autism Spectrum Disorder (ASD), is in the news and public view a lot in recent years.  According to some recent reports, it is now diagnosed in 1 out of 68 children (1.47%) in the U.S.  Reasons for the apparent increase in diagnoses over recent decades are complex, but they lead us to wonder what is happening and what are the causes?

Recent scientific literature suggests that the specific causes are largely unknown, but there is a very strong genetic component (heritability of 80%).  Unfortunately, even the genetic aspects are very uncertain and probably highly complex, not just a simple set of genes like the ones that determine your eye colour.  Although genetics may play a large role, there are also indications that environmental factors are involved, perhaps in some sort of interaction with the genetic factors.

The popular and social media keep going in circles about vaccines, a factor for which there is no reliable scientific evidence at all.  At the same time, there seems to be complete ignorance of a growing body of scientific literature linking ASD with air quality.  A quick search through peer-reviewed scientific literature using the Scopus database shows at least 160 papers that mention “autism and ‘air pollution'” somewhere in the publication over the past 20 years.

I don’t know a lot about ASD, but I can comment on air pollution and so here I’ll discuss what I see from some of this literature.  Much of the research literature is only fully available if you have access to a university library (like me), but I’ll try to provide some links to at least the summary or abstract of the studies.  Much of this literature is highly technical however, so don’t worry if it’s not so easy to digest.

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