COVID-19: Basic Concepts Regarding Carbon Dioxide (CO2) and its Measurement in Buildings

This document, written in the context of the COVID-19 pandemic, is intended for public building managers, building owners, and all organizations and individuals wishing to learn more about the topics covered in this overview of the scientific and grey literature. This document discusses the current knowledge on carbon dioxide (CO₂), its health effects, sources, and relation to building ventilation, as well as the different approaches used to measure this parameter and ways of interpreting the results. The methodology applied to produce this brief overview is set out in Appendix 1 of the full version (in French) of this document.

The objective of this document is to provide key information that may guide concerned organizations in selecting an appropriate measurement protocol and consistent approach for interpreting the results. These elements must meet clear objectives and be interpreted with caution, whether this involves selecting the type of device to use; the results of one-off, repeated, or continuous measurements; or the measurement context and methodology to use. When correctly determined, CO₂ measurements can be used for various purposes, including as an indicator of the emission of metabolic products (bioeffluents) by the occupants, a ventilation indicator, or to produce a quantitative analysis of the intensity of ventilation used in a given indoor environment. This document cites different evaluative approaches, guidelines, and recommendations currently in effect for these purposes. However, it should be noted that this document is neither a protocol for measuring CO₂ in an indoor environment, nor a best practice guide intended for this purpose.

It is also important to emphasize that the role of the COVID-19 Environmental Health Committee, of which the main authors of this document are members, is to conduct scientific monitoring, analyze the content of this monitoring, and report on the key findings that emerge from it. When required, the committee also writes expert opinions. It by no means has the mandate to supersede building decision-makers or managers to determine the preferred options or to resolve problematic situations that may result from applying the chosen options.

Highlights

Carbon dioxide (CO₂) is a chemically stable gas in usual environmental conditions. CO₂ is an omnipresent component of ambient air, the concentration of which has been increasing since the start of the industrial age due to fossil fuel combustion. In indoor environments, it essentially comes from exhaled air from occupants’ breathing. While its concentration in outdoor air is subject to slightly local variations, the concentration of CO₂ in indoor environments can vary significantly, according to occupant density, room volume, type of activity, duration of occupancy, ventilation used in the environment, and to a lesser extent, use of combustion appliances. Air exchanges with the outdoor environment, through both natural and mechanical ventilation, is mainly what modulates the concentration of this gas in occupied indoor environments.

The CO₂ concentrations normally found in indoor environments generally have no effect on the health of the occupants. The first physiological symptoms of CO₂ exposure, like an increase in blood acidity, dilation of the bronchi, and an increase in respiratory rate, are generally observed when ambient concentrations reach 10,000 ppm for a period of at least 30 minutes. The results of numerous studies also suggest that exposure to CO₂ at much lower concentrations (approaching 1,000 ppm) may cause adverse effects for certain individuals (dizziness, fatigue, headaches, etc.). The suggested associations between the manifestation of symptoms and specific exposure to lower concentrations of CO₂ still merits further documentation, especially with regard to the underlying biological processes. With the objective of explaining the observed effects, a number of authors have also referred to sick building syndrome, an ill-defined condition, potentially from multifactorial sources, with non-specific symptoms.

When looking more specifically at the potential link between high concentrations of CO₂ in a given environment and the risk of COVID-19 transmission, it is generally agreed that similar environmental and behavioural factors (inadequate ventilation, increased occupant density, etc.) are likely to cause an increase in concentrations of this gas and of infectious aerosols in the indoor air. However, any direct association between a high CO₂ concentration and an increased risk of transmission of the viral agent must be interpreted with caution, given that the principal mode of SARS-CoV-2 transmission is still direct or close contact between individuals over a prolonged period. It is, however, increasingly recognized that poor ventilation of occupied indoor environments may create an increased risk of aerosol accumulation, and consequently, of COVID-19 transmission.

In this context, assessing CO₂ concentrations in indoor environments through protocols designed for this purpose may be a promising approach for characterizing the intensity of ventilation applied in a given environment. Correctly measured CO₂ can be used for various purposes, including as a relative indicator of the emission of metabolic products (bioeffluents) by occupants, a ventilation intensity indicator (i.e., whether an environment is under-ventilated or has sufficient ventilation), or to produce a quantitative analysis of the intensity of ventilation used in a given indoor environment. Regardless of the situation, measuring CO₂ may provide indirect information about the volumes of fresh air (air intake from the outside) delivered in a given indoor environment, regardless of whether it is ventilated naturally, mechanically, or through hybrid ventilation (naturally and mechanically).

For the application of a validated measurement protocol, several reputable organizations have developed criteria that serve as different tools for interpreting and managing CO_2, concentration measurements for both usual building occupancy and pandemic contexts. While they are generally not enforced by law, except in workplaces, these criteria act as a guideline value for the use of proper ventilation in indoor environments (or for optimized ventilation in a pandemic context). These organizations suggest applying corrective measures when these criteria are not met. In summary, use of proper or optimized ventilation in indoor environments is a complementary measure for environmental monitoring and control that is considered effective at reducing the risk of exposure to contaminants in indoor air, including certain pathogens likely to be found in it.