Analysis of Indoor CO₂ Concentration Using Split Ventilation Systems as an Indicator of COVID-19 Transmission 

We measured the indoor CO₂ concentration in occupied areas with ventilation systems that recirculate air without an external air supply. The average time required to achieve the highest probability of contagion was also measured based on the number of participants in the group. Three different experimental groups were evaluated: Group One (G1), which included 5 participants; Group Two (G2), with 10 participants; and Group Three (G3), with 15 participants. Before the measurements, the CO₂ concentration was measured to be homogeneous and its sampled value was given by the difference between the indoor and outdoor CO₂ measurements (>5000 ppm or 0.5% CO₂ in air) averaged over an 8-h work day Time-Weighted Average (TWA.). G1 and G3 group participants performed low-intensity daily office activities, such as reading and talking. In contrast, Group Two (G2) was asked to perform moderate intensity activities, such as frequently lifting 10 kg items and walking quickly. The CO₂ concentration was measured with two instruments to compare the outdoor and indoor measurements. Both devices were configured to take one reading every second for 30 min. A mathematical model was developed from the CO₂ concentrations measured, the group size, and the retention factor of the mask being worn to determine the probability of inhaled air contaminated with an aerosol of SARS-CoV-2. We concluded that the likelihood of contagion in enclosed areas such as study areas, offices, and meeting rooms, among others, which use ventilation without a circulation of fresh air, is high. Despite proper distancing and masking, there is a 99% chance of contagion in one of the modeled extreme case scenarios in less than 10 min of exposure. The study took place in Albrook, Republic of Panama, which is a tropical developing coastal geographic location where split air conditioning units are widely used and, like many other countries in Latin America, where indoor air quality has only recently started being discussed publicly and enforced.