Ventilating indoor spaces is crucial to minimize the risk of transmitting COVID-19

The risk of airborne transmission of SARS-CoV-2 is proportional to the concentration of the virus in the air, the rate of inhalation, and the duration of exposure, increasing significantly indoors – indoor places where we spend about 90% of our time. In this sense, it was already expected that the autumn-winter period would bring reasons for concern. Low temperatures, the higher prevalence of other respiratory diseases or a long time spent indoors in spaces with little natural ventilation are some factors that make this period a particularly fertile period for the transmission of COVID-19.

It is therefore crucial to implement prevention measures that effectively break the transmission chains, namely by evaluating and ensuring the quality of the air we breathe indoors.

According to current scientific evidence, ensuring adequate ventilation conditions in all indoor environments, particularly shared ones, is critical not only to provide "fresh” air, but also to eliminate occupants' exhaled air that could be potentially contaminated. The correct functioning of the mechanical ventilation systems and their maintenance (if any), as well as the opening of doors and windows, must be ensured.

However, it is important to consider that ventilation needs vary from space to space. For example, an intermittently open or ajar window may be enough to air a room in our house, but not a classroom full of children, or a bar/dining room with a high occupancy of unmasked people.

Given this information, the question that arises is: how can we know if the ventilation conditions are adequate? The truth is that they depend on several variables such as the space occupancy rate, the activities carried out, the characteristics of the building or the room, or the weather outside, among others. The assessment of ventilation in multiple buildings requires technical knowledge, and the definition of corrective measures is typically multifactorial, taking into account aspects such as the nature of the building, the type of activity, the available equipment, occupants, etc.

In fact, unlike distancing and hygiene rules, ventilation requirements and related corrective measures cannot easily be converted into a simple approach that all buildings must comply with. However, the use of CO2 sensors is a relatively inexpensive and objective approach that allows you to indirectly measure the ventilation conditions of spaces with a high occupancy rate. Although there is not yet a complete consensus on the use of CO2 levels as an indicator of the concentration of infectious aerosols, the benefits of its use seem to far outweigh the limitations.

Due to the ability to measure air quality continuously over time, with regard to the accumulation of exhaled air, the usefulness of CO2 sensors is widely recognized by the technical-scientific community. It facilitates the adoption of corrective measures and the assessment of their effectiveness immediately, contributing to the safety of the use of indoor environments, and avoiding more costly interventions.

According to the scientific community, CO2 levels should, in the pandemic context, remain below 800 parts per million (ppm). A study carried out by INEGI, between 2014 and 2015, showed that the CO2 levels in 71 classrooms (20 primary schools) in the Porto region, assessed in the winter period, were far above this value (average CO2 value obtained : 1419 ppm)1. Overall, the results of the study revealed that a substantial number of schools had poor ventilation conditions in relation to their occupation, which may constitute an important risk factor for the transmission of COVID-19 in these spaces. In fact, based on DGS data, most outbreaks of COVID-19 in Portugal occur in schools. Based on the available information, the search for solutions should include measures that seriously consider the improvement of air quality in these spaces.