Addressing actual and community expectations on CO2 concentrations within indoor spaces – A reasonably practicable methodology using CO2 concentration to assess ventilation quality to indoor spaces

The ability to quickly assess the performance of a ventilation system to deliver an adequate amount of clean air to the space relative to the number of occupants is important as part of the overall goal of ensuring healthy indoor air. Current debate within the scientific community clearly aims to influence government to legislate a CO₂ concentration as an indoor air quality standard. However, to properly consider this, government will likely demand quantitative data on contemporary indoor CO₂ concentrations and a tested and reasonably practicable method for use by building occupants. Our study addresses this research gap with the aim of testing and documenting how to operationalise CO₂ monitoring for use by occupants as an infection risk reduction tool within mechanically ventilated spaces. The related research objectives were 1) to inform the current scientific debate on an indoor air CO₂ concentration action level for improved ventilation quality and infection risk control, 2) to pilot and document a method for characterising indoor CO₂ concentrations within occupied spaces serviced by heating, ventilation and air conditioning (HVAC) systems and demonstrate it was reasonably practicable in terms of instrument cost and deployment utility, 3) assess the performance of a ventilation systems to deliver an adequate amount of clean air to indoor spaces relative to the number of space occupants, and 4) to action improvements in ventilation quality where identified. The method was informed by a literature review of various standards that described optimal number, location, and layout of sensors, resulting in demonstration of deployment of CO₂ sensors. Sensors were deployed within 1439 teaching and office spaces across 78 mechanically ventilated buildings. In 1025 (72 %) rooms, CO₂ concentration was < 800 ppm, in 267 (18 %) between 800 ppm and 1000 ppm, and in 147 (10 %) > 1000 ppm, during room occupancy.