Air disinfection is an important part of the biological threat mitigation portfolio, but we currently lack highly-effective air disinfection tools that can be easily deployed to many indoor spaces in the near-term. Vapors of propylene, dipropylene, and triethylene glycol (PG, DPG, and TEG respectively) could be such a tool. They are commonly used in cosmetics, lighting effects, and food (in the case of PG), and in many settings they are very efficacious at inactivating many types of pathogens. At concentrations typically used for air disinfection, they are colorless, odorless, tasteless, and appear safe for humans to breathe.^1-6

While glycols showed promising results when they were first investigated in the 1940s, they fell out of popularity as the public health field turned its focus to surface and contact infections and pharmaceuticals. Because they are abundant and affordable, our initial assessment is that glycol vapors could be rapidly deployed as an additional layer of defense against airborne pandemics, and warrant further consideration.

A gap in our pandemic toolkit

Successfully suppressing indoor pathogen transmission will require a portfolio of interventions that layer together to protect occupants.

Air cleaning and disinfection technologies are a valuable component of this portfolio. Besides helping reduce risk in indoor spaces on their own, they can also compensate for the disadvantages of other transmission suppression interventions. For example, while personal protective equipment (PPE) is another necessary part of the transmission suppression portfolio, current respiratory PPE is often burdensome, must eventually be removed (e.g. for eating and drinking), and requires advance stockpiling to avoid shortages.

Existing air cleaning and disinfection technologies each have their own strengths and weaknesses:

• Building-scale ventilation can be expensive due to high installation costs and energy costs for conditioning outdoor air.
• Filtration and portable air cleaners can be affordable and accessible, but require very high airflow rates to achieve significant protection. Depending on the scenario, noise and air currents from air purifiers could cause significant disruptions to work environments.
• Germicidal UV light is promising, but is currently expensive and will likely take time to achieve economies of scale.

There is a gap in the toolkit for an efficacious, low-cost, and minimally-disruptive air disinfection technology that can be deployed quickly if another pandemic occurs soon. 

Assessing glycol vapors as a potential solution

PG, DPG, and TEG are abundant organic compounds that are extremely hygroscopic, meaning they absorb moisture from the air. They are registered for use in air sanitization and deodorization by the US Environmental Protection Agency (EPA)⁷, and are widely used in food, drug, cosmetic, and industrial applications (from salad dressings to perfumes to toothpaste to natural gas refining). In aerosol and vapor form, these glycols have been used for decades in theatrical effects and as non-hazardous smoke simulants.⁸

When vaporized into a room, they have been demonstrated to rapidly inactivate viruses, bacteria, and fungal spores, including common pathogens like SARS-CoV-2, influenza A, and Streptococcus (see the Appendix for a complete list).⁹ Glycol vapors can achieve 1,000+ fold reduction of pathogens in an hour (i.e. 3+ log reductions per hour) against many aerosolized pathogens, depending on the indoor environment. In the field, a study conducted over three winters from 1941 to 1944 in a healthcare facility for bedridden children, where direct contact between patients was limited and any infections were most likely airborne, showed a 90% reduction in infections in glycol-treated wards compared to controls. This included a 96% reduction in colds and a 90% reduction in tracheobronchitis, otitis media, and acute pharyngitis.¹⁰

Because glycol vapors are hygroscopic, they quickly dissolve into any moisture in particles. While their mechanisms of action are still not yet entirely understood, it appears that the condensed glycols subsequently dehydrate pathogens, biophysically disrupt membranes and structural proteins, and inactivate mechanisms of cellular entry.^5,11 These effects have been demonstrated to work against both airborne and surface-borne pathogens.⁵ Significant inactivation of airborne pathogens can occur at low concentrations that are largely invisible, odorless, and tasteless.

Evidence to date suggests that inhaling these vapors at the concentrations typically used for air disinfection is safe for healthy adults, although more research may be needed for sensitive populations.¹² After reviewing in vitro studies, animal studies, and human exposure reports for PG and DPG (in 2006) and TEG (in 2003), the EPA found no significant evidence for negative health concerns from these vapors.^13,14 While there are some case reports of adverse health effects from fog and haze effects in theater performers (such as throat, skin, and eye irritation, laryngitis, and allergies), these are from mixtures of glycols and other compounds at much higher concentrations than would be used for air disinfection, and occur in only a small fraction of people.¹⁵

TEG is likely the most economical glycol to deploy, as PG and DPG have lower boiling points and thus require higher concentrations to saturate the air enough to be efficacious.⁵ We expect it to cost ~10-50¢ per day to protect a 1000 ft² room, including TEG liquid and energy costs.

US and UK researchers initially investigated glycol vapors as an air disinfection tool in the 1940s and 1950s, alongside germicidal UV.^17,18 Unfortunately, interest in glycols and germicidal UV waned as consensus incorrectly grew that airborne transmission was not a significant mode of infection, and as antibiotics and vaccines appeared to be panaceas for infectious disease. At the same time, post-World War II funding dwindled and much remaining air disinfection research in the US was classified as part of biological weapons programs. These pharmaceutical approaches shifted the focus from the public to the individual and became the domain of medical doctors over public health professionals like sanitary engineers. There was a resurgence of interest in glycol vapors during COVID-19: Bleu Garde developed a TEG-based product series for continuous use and achieved emergency approval in six US states for use in occupied spaces, while Reckitt Benckiser released a DPG-based spray-can product for temporary disinfection of unoccupied spaces.²⁰

However, public health agencies involved in pandemic response (like CDC in the US and SAGE in the UK) largely declined to promote glycol vapors for air disinfection during COVID-19. Since the safety and efficacy profile of glycol vapors had not been fully studied at that point, public health experts were understandably skeptical about adding substances to indoor environments when other tools like ventilation, filtration, and PPE had not yet been fully employed.^21,22

Conclusion

Glycol vapors appear to be a promising tool for quickly and affordably surging air disinfection capacity during future airborne pandemics. We believe they warrant further investigation to fully understand and realize their potential. We recommend studies on safety of degradation products in controlled buildings; real-world efficacy in a range of environments; toxicology; industrial surge production; layerability with filtration and germicidal UV; and scalable dispersion methods.

Citations

1. O Gomez et al, “Airborne murine coronavirus response to low levels of hypochlorous acid, hydrogen peroxide and glycol vapors.” Aerosol Science and Technology, 2022. 
2. Z Sultan et al, “Effectiveness of triethylene glycol disinfection on airborne MS2 bacteriophage under diverse building operational parameters.” Indoor Environments, 2024. 
3. K Ratliff et al, “Efficacy of Triethylene Glycol (Grignard Pure) Against Bacteriophage MS2 in the Air and on Surfaces.” SSRN, 2023. 
4. M Mellody and E Bigg, “The Fungicidal Action of Triethylene Glycol.” Journal of Infectious Diseases, 1946.
5. CT Styles et al, “Propylene glycol inactivates respiratory viruses and prevents airborne transmission.” EMBO Molecular Medicine, 2022.
6. ACGIH, “Triethylene glycol.” 2024.
7. U.S. Environmental Protection Agency, Pesticide Product and Label System. 2024. 
8. ML Boroson et al, “Propylene Glycol as a Fire Smoke Simulant.” Naval Research Laboratory, 1981.
9. K Duggan et al, “Reviewing the evidence of antimicrobial activity of glycols.” Journal of Applied Microbiology, 2024.
10. TN Harris and J Stokes Jr, “Summary of a 3-Year Study of the Clinical Applications of the Disinfection of Air by Glycol Vapors.” American Journal of Medical Sciences, 1945.
11. Y Hirama et al, “Antiviral Effect of Propylene Glycol against Envelope Viruses in Spray and Volatilized Forms.” Viruses, 2023.
12. U.S. Environmental Protection Agency, “Propylene Glycol, Dipropylene Glycol and Triethylene Glycol Interim Registration Review Decision, Case Numbers: 3126 and 3146.” Regulations.gov, 2018.
13. U.S. Environmental Protection Agency, “Reregistration Eligibility Decision (RED) for Propylene Glycol and Dipropylene Glycol.” EPA Archive Document, 2005.
14. U.S. Environmental Protection Agency, “Reregistration Eligibility Decision (RED) for Triethylene Glycol.” EPA Archive Document, 2003.
15. SR Magari and CJ Wesley, “Theatrical Fog Exposure Assessment: Methods, Exposure Limits, and Health Effects – Literature Review” Colden Corporation, 2017.
16. W Lester Jr et al, “The Rate of Bactericidal Action of Triethylene Glycol Vapor on Microorganisms Dispersed into the Air in Small Droplets.” American Journal of Epidemiology, 1949. 
17. AD Langmuir et al, “Progress in the control of air-borne infections.” American Journal of Public Health, 1950. 
18. AD Langmuir, “Keynote Address: Epidemiology of Airborne Infection.” Bacteriology Reviews, 1961.
19. Personnel of the United States Naval Medical Research Unit No. 4, “The use of triethylene glycol vapor for control of acute respiratory diseases in Navy recruits.” American Journal of Epidemiology, 1952.
20. U.S. Environmental Protection Agency, “EPA Decision Documents for Emergency Exemption Requests for Use Of Grignard Pure.” 2021. 
21. C Noakes, Testimony to the UK Covid-19 Inquiry. 2024. 
22. U.S. Centers for Disease Control and Prevention, “Safety Precautions When Using Electrostatic Sprayers, Foggers, Misters, or Vaporizers for Surface Disinfection During the COVID-19 Pandemic.” 2024.