Talking, shouting, sneezing or coughing are common actions that generate aerosols. In enclosed spaces, these aerosols can remain suspended in the air for hours. To reduce the spread of COVID-19, clean air must be brought in and potentially contaminated air removed.
Such ventilation is occurring in many schools, universities and offices by keeping windows open continuously. This has increased the energy consumption of heating and air conditioning with a negative effect on the environment.
To verify the efficiency of indoor ventilation, avoid contagions and reduce energy waste, it is necessary to install CO2 meters.
How is air quality defined?
Carbon dioxide is an objective measure of air quality. Humans breathe this colorless, odorless gas, which becomes more active in direct proportion to age and corpulence.
The concentration of carbon dioxide outdoors ranges from 360 ppm (parts per million) in clean air areas to 700 ppm in cities. The maximum recommended value for indoors is 1,000 ppm and the limit value for offices is 1,500 ppm.
Caution should be exercised as this limit value is easily reached. For example, in a 25 square meter office with four adults working in it and which has been freshly ventilated, the carbon dioxide concentration rises to 2,000 ppm one hour after the insulating windows have been closed.
What are acceptable CO2 levels?
The World Health Organization recommends that indoor CO2 concentration levels should not exceed 1,000 ppm. But in a pandemic situation, to avoid contagion, the risk is considered to decrease with lower levels.
Acceptable levels in an indoor space would be between 500 ppm and 700 ppm, taking into account that in outdoor air it is 400 ppm. Thus, when the CO2 meter shows a value of 800 ppm or more, ventilation of that enclosed space would be mandatory. In the case of levels of 1,000 ppm, it would be necessary to ventilate immediately and as much as possible.
Meters recommended by CSIC
The Spanish National Center for Scientific Research (CSIC) recommends that meters should be capable of providing data without having to download files (txt, Xls, CSV or similar). That is, the display should show CO2 levels in real-time, with a frequency of at least one data per minute, and use NDIR (non-dispersive infrared) technology. It also indicates that calibration standards must be followed and that the cost should be between 100 and 300 €.
Calibration and measurements
The following guidelines are recommended for calibration:
1. Perform calibration of the device by exposing it to the outside air.
2. Note the reading it gives in the open air (e.g., 400 ppm).
3. Keep the analyzer in a place with CO2 levels of at least 700-800 ppm.
4. Take it outside again and wait until the analyzer drops to a value that is about 20 ppm higher than the calibration value in the street (420 ppm if the initial outdoor measurement was 400 ppm). This is the time that you will normally have to wait before taking a valid reading after placing the analyzer.
If we are close to the device when taking the reading, it can detect the CO2 we exhale. Therefore, it is advisable to take the reading relatively quickly and spend as little time as possible next to the calibrator.
As for its location, it is recommended to place them in the worst ventilated area of the room, on the wall opposite the windows if there are any, at a height of 1.5 meters from the floor and 1 meter away from people.
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