Tuesday, September 7, 2021

A Fresh Air Look at Ventilation

Environmental factors contribute to the spread of microorganisms causing diseases. Sunlight can kill viruses in minutes, while increased air pollution could be one of the risk factors of more severe outcomes. Humidity is also thought to be important. A team of scientists from Northeast US analyzed COVID-19 cases from 2669 counties and found that cold and dry weather and low levels of ultraviolet radiation are moderately associated with increased SARS-CoV-2 transmissibility, with humidity playing the largest role. 17.5% of the virus’ reproductive number was attributable to meteorological factors, with temperature accounted for 3.73%, humidity accounted for 9.35%, and UV radiation for 4.44%. This is in line with earlier findings about SARS-CoV-2 being less stable at higher humidity and warmer temperatures in human nasal mucus and sputum. Like in previous environmental studies, however, these fractions were not the same everywhere and were higher in northern counties. 

20 years ago, American scientists Wells and Riley developed a model of the airborne transmission of infectious diseases such as tuberculosis and measles. A novel modified version of this model was used to estimate the impact of relative humidity on the removal of respiratory droplets containing infectious virus particles. The results showed that this impact depended on the ventilation rate and the size range of virus-laden droplets.  It was concluded that increasing the ventilation rate is more beneficial, while installing and running humidifiers may not be an efficient solution to reduce the risk of COVID-19 disease in indoor spaces. 

A popular metric for airflow is Air Changes per Hour (ACH, also called Air Change Rate).  It tells how many times the air within a space can be replaced with fresh air each hour. Increasing the ventilation rate from 0.5 ACH to 6 ACH was predicted to decrease the infection risk by half. Studies of US houses and apartments found typical ACH values between 0.5 and 2.0 (with open windows). 4.0 ACH is the minimum air exchange rate acceptable for commercial buildings, but 1.5 ACH is the reality for most schools. Opening a car window raises ACH to 6.  Natural ventilation combined with novel technologies could help to increase fresh air intake with minimal energy cost.

Virus clouds can, indeed, be dispersed with some fresh, clean air. But fresh air isn’t going to stop the spread of microorganisms in high-density crowds. Effective ventilation is only one of basic infection control strategies along with hand/environmental hygiene, social distancing, case surveillance and other evidence-based measures. 


Aganovic A, Bi Y, Cao G, Drangsholt F, Kurnitski J, Wargocki P. Estimating the impact of indoor relative humidity on SARS-CoV-2 airborne transmission risk using a new modification of the Wells-Riley model. Building and environment. 2021 Aug 23:108278.

Ma Y, Pei S, Shaman J, Dubrow R, Chen K. Role of meteorological factors in the transmission of SARS-CoV-2 in the United States. Nature Communications. 2021 Jun 14;12(1):1-9.


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