UV disinfection lights can produce pollutants in the air

Air sanitisers that use ultraviolet (UV) light to kill pathogens, such as those that cause the flu and COVID-19, can also cause unintended chemical reactions in the air, producing pollutants that could be a health risk. That’s according to new research out of the US, which has been published in the journal Environmental Science: Processes & Impacts.

The most common type of UV air sanitation has a wavelength of 254 nm, because that energy level is the most effective at breaking apart DNA and killing pathogens. But this high-energy UV can cause sunburn or temporary blindness, and so has to be directed away from people while still shining on the air they breathe. This is one of the main reasons why this form of air purification isn’t used more widely, and why there has been a lot of interest in using less-damaging UV light.

UV light with a wavelength of 222 nm is still effective at killing pathogens, but it doesn’t penetrate as deeply into human skin, so it’s safe to shine directly on people. Because of this, 222 nm UV light can be used to sanitise the air in small, crowded places like classrooms, lifts and public transport, where 254 nm UV sanitisers would be difficult to install easily.

But this wavelength has its own drawback: it turns oxygen in the air into ozone. Ozone itself can be concerning, and it is also highly reactive with other chemicals in the air and creates new secondary compounds. Researchers at the US National Institute of Standards and Technology (NIST), Michael Link and Dustin Poppendieck, wanted to know whether these new compounds could pose a health risk, and so set up an experiment in a real-world environment that would feature several different chemicals.

The researchers opted to conduct their study in a men’s restroom because it had urinal screens — those brightly coloured pads placed over the drain to prevent splashing. These screens release fragrances (to mask the odour of urine) that include terpenes, a group of chemicals that reacts strongly with ozone. The study primarily focused on terpenes, but most restrooms have air filled with chemicals such as cleaners and other scented hygiene products that would also react with ozone.

The team installed 222 nm UV sanitisers in the restroom along with a suite of chemical sensors. After analysing their results collected over two weekends — one with ventilation and one without — they found that the 222 nm UV light was producing enough ozone to react with chemicals in the air. The by-products included formaldehyde, volatile organic compounds and nanoparticles.

The researchers will need to conduct additional studies to determine how dangerous these pollutants might be, including on a wide variety of indoor spaces and environments that could benefit from the use of UV sanitisers. Other environments are expected to create different kinds of chemicals at different rates, and ventilation could affect how long they will stick around in the air. These studies could be used to develop tests for UV air purifiers and guidelines for how to minimise their production of volatile chemicals, which could ultimately guide decisions on how to best deploy UV sanitisers to optimise their benefits to human health.

Image caption: Dustin Poppendieck and Michael Link carefully suspend state-of-the-art air quality measuring equipment over bathroom stalls. Image credit: R Eskalis/NIST.