Factors that affect indoor air quality

Indoor air quality has always been a concern in healthcare settings and controlled industrial environments (e.g. ‘clean rooms’) but the COVID-19 pandemic has raised awareness of the threat of indoor air contaminants.

That can include viruses and bacteria, but also other irritants like dust and pet hair, and potentially harmful gases like carbon monoxide, carbon dioxide, nitrogen oxides and ozone.

But what are the main factors that affect indoor air quality? There are three main groups of risks – and not all of them derive from inside the building.

In this article we’ll take a look at the three major factors that affect indoor air quality, and break them down into some specific sources of contaminants.



Indoor activities can have a direct impact on the quality of the air in the room, and this is especially true in sealed environments, such as interior rooms with no windows or artificial ventilation.

Some examples of indoor sources of air contamination include:

  • Human activity (personal hygiene, smoking)
  • Maintenance work (new carpet/furniture, pest control and redecoration)
  • Cleaning (surface sprays, agitated dust, artificial odours)
  • Equipment (e.g. toner particles from printers and photocopiers)
  • Leaks and spills of liquids
  • Number of people in the room (especially if very high)
  • Temperature/humidity – see HVAC Systems below

Behavioural change and best practice can mitigate some risks, but in any indoor environment you should monitor air quality for any dangerous deviations, even where HVAC systems are used to improve ventilation.



It’s quite possible for activities outside the premises to have an impact on indoor air quality, especially around main entrances and open windows. This is one reason why designated smoking areas are not immediately adjacent to entrances and exits.

Other risks in this category include:

  • Airborne contaminations (pollen, vehicle exhaust particles, other air pollutants)
  • Nearby emissions (delivery/loading bays, waste disposal activities)
  • Building exhaust emissions that re-enter the premises
  • Mould and mildew from stagnant water (Legionella risk)
  • Soil gas (e.g. from pesticides or underground storage tanks)

Always monitor air quality around entries and any large windows that are routinely opened, to highlight risks arising due to nearby outdoor activity, especially persistent activity that creates an ongoing threat.


HVAC Systems

Finally, HVAC systems are intended to improve the comfort and safety of occupants, but when poorly maintained they can lead to a variety of indoor air pollution risks:

  • Poor distribution of fresh air around building
  • Transported dust from inside ventilation ducts
  • Poorly maintained filters in need of cleaning or replacing
  • Biological risks from poorly cleaned ducts and humidifiers

Again this last risk raises the chance of a Legionella outbreak, in the presence of stagnant water and moist dirty surfaces, and HVAC systems should not be overlooked in the pursuit of clean, dry indoor air.


Find out more about how you can improve the indoor air quality of your property with a HVAC system by giving The hebs team a call on 0151 2360707.

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