Put people in poorly ventilated rooms, where coronavirus-containing aerosols are trapped in the air with nowhere to go, and their risk of getting COVID-19 skyrockets. Research has shown that you can decrease the risk by ventilating the room and filtering viruses from the air. Now, a new CIRES and CU Boulder-led study helps shine a light on another approach—germicidal ultraviolet light (GUV), which can inactivate airborne pathogens but also has potential to create an unhealthy indoor “smog.” The work, published today in Environmental Science & Technology Letters, finds that after GUV disinfection, the amounts of harmful secondary chemicals in indoor air have a significant but not overwhelming impact, suggesting that GUV can be used to fight the ongoing COVID-19 pandemic in environments at high risk of virus transmission.

“It’s been known for a long time that GUV can kill airborne pathogens. The technique regained attention during the pandemic, but very little was understood about the potential impact of secondary chemistry,” said Zhe Peng, a research scientist with CIRES and the chemistry department at CU Boulder and lead author of the paper. 

Indoor GUV disinfection inactivates the genetic material of airborne pathogens like SARS-CoV-2. But for the same reasons, it also oxidizes and breaks down ozone and volatile organic compounds, which can lead to the production of secondary chemicals that have negative health effects. Despite the recent push to employ GUV in more indoor spaces, this study is the first to look at the impact of GUV on indoor air quality.

“People have to pay attention to the pros and cons of any air cleaning or disinfection technique before using it,” Peng said. “If GUV kills viruses but also turns airborne VOCs into something more toxic like particulate pollution, then we need to weigh the benefit of reducing pathogens with the increased risk of air pollution.”

Peng and coauthors adapted a computer model that simulates realistic air chemistry to evaluate the impact of GUV on the chemistry of indoor air under typical room conditions. Their work focused on two commonly used wavelengths of GUV—254 nm, which is harmful for people and typically installed near the ceiling or in ventilation ducts, and 222 nm, which some research suggests is safe when shone directly on people, and thus may be usable for whole room exposure. 

The team found that both wavelengths of GUV triggered the formation of highly oxidizing hydroxyl radicals, which in turn oxidized volatile organic compounds (VOCs) in the air. This chemistry resulted in the production of gases and particles that can have negative health impacts. But their results suggest that the concentration of these secondary chemicals was not so large as to be a major concern at present.

“These results give us some more confidence to recommend the use of GUV immediately, given the pandemic and high incidence of the flu and RSV, in high-risk environments such as emergency room waiting areas, restaurants, gyms, choirs, and churches, where there is a high potential for superspreading of airborne pathogens, especially if adding sufficient ventilation and filtration is not feasible” said Jose-Luis Jimenez, CIRES fellow, distinguished professor at CU Boulder, and co-author of the paper. “However, in many lower risk settings or during periods of low respiratory disease transmission, we may be better off using simpler techniques such as ventilation and filters, which remove viruses and reduce indoor pollution rather than increase it.” 

The authors noted that more work is needed to evaluate the impact of secondary pollutants in different conditions: In some areas, GUV “smog” formation could be problematic, they wrote, and there could be surprises, such as the formation of more toxic pollutants.

Still, co-author and CU Boulder professor Shelly Miller said, the new paper could help inform policy, much as previous work by her and her colleagues, informed CDC’s guidance for GUV use in healthcare settings. “GUV has been used in schools against measles, in hospitals for tuberculosis…” Miller said, “and now for COVID-19.”