Ventilation and air conditioning systems can introduce outdoor pollutants; in themselves, they can become a source of pollutants. Through their ducts they allow pollutants to circulate from one area of a building to another, and ultimately, they may be ineffective in removing or diluting pollutants from a building.
The combination of all these factors gives rise to one of the main reasons for indoor air quality problems: Unsuitable design, installation, operation or maintenance of ventilation systems.
The most common outdoor pollutants are the following: SMOG, fungal spores and pollen (depending on the time of year), vehicle exhaust emissions and the incorrect installation of air intakes, Legionnaires’ disease, bacteria or biocides from adjacent cooling towers, etc.
The most common pollutants generated by ventilation and air conditioning systems are fungal and bacterial spores, due to the water used by some of these systems. Fibreglass is also produced by the air flow eroding duct insulation and dirt build-up in the ducts due to lack of maintenance and/or filtration.
Poor management of indoor air pressures may cause pollutants to be transmitted between different areas of a building. For example, in bathrooms there are usually extractors to eliminate fungi and bacteria, and if a bathroom has an air supply with a higher pressure than the extractor, then the air will flow from the bathroom into the adjacent areas. Ducts with loss of air flow located in false ceilings may cause air to travel from false ceilings into occupied areas.
“Airtight buildings” are constructed to reduce energy costs and depend on outdoor air to dilute pollutants that are transmitted from or are present inside the building. When the outdoor air supplied is insufficient, pollutants may build up and become a serious problem for the occupants of a building.
The most common causes of insufficient outdoor air supply are listed below, starting with the most regular:
- Thermostat fans in “auto” mode that only work when heating or air conditioning is in operation.
- Units turned off or have clocks set incorrectly, causing the system to be inoperative during building occupancy hours.
- Absence of outdoor air intakes or outdoor air intakes are completely closed.
- Buildings constructed according to obsolete standards with insufficient outdoor air supply (1970s’ oil crisis and exploring energy conservation).
- Use of economisers to reduce energy consumption in the systems.
