On average (according to seasons, region, etc.), we spend over 75% of our time indoors. But the indoor air is up to 20 times more polluted than the outdoor air (30 pollutants per accommodation on average). On average, a person breathes about 15 000 liters of air each day. To ventilate, but also to filter and to improve the indoor air in an urban and industrial environment is a major health challenge of all users and inhabitants. According to the French observatory of the quality of indoor air (#OQAI) the estimated economic and social costs related directly to a poor quality of air in France are nearly 20 billion € per year (and almost 20000 deaths).

In terms of indoor air pollution we are talking about chemical and biological pollutants. The pollutants are very diverse as for their number, their origin and their chemical composition. There are mainly:

• airborne particulates and organic compounds:
  • exhaust gas,
  • stack effluents and combustion residues, fire smoke, tobacco smoke,
  • cement dust,
  • heavy metals (lead, copper, zinc, iron, cadmium, etc),
  • water vapour from domestic use and excess moisture,
  • aerosols, particulates can be formed (in the atmosphere) from gaseous organic substances such as sulfur dioxide, methane, nitrous oxide or ammonia,
  • VOC (volatile and semi-volatile organic compounds). They are for example linked to cleaning products, construction materials, some candles, incenses, furniture, air fresheners, paints or varnishes, flooring, etc.
• organic particulates and allergens (pollen, insects, mites, moulds, viruses, bacteria, spores, hair, pesticides, animal fibres, vegetable fibres, cockroaches, etc.),
• mineral particulates (dust, clay, sand, sea salt, etc.),

There are perceptible pollutants (some smells – kitchen, tobacco, human, essential oils, etc.) and imperceptible pollutants (for example carbon monoxide).

Some notorious pollutants and sometimes classified as a human carcinogen: formaldehyde, limonene, phenol, benzene, hexanal, phtalate, ozone, bisphenol, carbon monoxide (CO), sulfur and derivatives, radon, heavy metals, NOx, ammonia… etc.

The fine particulates are often classified by size (PM-Particulate Matter) according to WHO criteria and ISO global standards:

• PM10 also called “gross particulates” (whose diameter is less than 10 µm = 10 microns): sand, earth, construction or quarry dust, coal ashes, exhaust gas, tyres, brake linings, asbestos, etc.
• PM2.5 (whose diameter is less than 2.5 µm): emissions related to transports +  industrial waste combustion.
• PM1 (whose diameter is less than 1 µm = 1 micron = 0.001 mm) are the smallest but also the most dangerous particulates. They are responsible for severe or even mortal diseases. They can enter the pulmonary alveoli and some enter the blood. Many people are very sensitive to them and have cardio respiratory and eye allergic reactions (asthma, chronic bronchitis, etc.). That’s the case of particulates associated with diesel with approximately 0.1 µm in exhaust outlet! This is also the case with viruses and nanoparticulates.

Note, the levels of pollution tolerances are calculated based on the mass of particulates that must be assessed. So, smaller the particulates are, more complex they are to quantify (too light, too thin). Yet these are the most dangerous to our health. The bigger amount of fine particulates is in the atmosphere, smaller is the size of water droplets in the clouds and less rainfall we have. Ultimately, the clouds grow to make an “umbrella” effect, which is the famous London’s “fog”!

Two main points are to be privileged in order to limit the pollution of the indoor air: limit the pollutant sources (from the inside and outside) and refresh the air.

It is therefore necessary to set up ventilation, treatment, filtration and purification systems of the indoor air.

The filters must especially satisfy several constraints:

• reduce the energy consumption of the air handling systems while maintaining a consistently high performance. Fall within the scope of low energy buildings
• have a long lifetime in order to limit the changes and the maintenance costs
• have the optimised filtration performances certified by LCC
• not have any side effects to health
• respect the acoustic comfort
• be light, compact, resistant to moisture, limit the impact on the environment!

The main air purification methods are: filtration by foam and synthetic fibres, by activated carbon, by ionisation, the photocatalysis, the chemical captation. Depending on circumstances the method used degrades the chemical compounds, the microorganisms, the volatile organic compounds and the inorganic gases present in the air.

If the filters do not work as they should, the accumulated deposit of dust and pollutants reduces significantly the air flow at the outlet increasing the overall system consumption. The HEPA air purifier filters and activated carbon filters must be changed once saturated.

It is important to balance efficacy and harmlessness of the procedures used for the air filtration (including by product formation that may have adverse health effects). Some methods are discussed by experts.

The system used must be proven comparing to the specificity of the air to handle. Regarding the housing, the indoor air contains a variety of pollutants but with lower concentration than in the industrial environments for instance. The complexity lies in the fact that the system, to be effective, must adapt to the moisture and the room temperature, in addition to the specific nature of pollutants.

The air purification system to choose must be adapted to this type of pollution very specific for the housing concerned. Indeed, significant differences in the efficiency depending on the environments have been identified on real tests conducted by the French CSTB (Scientific and Technical Centre for Building) laboratories. Due to this diversity, the FPA (flow of purified air), calculated in the laboratory by the manufacturers, is not always a reliable indicator. Regarding the filtration of air coming from the outside, the constraints and solutions differ depending on whether the housing is located in an urban or rural environment.

We distinguish:

• Autonomous systems for use in domestic housing commonly called #air purifiers. The devices are fixed, mobile, wall or ceiling mounted. They operate on the principle of recycling the air contained in the room (detection, suction, filtration-purification, ventilation).
• Depolluting paints or wall coverings based on the principle of photocatalysis (photo-active to degrade the pollutants).The efficiency is rather controversial depending on the brands, due to the use of substances such as the titanium dioxide (classified carcinogenic) or the production of other volatile organic compounds (sources OQAI).
• The HVAC systems (Heating Ventilation Air Conditioning) dedicated to offices and premises receiving public. These are filtration modules or cartridges installed at the output of the conditioning system and air distribution, or at the air vents.

As for the air filtration, the efficiency of the device must be high so that the actual impact on the air purification is significant (lot of air passages cycles). We measure the FPA: flow of purified air or purified air volume per time unit – m3/s or m3/h. This allows to evaluate the efficiency of an air purification system comparing to the ventilation rate of the room.

The level of fineness of the filtration is essential in order to retain the finest particulates, therefore the most dangerous. The important indicator is the % of fine particulates filtered for the size of the given particulate with a HEPA filter – High Efficiency on Air Particulates – (which means it filters 99.97% of particulates having a size greater than 0.3 microns).

The main autonomous air purifier manufacturers are, among others, DAIKIN, PHILIPS, DYSON, ROWENTA, etc.

The lack of effective air exchange causes a concentration of pollutants in the stale air of buildings and living spaces. This is why a ventilation system is essential. However, on its journey in an existing filtration system, the dust, smoke, pollen and pollutants accumulate on the fans, batteries and ducts. That’s why an efficient filtration system must be set up.

A ventilation and a filtration system, to produce good quality indoor air, must meet minimum performance, follow the recommendations of the European Standard EN 13779 and the new global standard ISO 16890, and if applicable have certifications. We measure the Quality of Indoor Air (QIA) depending on the quality of outdoor air or fresh air.

Remember that to meet the global standard in force ISO 16890, the air filter must have a minimum efficiency of 50% for PM1 particles (which are the finest and most dangerous). For example, a filter with 55% of efficiency in filtering PM1 particulates will be classified to 66% in the category ePM1 (same for ePM2 and ePM10). The effectiveness (for synthetic medium) is given for a static electricity discharged medium in order to make the measure realistic.