Air is the most important ingredient of life. We breathe about 12,000 liters of air every day and therefore we need to ensure that the air we breathe is clean. Indoor Air Quality is considered to be ‘healthy’ when the air does not contain contaminants in harmful concentrations and is acceptable when the majority of people feel satisfied.

Outdoor air is the biggest indoor air pollution source in many parts of the world if the outdoor air is not properly purified in the air intake. Outdoor air brings the particulate matter and many e.g. traffic based gases into indoor spaces. Here are the results of European EnVie project (2010).


Other unwanted impurities in the indoor air are carbon dioxide and other human based emissions as well as the building material based emissions like volatile organic compounds.

It is important that the indoor air does not contain any harmful substances but at the same time the solution needs to ensure a balance between a clean environment and the protection of immunity in its users. Technologies chosen to clean the indoor air have to be safe for users and well tested. Indoor air quality targets have to be achieved without major increase in energy use of buildings.

It is very important to have a correct benchmark value when evaluating the indoor air quality. There are several standards and guidelines available like Indian ISHRAE IEQ standard, WELL building standard, Australian NABERS or the Finnish Indoor Climate Classification.

Indoor air impurities can be categorized to two major group: Particulate Matter (PM) and Gases. Both main groups can be divided to subgroups as the health impact and removal or mitigation methods are different. Particulate matter can be divided to ‘Suspended Particulate Matter (SPM)’ and ‘Bio-aerosols’. Carbon dioxide is the gas that cannot be removed from the air with molecular filters and therefore it needs a separate treatment.

Suspended Particulate Matter

Suspended particulate matter (SPM) refers to particles in the air of all sizes. SPM is a complex mixture of organic substances, present in the atmosphere both as solid particles and liquid droplets. They include fumes, smoke, dust and aerosols. Health impacts of respirable suspended particulate matter (RSPM) vary depending on the size and the concentration of particles:

  • PM10 refers to particulates with a diameter less than 10 μm. These are commonly called coarse particulates – they contain dust from roads and industries as well as particles formed under combustion. Depending on their size, coarse particles can lodge in the upper throat or in the bronchi.
  • PM2.5 refers to particles with a diameter of less than 2.5 μm. These are usually called fine particles and contain secondary aerosols, combustion particles and re-condensed organic metallic vapour, and acid components. Fine particles can reach all the way down to the alveoli in the lungs.
  • PM0.1 refers to particles with a diameter less than 0.1 μm, and are called ultra-fine particles. Ultra-fine particles are the most harmful to our health as they can penetrate into the bloodstream.

As a reference, dust and fibres that are visible to human eyes are more than 80 microns in diameter. Main source of indoor air suspended particulate matter (SPM) in most of the buildings is the outdoor air unless the outdoor air is properly filtered in the air intake. In residential buildings the cooking, especially open-fire cooking and frying, creates lots of particulate matter.

The long-term health effect of high particulate pollution is the reduced lung capacity and asthma. Exposure to particulate pollution also increases a person’s risk of stroke, heart disease and chronic bronchitis. The International Agency for Research on Cancer (IARC) together with WHO has classified particulate pollution as carcinogenic to humans (group 1) in 2013.


A bio-aerosol is a suspension of airborne particulates that contain living organisms or were released from living organisms like bacteria, yeast, mould, fungi, virus and pollen or their toxins and by-products. They are small in size ranging from less than 1 μm to 100 μm.

Exposure to bio-aerosols is associated with a wide range of health effects including infectious diseases, acute toxic effects and allergies. Respiratory symptoms and lung function impairment are the most widely studied and the most important bio-aerosol-associated health effects. In addition to these adverse health effects some protective effects of microbial exposure on atopy and atopic conditions has also been suggested.

Humans have a relatively good immunity against infectious or allergenic microbiological contamination. In too sterile environment our immunity may be reduced. Therefore, sterilization of air is not recommended among the healthy population or in the environments where the risk of infection is small. But in case there are already people with reduced immunity, all bio-aerosols should be removed from the air and killed as the live bacteria and viruses can go through the filter media and get back to the air stream.


The gaseous air pollution can be either outdoor or indoor air based. Outdoor air can contain high concentrations of e.g. carbon monoxide (CO), sulphur dioxide (SO2), nitrogen dioxide (NO2), ozone (O3), hydrogen sulphide (H2S) ammonia (NH4) and volatile organic compounds (VOC) depending on the location of an air take of building. Most of these gases are related to vehicle emissions, landfills and open sewage systems.

The indoor air gases are mainly building material or cooking based gases. Many new building materials, paints and furniture emits formaldehyde (CH2O) and other volatile organic compounds (VOC). Poorly burning biomass can release carbon monoxide (CO) into the air.

Carbon monoxide is toxic to humans when encountered in concentrations above about 35 ppm. It is a serious health risk and may cause an immediate death. Also other gases in high concentrations can create various health effects, like increased respiratory symptoms in people with asthma. Sometimes the combination of particulate matter with gases like NO2 are harmful as particulates carrying NO2 penetrate deeply into the lungs and can cause together respiratory disease. Ozone is a lung irritant and is harmful to our health in high concentrations.

Carbon Dioxide

Main source of carbon dioxide (CO2) inside the building is people. Carbon dioxide is not an air pollution in a similar way than other gases as it is not causing permanent health effects in the typical indoor concentrations (600 – 3,000 ppm). However, in levels above 1,000 ppm it reduces significantly human productivity and cognitive performance as well as causes discomfort like drowsiness and headaches.

Carbon dioxide cannot be filtered from the air with active carbon or molecular filters like other indoor gases. Therefore it needs to be diluted by bringing low CO2 concentration outdoor air into the building or having plants in the space as they removes carbon dioxide and releases oxygen.


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