From the Hong Kong - Guidance Notes for the Management of Indoor Air Quality
3.5.1 Carbon Dioxide (CO2)
Carbon dioxide has been commonly adopted by IAQ researchers as an indicator with levels above 1,000 ppm in indoor environments indicating that the ventilation rate is low and that other airborne contaminants are accumulating.
For example, levels of carbon dioxide are normally highest in the late morning and late afternoon and vary with occupancy during the day in offices.
Also, fresh air intake is usually at a minimum during the peak heating and cooling seasons.
Measurements should be taken at the fresh air intake, places of high occupancy density, and locations where there are complaints of poor air quality.
Carbon dioxide level can be measured using real-time monitors, such as Non-Dispersive Infra-Red (NDIR) analysers or electrochemical oxidation devices.
The user should select monitors with appropriate start-up and response time as some of them are designed for continuous monitoring and require longer time to reach equilibrium before operation.
For more information click on this link:https://www.iaq.gov.hk/wp-content/uploads/2021/04/gn_officeandpublicplace_eng-2019.pdf
