In indoor spaces without adequate air change, high concentrations of respiratory particles, pollutants and CO₂ may accumulate. It is essential to monitor these parameters in order to evaluate and subsequently improve IAQ in buildings.

Parameters to be monitored to ensure indoor air quality

The Air Quality Guidelines AQG, published in 2021 by the WHO, list six parameters that need to be monitored in order to determine indoor air quality, while the EEA guidelines list 12 different parameters. In addition to these parameters, when monitoring IAQ, it is recommended to also measure CO₂, as this is generated by the respiratory activity of living beings and can reach high concentrations in closed spaces. Low-cost sensors are available on the market to monitor some of the pollutants covered by the WHO and EEA guidelines. However, it is not always possible or meaningful to monitor all pollutants in closed spaces. For commercial buildings, in addition to CO₂ and fresh air flow, it is also recommended to monitor the following pollutants:

• fine particulate matter (PM2.5);
• carbon monoxide (CO).

Fine particulate matter (PM2.5)

PM2.5, or particulate matter with an equivalent aerodynamic diameter of less than 2.5 μm, was recognised in 2013 by the International Agency for Research on Cancer (IARC) as one of the causes of lung cancer. According to the EEA, in 2024, 96% of the European population living in cities will have been exposed to unsuitable concentrations of PM2.5. In 2021, awareness of the dangers of particulate matter prompted the WHO to lower the annual average allowable concentration to 5 μg/m³. Using a shorter reference time interval of one day, the WHO guidelines suggest a concentration of 15 μg/m³. Recent studies, however, propose a concentration of 15 μg/m³ for a time of just one hour, as 24 hours correspond to a longer period of time than people normally spend in closed public spaces.

Limit values for the pollutant PM2.5 according to different guidelines and directives

Carbon monoxide (CO)

Carbon monoxide (CO) is a colourless, odourless gas that, when inhaled, binds more tightly to haemoglobin than O₂, reducing the human body’s ability to transport oxygen in the blood and, in high concentrations, can be fatal.

Chemical and physical properties of CO

Carbon monoxide (CO) must be carefully monitored in buildings situated in geographic areas where regulatory limits may be exceeded. This gas can also be generated directly inside buildings, being one of the by-products of incomplete combustion that may occur when solid and liquid fuel heating systems (e.g. wood stoves for heating and cooking) are lit and go out. For this parameter too, the limit values specified in various guidelines and regulations highlight a downward trend in terms of maximum concentration.

Limit values for the pollutant CO according to different guidelines and directives

Carbon dioxide (CO₂)

Carbon dioxide, at atmospheric pressure and temperature, is a colourless, odourless gas that is naturally present in the Earth’s atmosphere. The concentration in the air is approximately 450 ppm. CO₂ is generated by industrial processes, which involve the combustion of fossil fuels, by the respiration of living beings, and by the decomposition of organic matter.

Chemical and physical properties of CO₂

Prolonged exposure to levels exceeding 1000 ppm (1800 mg/m3) of CO₂ can cause the following effects:

• mucous membrane and respiratory apparatus symptoms (e.g. eye irritation, sore or dry throat, blocked, congested or runny nose, sneezing, coughing, and rhinitis);
• decreased cognitive performance (e.g. decision making, execution of tasks);
• neurophysiological symptoms (e.g. headache, tiredness, fatigue, dizziness, or difficulty concentrating).

For this reason, even if not toxic, it is worth monitoring CO₂ in the same way as the other pollutants examined here.

CO₂ as a proxy for IAQ

Regardless of its effects on human health, it is important to monitor CO₂ as this can act as a proxy for the presence of pathogens emitted by occupants. Pathogens, together with CO₂, are emitted during respiration, so it is much easier to infer pathogen concentration by measuring the CO₂ concentration than it is to model/measure pathogen emissions. The importance of monitoring CO₂ concentration is just as crucial as identifying a maximum concentration value that statistically ensures no secondary infection, as recommended by the recently published study by Morawska et al, 2024 - Mandating indoor air quality for public buildings.

The contents of this blog post regarding IAQ can be examined more in depth by reading the white paper
“Indoor air quality - Guaranteeing health and comfort in buildings”

Download the white paper

Bibliography:

• EEA, 2024. https://www.eea.europa.eu/publications/europes-air-quality-status-2024 (visited 22/10/24).
• Health Canada, 2021. Residential Indoor Air Quality Guidelines. Carbon Dioxide.
• IARC,2013. Press release 221. Outdoor air pollution a leading environmental cause of cancer deaths.
• Lidia Morawska , Joseph Allen, William Bahnfleth, Belinda Bennett, Philomena M. Bluyssen, Atze Boerstra, Giorgio Buonanno, Junji Cao, Stephanie J. Dancer, Andres Floto, Francesco Franchimon, Trish Greenhalgh, Charles Haworth, Jaap Hogeling, Christina Isaxon, Jose L. Jimenez, Amanda Kennedy, Prashant Kumar, Jarek Kurnitski, Yuguo Li, Marcel Loomans, Guy Marks, Linsey C. Marr, Livio Mazzarella, Arsen Krikor Melikov, Shelly L. Miller, Donald K. Milton, Jason Monty, Peter V. Nielsen, Catherine Noakes, Jordan Peccia, Kimberly A. Prather, Xavier Querol, Tunga Salthammer, Chandra Sekhar, Olli Seppanen, Shin-ichi Tanabe, Julian W. Tang, Raymond Tellier, Kwok Wai Tham, Pawel Wargocki, Aneta Wierzbicka, and Maosheng Yao, 2024. Mandating indoor air quality for public buildings.