Long term monitoring of CO2 levels and ventilation rates in a naturally ventilated residential apartment

Indoor CO₂ levels became particularly topical during the recent COVID-19 pandemic. In this study a long-term investigation of indoor CO₂ levels in a 1970s built residential apartment in single occupancy is presented. Three NDIR CO₂ sensors were used to measure CO₂ levels over a one-year period. Mean CO₂ levels over this period were 1278 ± 504 ppm, with elevated CO₂ levels of greater than 2000 ppm not uncommon. Subsequent investigations used the single zone mass balance model and the decay of CO₂ in the absence of occupants to estimate the ventilation rates in various configurations. A mean natural ventilation rate of 0.16 ACH was estimated with all windows closed. Opening fan light windows resulted in a mean ventilation rate of 2.86 ACH whereas opening all windows increased the mean ventilation rate to 19.1 ACH. Evidence was observed of the effect of both wind speed and indoor-outdoor temperature difference on the ventilation rates. It was concluded that with all windows closed the natural infiltration rate was insufficient to maintain optimal indoor air quality even in single occupancy. Opening the fan light windows was sufficient to achieve satisfactory indoor air quality but insufficient for the effective inhibition of airborne disease transmission.