What Indoor Environmental Factors affect the spread of COVID-19?
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the virus that causes COVID-19. This blog highlights some of the key parameters which could affect the spread of COVID-19 in indoor environments.
Does indoor air temperature and humidity affect the spread of COVID-19? We know that people spend a lot of their time indoors and as winter approaches workers will spend more time indoors.
According to the Mayo Clinic the quality of the indoor environment has a major impact on human health.
It affects risk of disease through multiple factors, including indoor air pollutants; surface contamination with toxins and microbes; and contact among people at home, at work, in transportation, and other indoor public and private places.
A major question we need to explore is does the indoor environment affect the spread of COVID-19?
It is well established that environmental factors such as temperature, humidity, and ventilation, play a role in the persistence, infectivity, dispersal and removal of viruses and can alter human defence mechanisms that protect against respiratory pathogens.
Cold temperature and low relative humidity are believed to increase the transmission of respiratory viruses, including human rhinovirus, and avian influenza virus.
SARS-CoV-2 most often spreads via droplets when infected people exhale, cough, sneeze, or talk.
Research at the University of Sydney indicates that dry air appears to favour the spread of COVID-19. This research concluded that when the humidity is lower, the air is drier, and it makes the aerosols smaller. Aerosols are smaller than droplets. When people sneeze and cough those smaller infectious aerosols can stay suspended in the air for longer. This increases the exposure for other people. When the air is humid and the aerosols are larger and heavier, they fall and hit surfaces quicker.
The study estimated that for a 1 percent decrease in relative humidity, COVID-19 cases might increase by 7-8 percent.
This increases the argument for people to wear a mask, both to prevent infectious aerosols escaping into the air in the case of an infectious individual, and exposure to infectious aerosols in the case of an uninfected individual. More detail on the University of Sydney study can be found here.
The virus may remain infectious in aerosols for hours and on surfaces up to days.
Ventilation and air filtration are two obvious areas which we could control to reduce the risk.
The American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) recommends maintaining indoor relative humidity (RH) within the range of 40% to 60 % RH, and the United States Environmental Protection Agency recommends maintaining 30% to 50% RH, ranges that optimize thermal comfort, air quality, and reduce growth of mould.
The Health and Safety Review magazine (Nov 2020) recently published an article on how building ventilation and filtration can help beat coronavirus.
This references a few recently published guidelines on ventilation:
“Guidance on non-healthcare building ventilation during COVID-19” by the Health Protection Surveillance Centre (HPSC) aimed at commercial and public buildings https://www.hpsc.ie/a-z/respiratory/coronavirus/novelcoronavirus/guidance/educationguidance/Guidance%20on%20non%20HCbuilding%20ventilation%20during%20COVID-19.pdf
The American Conference of Governmental Industrial Hygienists (ACGIH) White Paper on Ventilation for Industrial Settings during the COVID-19 pandemic https://www.acgih.org/tlv-bei-guidelines/vent-comm-position-paper
“CIBSE COVID-19 Ventilation Guidance”, (Version 4, 23rd October 2020) The Chartered Institute of Building Engineers (CIBSE) (available to purchase) https://www.cibse.org/knowledge/knowledge-items/detail?id=a0q3Y00000HsaFtQAJ
The HPSC guidance advises that relative humidity be maintained between 20% to 60% if feasible and in fact points out that that lower humidity (less than 20%) is known to increase an individual’s susceptibility to infection.
This Mayo Clinic study indicated that higher temperature increased COVID-19 growth rate, relative humidity was associated with a nonsignificant decrease in growth rate, and ultraviolet light had the strongest effect in decreasing COVID-19 growth rate.
Options to control indoor humidity include in-duct humidifiers in central air systems or portable humidifiers, which are attractive because they are generally easy to install.
The CIBSE guidance strongly suggests that building owners and operators increase the air supply and exhaust ventilation, supplying as much outside air as is reasonably practicable. This is needed to dilute and remove airborne pathogens and exhaust them to the outside air. The guidance also advises against the use of rooms or spaces which do not have direct access to fresh air.
Further research is ongoing on the relationship between environmental factors and the spread of COVID-19. This will add to our understanding of how the virus is spread in indoor environments and allow for the development of practical control measures.
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