With the term Indoor Air Quality (IAQ – Indoor Air Quality) we refer to the quality of the air inside and around buildings, especially in relation to the health and comfort of occupants.
In Italy, the Ministry for the Environment defines indoor pollution as “the presence, in the air of confined environments, of chemical, physical or biological pollutants that are not naturally present in outdoor air”.
In almost all cases, over the daily average, indoor environments are more polluted than outdoor ones: indoor air is taken from outside air, which may already be contaminated, and further pollutants are added indoors.
This type of pollution can be caused by personal activities, work activities, inadequate ventilation, building materials, furniture, cleaning methods and the products used.
More specifically, IAQ can be influenced, for example, by: gaseous substances (such as carbon monoxide, radon, volatile organic compounds – VOCs – and polycyclic aromatic hydrocarbons – PAHs); dust and particulate matter; microbial contaminants (fungi and bacteria); as well as any other element that may have adverse health effects (tobacco or wood smoke, detergents, pesticides and insecticides, man-made mineral fibres, etc.).
The groups most sensitive to the health effects of indoor pollutants – and therefore at greater risk – include older people, children, asthmatics and people with heart or lung disease.
Indoor air quality can be controlled by acting both on the pollution sources and on ventilation.
Adequate ventilation, whether mechanical or natural, makes it possible to remove and/or dilute contaminants, by mixing outdoor air with the air already present indoors.
If too little outdoor air enters a dwelling, pollutants can accumulate to concentrations that cause health and comfort problems.
Outdoor air enters and leaves an enclosed space in different ways: infiltration, natural ventilation, and mechanical (forced) ventilation. In the case of “infiltration”, outdoor air enters through cracks and gaps. In natural ventilation, air enters when doors and windows are opened. Mechanical ventilation can be provided by intermittent local exhaust fans (typically installed in kitchens and bathrooms) or by full ventilation systems that extract indoor air and introduce outdoor air, which may be filtered and conditioned.
In general, the quantity used to measure ventilation is the airflow rate Q of fresh air supplied to the space (expressed in m³/h).
However, it is often more useful to refer to another parameter – the “air changes per hour” (ACH) – which is independent of the specific size of the room and provides indications that can be applied to different spaces under similar indoor pollution conditions.
The air change rate is defined as the ratio between the airflow rate Q and the volume V of the space considered.
In residential environments, the minimum air change rate is usually determined by the need to remove, under normal conditions, body odours, human-origin indoor pollutants and moisture. However, this minimum value may be insufficient in particular situations – for example, where materials that emit radon (certain types of masonry) or formaldehyde (furnishings) are present, where combustion appliances are used, or where there are many smokers. For each type of environment, and depending on the activities carried out there, appropriate ventilation rates must therefore be provided, proportional to the amount and toxicity of the substances emitted per unit of time, in order to maintain acceptable air quality levels.
Ventilation, understood as the renewal of air in a space, is meaningful only if we assume that contaminants are produced inside the space and that outdoor air is relatively clean. This assumption is not always strictly true, but is generally acceptable for design purposes.
For airtight buildings this assumption is, by definition, satisfied as regards uncontrolled leaks. However, it is important to remember that in such buildings indoor air quality may actually worsen when the ventilation system is switched on: ventilation units can act as amplifiers of microbial growth, especially in the absence of proper inspection and maintenance, and may become responsible for spreading microorganisms and introducing contaminants into occupied spaces.
To prevent this and ensure proper performance, regular maintenance is essential, especially for filters, heat exchangers and humidifiers. In this context, the choice of high-efficiency filters and their scheduled replacement – for example, using quality filters for mechanical ventilation and Heat Recovery Units (HRUs) such as those supplied by AEROFEEL – plays a key role in keeping particulate and outdoor pollutants under control while maintaining system efficiency over time.
In addition, if the emission characteristics of pollutant sources vary significantly over time (for example, spaces with large variations in occupancy), the mechanical ventilation system should be equipped with suitable control devices to compensate for these variations. In such cases, controlled mechanical ventilation systems – in which the airflow rate is adjusted according to the concentration of a selected indoor pollutant used as a control variable – can be particularly useful.