Ventilation, CO2 and Risk
We know Covid is airborne, but how do we know how likely it is to be in the air we breathe?
We know Covid is airborne, but how do we know how likely it is to be in the air we breathe?
Measuring indoor carbon dioxide levels can provide an approximation of the risk of contracting SARS-CoV-2. In any given indoor environment, when excess carbon dioxide levels double, the risk of transmission also roughly doubles.
SARS-CoV-2 is airborne. Infectious virus particles accumulate and remain suspended in the air for many hours. If you are sharing air in an enclosed space the risk of contracting SARS-CoV-2 is possible.
Every person exhales carbon dioxide. SARS-CoV-2 positive people exhale airborne virus particles at the same time. While we cannot detect virus particles in the air, we can detect CO2.
This means measured indoor CO2 levels can serve as an approximation of the quantity of virus in the air you are breathing.
As the measured CO2 level in an enclosed space increases, you are breathing more air that has been in other people's lungs. This is referred to as "rebreathed" air.
An increase in rebreathed air increases risk of exposure to and transmission of SARS-CoV-2 (not to mention cognitive impairment (1,000+ ppm) and cognitive dysfunction (2,500+ ppm)).
High CO2 levels are an indication of poor ventilation.
While there is no explicit CO2 level at which one can assume a shared indoor space is without risk, we can determine relative risk (what is more safe and what is more risky) by knowing current CO2 concentration and interpreting its implications effectively.
Commercially available CO2 monitors have become more popular, though their cost can be prohibitive.
For this reason we've tabulated estimated CO2 concentrations for various space sizes, ventilation levels, activities levels and occupation levels.
Indoor spaces can be made more safe through ventilation; intentionally bringing outdoor air into a building to reduce air contaminants.
Ventilation can be achieved mechanically or naturally.
Mechanical ventilation brings outdoor air into the space via a forced air delivery system, such as a HVAC system.
Natural ventilation allows in outdoor air via open windows and/or doors that are designed to serve the space’s ventilation needs.
Natural ventilation can be inconsistent and is most effective on windy days or when there is strong temperature difference between indoor and outdoor air. Other factors such as space geometry can limit its effectiveness.
The use of High Efficiency Particulate Air (HEPA) air filtration is recommended in any shared space as they have been shown to reduce the quantity of airborne virus.
The harsh reality is that ventilation in many spaces is beyond our control. Knowing the estimated CO2 concentrations can assist you in assessing risk.
There are no situations where sharing indoor air with another individual is guaranteed to be safe. Wear a good mask whenever you must enter an indoor space or, if possible, avoid those spaces altogether.
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