There are many things we have learned over time about COVID-19. What we didn’t know so far was that the immensely important role that well-designed HVAC systems have in the fight against the spread of the coronavirus pandemic.HVAC-COVID-19-Ventilation-superweapon-infographic

This article will take a look at the latest insights on how proper ventilation affects the spread of the coronavirus indoors. Since there is a lot of misinformation widely spread, we’ll use facts that come from experiments in credible medical research journals and probable theories from reputable experts. You can also download our printable infographic on Ventilation against COVID-19.

Let’s get started.


There are three vehicles for COVID-19 transmission

  • Contact, fomites are contaminated objects that can infect us if we touch them and don’t wash our hands afterwards.
  • Droplets which spread from a carrier’s mouth or nose when they breathe, talk, sneeze or cough.
  • Aerosols, which are miniscule droplets that float in the air for hours.

Coronavirus Transmission HVAC

When the virus from any of these three contamination sources reach our eyes, mouth, or lungs it is likely that we will become infected.

Coughing, sneezing, talking or breathing are activities that can result in droplet, aerosol or fomite infection. The use of masks can help reduce some the spread of these but some aerosol circulation is inevitable even with masks.

Droplets and aerosols

According to the WHO, respiratory droplets are >5-10 μm in diameter and droplets <5μm in diameter are aerosols (1).

Droplets fall rapidly to the ground and are transmitted over a short distance (some experts say ≤1m, others ≤2m). Aerosols on the other hand – (aka droplet nuclei) – can remain suspended in the air for long periods of time. This allows them to transmit the disease over a greater area compared to droplets (2) (3).HVAC-Covid-19 transmission through Aerosols

“Particles that are 5 μm or smaller in size can remain airborne indefinitely under most indoor conditions unless there is removal due to air currents or dilution ventilation 12.


In the early days of the COVID-19 pandemic there was some debate as to whether the coronavirus was an aerosolized virus or not but the current research shows that it is indeed airborne (4).

Studies found live SARS-CoV-2 virus (the virus causing COVID-19) in aerosols form for up to 3 hours in one study (5) and 16 hours in another (6). Even though in these experiments the aerosols were artificially created, they demonstrate that the virus can indeed be alive in aerosolized form.

Aerosol plumes containing the highest concentration of particles can travel up to 7–8 m from their source (7).

Since many people infected with SARS-CoV-2 remain asymptomatic for a prolonged period but also carry a similar viral load to symptomatic patients, aerosols are always going to be a transmission concern (8) especially since shedding of the infectious virus can last up to 18 days from the onset of symptoms (9).

To add to that, toiled flushing may also aerosolize the virus or disseminate droplets, since live SARS-CoV-2 has been found in saliva, urine and feces (10) (11).

Aerosolized particles (i.e., <5 μm) deposit in the lower respiratory tract in humans (12)


The important role of ventilation against COVID-19

Face masks offer limited protection for the wearer but when worn by the infected they help reduce exposures to infectious aerosols to health-care workers and other individuals (12). This is the reason we need to find additional tools to fight the spread of the virus.

 Ventilation can reduce the spread of COVID-19

According to the Lancet: “Whether droplet or airborne transmission is the main route, the risk of infection is known to be much lower outside where ventilation is better. […] Advice on spending time indoors should also focus on improved ventilation and avoiding crowded spaces. (13).

Atmospheric chemist Jose-Luis Jimenez of the University of Colorado Boulder has put together a pilot tool that provides example simulations using input data14. This tool is a theoretical estimator but may help give an idea of how ventilation and other measures can actually help aid in the fight against COVID-19.

An El Pais article presented results using this tool to simulate a bar with 18 people, one of whom was infectious with COVID-19 15. With no precautions, one person could infect up to 14 others within 4 hours. With the use of masks this could drop down to 8 people. Using masks, good ventilation and by limiting stays to two hours it is likely that there would be only one transmission.


Good ventilation may result in less severe coronavirus disease symptoms

Although there is no proof of this, theoretically it is likely that someone can get a less severe case of COVID-19 if they don’t get it from a poorly ventilated space. Here’s why:

The severity of COVID-19 can be dependent on:

  1. The lower initial viral load: the greater the exposure, the greater the strain on the immune system to fight back. “Infection control measures might not only reduce the probability of infection, but might also reduce the size of the inhaled inoculum, which has been associated with disease severity in influenza and other diseases 12”.
  2. Whether the lower respiratory tract was infected. The smaller infectious particles of aerosols can reach deeper in the lower respiratory tract in contrast to the larger particles from droplets which infect the upper airways of the neck and head 12.



We’ve put together a neat facts-based infographic on Ventilation and COVID-19 that you can print or share online with your clients.  



  1. WHO Guidelines: Infection Prevention and Control of Epidemic-and Pandemic-prone Acute Respiratory Infections in Health Care. Geneva: World Health Organization; 2014 (pp 17).
  2. Stetzenbach LD, Buttner MP, Cruz P. Detection and enumeration of airborne biocontaminants. Current Opinion in Biotechnology. 2004;15(3):170–174 as cited in Natural Ventilation for Infection Control in Health-Care Settings, Geneva: World Health Organization; 2009. ISBN-13: 978-92-4-154785-7.
  3. Wong KC, Leung KS. Transmission and prevention of occupational infections in orthopaedic surgeons.Journal of Bone and Joint Surgery America. 2004;86-A(5):1065–1076 as cited in Natural Ventilation for Infection Control in Health-Care Settings, Geneva: World Health Organization; 2009. ISBN-13: 978-92-4-154785-7.
  4. Lednicky JA, Lauzardo M, Fan ZH, Jutla AS, Tilly TB, Gangwar M, et al. Viable SARS-CoV-2 in the air of a hospital room with COVID-19 patients. medRxiv 2020.
  5. Van Doremalen N, Bushmaker T, Morris DH, Holbrook MG, Gamble A, Williamson BN, et al. Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1. N Engl J Med. 2020;382:1564-7.
  6. Fears AC, Klimstra WB, Duprex P, Weaver SC, Plante JA, Aguilar PV, et al. Persistence of Severe Acute Respiratory Syndrome Coronavirus 2 in Aerosol Suspensions. Emerging Infectious Diseases 2020;26(9).
  7. Bourouiba L. Turbulent gas clouds and respiratory pathogen emissions: potential implications for reducing transmission of COVID-19. JAMA 2020; published online March 26. DOI:10.1001/ jama.2020.4756. as cited in Fennelly KP, Particle sizes of infectious aerosols: implications for infection control, Published: July 24, 2020, The Lancet, Volume 8, Issue 9, P914-924.
  8. Lee S, Kim T, Lee E, et al. Clinical Course and Molecular Viral Shedding Among Asymptomatic and Symptomatic Patients With SARS-CoV-2 Infection in a Community Treatment Center in the Republic of Korea. JAMA Intern Med. Published online August 06, 2020.
  9. Liu W-D, Chang S-Y, Wang J-T, Tsai M-J, Hung C-C, Hsu C-L, et al. Prolonged virus shedding even after seroconversion in a patient with COVID-19. Journal of Infection. 2020;81(2):318-56. 2020
  10. Jeong HW, Kim SM, Kim HS Viable SARS-CoV-2 in various specimens from COVID-19 patients Clinical Microbiology and Infection. 2020 Jul 22:S1198-743X(20)30427-4. doi: 10.1016/j.cmi.2020.07.020.
  11. Lo IL, Lio CF, Cheong HH et al Evaluation of SARS‐CoV‐2 RNA shedding in clinical specimens and clinical characteristics of 10 patients with COVID‐19 in Macau. International Journal of Biological Sciences, 2020;16: 1698–707.
  12. Fennelly KP, Particle sizes of infectious aerosols: implications for infection control, Published: July 24, 2020, The Lancet, Volume 8, Issue 9, P914-924.
  13. COVID-19 transmission—up in the air, The Lancet Respiratory Medicine, Published: October 29, 2020 DOI: 10.1016/S2213-2600(20)30514-2
  14. Jimenez JL, COVID-19 Airborne Transmission Tool Available: New model estimates COVID-19 transmission in classrooms, buses, protests, more, June 25, 2020, Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder.
  15. Zafra M, Salas, J, A room, a bar and a classroom: how the coronavirus is spread through the air, 29 October 2020, El Pais.