Indoor air quality (IAQ)
Nadja von Hahn, Institute for Occupational Safety and Health of the German Social Accident Insurance
- 1 Introduction
- 2 Indoor air quality and well-being
- 3 Sources of pollutants
- 4 Most common chemical and biological IAQ-related hazards
- 5 Legislation and guidelines
- 6 Prevention and protective measures
- 7 References
- 8 Links for further reading
On average, a European spends 90% of his or her life indoors. Together with the time spent at home, the time spent indoors also includes time at work, for instance in offices, sales areas, educational establishments, libraries, hospitals, restaurants and bars, in private vehicles and on public transport. In this article, only indoor air quality in offices will be considered.
It is without controversy that poor air quality is recognised as having a negative impact upon human health and well-being. Beside a good indoor climate, the air should also be free of substances hazardous to human health. Sources of such substances are the products used for construction and the interior furnishings, as well as the human beings themselves, the tasks they perform and the work equipment used, and the chemical or biological substances used or released as incidental by-products of human activities or work processes. The use of no-emission, or as low as possible emission equipment and products therefore helps improve indoor air quality, as does good ventilation. In contrast to outdoor air, indoor air is subject to very few statutory provisions governing its quality.
Indoor air quality and well-being
Health complaints at indoor workplaces are frequently associated directly with the presence of harmful substances in the breathing air.
Odours, among other things, may lead to assumptions that hazardous substances are present, or be used as evidence of them. An individual's subjective impression of an odour is not however generally a reliable indicator of its relevance to health. Some odours that are easily sensed by human beings cannot be detected by recent analysis methods. The reverse is also the case: not every hazardous substance has a perceptible odour. If an odour is noticed, it is automatically assessed based on previous experience. As a result, the mere odour of a substance may be sufficient to trigger a feeling of well-being, indisposition or even complaints, even where its concentration is completely harmless.
In 1983, the World Health Organization coined the term sick building syndrome (SBS). The term describes the reports of impaired well-being in indoor areas, which had been on the rise since the mid-1970s. SBS refers to complaints by one or more users of a building of one or more non-specific symptoms, such as: irritation of the eyes, nose and throat, frequent infections of the airways and cough, skin irritation, headaches, tiredness, indisposition, nausea, vertigo, fatigue, difficulty in concentrating or sensitivity to odours. The symptoms generally subside a few hours after the affected individual has left the building. Beside poor indoor air quality, the indoor climate, noise, lighting, electromagnetic fields, ionising radiation, design of the workplace, and psychosocial factors such as stress must also be considered as possible causes of the complaints.
In contrast to SBS, the term building-related illness covers clearly defined clinical pictures that have their causes in the building. These include allergies to house-dust mites or fungi, including asthma, legionellosis and humidifier lung.
Sources of pollutants
A number of different factors influence indoor air quality. Firstly, vapours given off by construction materials and interior furnishings contaminate the air. Secondly, undesirable substances are emitted into the room by the substances used by human beings (e.g. perfumes), their transpiration, or by the tasks they performed (such as cleaning, smoking and printing).
Construction materials and indoor equipment
The indoor air quality is influenced by the design of the building and by the furnishings and equipment within the rooms. Beside the design of the building envelope and the layout of the individual rooms, the construction materials used, the materials employed for the interior furnishings and the technical equipment are therefore of decisive importance. Vapours given off by the materials employed (e.g. volatile organic compounds, formaldehyde) may impair the quality of the air, particularly in new buildings and in consequence of redecoration work.
In order to reduce energy consumption, the sealing of modern buildings is becoming increasingly tight. This prevents the natural exchange of air. In order to retain high indoor air quality, many experts recommend that ventilation equipment be installed in all such buildings. This also enables exposure to damp and the ensuing mould build-up to be prevented.
The ventilation systems themselves may however be a cause of complaints of poor air quality or indoor climate. This may be the case where such equipment is not adequately maintained and cleaned, the air is routed unsatisfactorily, or the installation has not been properly adjusted. In some cases, persons in the working area may not benefit from the fresh air supplied because it disappears again immediately through the discharge air outlet.
Human beings themselves are one of the main emitters of pollutants in indoor areas. Besides respiration, which causes large quantities of carbon dioxide to be emitted into the indoor air, the human body gives off various odorous vapours, both of its own and of foreign origin (such as perfumes). This phenomenon is exacerbated by perspiration caused by high temperatures or strong physical exertion. The release of intestinal gas and of bacteria and viruses during sneezing are further examples. Flaking of the skin and loss of hair, and their contribution to the dust, which also impacts negatively upon indoor air quality, are frequently overlooked.
Work equipment such as writing utensils, paper and electrical appliances (such as computer screens, laser printers, photocopiers) may emit a whole range of organic compounds of high or low volatility to the ambient atmosphere when in use. The substances emitted are essentially solvents, degreasing agents, release agents and coatings which outgas from the materials. Flameproofing agents of lower volatility and plasticising agents, which are added to the materials from which the equipment is manufactured in order to produce certain material properties, may also enter the ambient atmosphere, particularly under the influence of heat. Ozone, paper and toner dusts, and volatile toner constituents may also be emitted from certain equipment during operation.
Residues from cleaning agents may contaminate the indoor air over longer periods when their constituent substances vaporise or outgas. Such substances are often preservation agents, disinfectants (such as aldehydes), solvents (such as glycols, isopropanol), organic acids, propellants, and flavours. Furthermore, secondary products could be formed for example by the reaction of some fragrances with ozone.
In Europe, the contamination of air by cooking and heating processes is likely to be of minor significance at indoor workplaces in offices. Tobacco smoke was for long time the most significant source of pollutants in indoor areas. Nowadays in many countries smoking at workplaces is forbidden. Where workers smoke at their workplaces, the dominant pollutants in the indoor air are those attributable to cigarette smoke. Cigarette smoke contains an extensive number of chemical compounds with a wide range of effects upon the human organism, including many carcinogens. To date, over 4,000 discrete components have been identified in tobacco smoke.
In this context the burning of candles and also the fashionable use of aroma lamps to vaporise aromatic oils appear to be problematic. This may cause sensitisation with subsequent triggering of allergic reactions, and possibly even asthma attacks.
Atmospheric contaminants from external sources may impair the indoor air quality following ingress through either the ventilation system or open windows. These contaminants include car exhaust fumes next to busy roads, and waste gases emitted from nearby industrial plants. The ingress of agricultural odorous substances and of natural pollen and spores must also be considered. In addition, radon may enter indoor areas in geographical regions with specific underground geological formations via the air coming from the soil.
Carbon dioxide is a natural component of the ambient air, with an average concentration of 400 ppm. Since human respiration is generally the primary source of carbon dioxide emissions in indoor areas, the carbon dioxide concentration rises essentially as a function of the number of users of an indoor area and their activity. The carbon dioxide emissions are approximately proportional to the rise in odorous substances given off by human beings by perspiration. In rooms in which no combustion processes are taking place, the carbon dioxide concentration can therefore be regarded as an indicator of the indoor air quality. A value of 1,000 ppm (1,800 mg/m3) is stated as a guideline value for the carbon dioxide concentration in some European countries. Concentrations above 1,000 ppm cause general feelings of unwellness such as fatigue, headaches and loss of concentration.
Carbon monoxide is produced by the incomplete combustion of organic material. Beside gas-fired cookers and heaters with poor or no exhaustion, smoking is therefore the primary source of carbon monoxide in indoor areas in Europe. In the vicinity of busy roads, the ingress of carbon monoxide together with the outside air must be considered. Carbon monoxide decreases the oxygen transport capacity of the blood. Primarily this could be problematic for people with cardiovascular diseases and pregnant women. More generally, the decreased oxygen transport in the blood reduces people’s exercise ability. The WHO proposes an eight-hour mean guideline value of 10 mg/m³ for indoor areas, particularly indoor workplaces.
Volatile organic compounds
A large number of volatile organic compounds (VOCs) can be detected in indoor air. These substances differ in their chemical characteristics and thus also in their potential effects. The possible effects of individual VOCs upon the health and well-being of human beings thus cover a very broad spectrum. Beside the carcinogenic, mutagenic or reprotoxic effects of some VOCs (e.g. formaldehyde, some ethylene glycols, etc.), possible health effects include irritation of the eyes, nose and throat, respiratory disorders – including asthma - nosebleeds, allergic skin reactions, nausea and vomiting, headaches and vertigo, fatigue, and shortness of breath.
Since the mutual and combined action of the individual substances is not generally known, assessment of health effects continues to present difficulties. The sum of the concentrations of the individual compounds is used as an indicator for the concentration of VOCs. This sum is termed the TVOC (total volatile organic compounds) value. As early as 1992, the European Commission published a target concentration of 300 µg/m3 for the TVOC value in indoor areas.
Virtually all materials used in modern buildings are potential sources of VOCs. Modern bricks, mortar and other building elements include additives containing plastics and solvents. The range of potential sources include wall panelling and flooring products, insulation materials, furniture, paints and coatings, and solvents for interior work. VOCs are also emitted into indoor areas through cleaning and care materials, cosmetic products, disinfectants, pesticides and tobacco smoke. Already at the purchase stage it should be taken into account only to choose materials not susceptible to release any harmful VOCs. However, this is challenged by the fact that even today, products which have inadequate, if at all, hazard labelling and chemical composition listings, are found on the market. These products include hobby and do-it-yourself materials.
For the identification of possible sources of detected VOCs, the first to be considered is recent redecoration work or the introduction of new furniture or equipment into the rooms. In such cases, the VOC concentrations can often be reduced by thorough ventilation and heating of the rooms. Certain cleaning agents and air fresheners should also be considered as possible sources.
Significant causes of the release of formaldehyde in indoor areas are tobacco smoking and other incomplete combustion processes, such as open flames and fireplaces, and gas-fired heaters. Furniture manufactured from chipboard, coatings (especially acid-hardening lacquers on parquet floors and furniture), veneers, textiles and carpets are further relevant sources of formaldehyde. Formaldehyde is also found in water-based preparations as a disinfectant and preserving agent, and can be found in bodycare products, cleaning and washing agents and disinfectants. Secondary sources are the oxidation of volatile organic compounds and the reaction between ozone and alkenes (especially terpenes).
In 2004, a working group of the International Agency for Research on Cancer (IARC) classified formaldehyde as 'carcinogenic to humans'. At EU level, formaldehyde is classified as suspected carcinogen in the so-called CLP regulation. The WHO recommends a guideline maximum value for the indoor air of 0.1 mg/m3 for 30-minute average exposure in order to protect the majority of the population against sensory irritation. Eye irritation was demonstrated after only four hours of exposure to a concentration of 0.36 mg/m3 .
A range of options are available for substituting formaldehyde with non-hazardous, or less hazardous substances in paints, coatings, furniture, etc.. When furnishing and decorating indoor workplaces, purchasing such formaldehyde-free materials and equipment containing no-hazardous, or less hazardous substances should be given priority.
Radon is an inert gas and can be detected neither by its odour, nor by the other human senses. It is produced under normal ambient conditions by the radioactive decay of uranium and thorium, which are ubiquitous in small quantities in the soil. Radon enters the atmosphere from the soil through convection and diffusion, resulting in a minor radon component in the atmosphere.
Radon gas may however not only enter the outdoor air, but also penetrate the cellar walls of buildings. Minor differences in pressure, arising in particular during the period when the building is heated, may cause radon to rise from the cellars to the storeys above them. Within the rooms of a building, the radon concentration is dependent upon a number of factors, including the geological structure of the ground, the storey, the airtightness of the building (floor slab, joints, etc.), and the ventilation of the rooms.
Radon may also enter buildings through the groundwater. While using the water, radon is released into the indoor air. The inhalation of radon and radon decay products does not lead to acute symptoms. Delayed health effect may however occur, above all lung cancer. After smoking, the inhalation of radon is the second most common cause of lung cancer worldwide. The probability of such damage occurring is dependent upon factors including the level of the absorbed radon decay product activity and thus also upon the radon concentration in the ambient air.
Airborne particulate matter
Airborne particulate matter is solid or liquid particles distributed evenly in the air which remain airborne for prolonged periods owing to their particle size. The time to deposition is inversely proportional to the particle size.
Airborne particulate matter enters indoor areas primarily by the ingress of particles from the outdoor air through the ventilation system, and secondarily by adhesion to shoes and clothing. Within the rooms themselves, particulate matter in the air arrives through the dispersal of dust and in some cases by mechanical processes (such as paper dust during the handling of papers).
In recent years, concern has increased relating to possible health hazards caused by the presence of ultrafine particles in indoor areas. The particles concerned are less than 100 nm (= 0.1 μm) in diameter. They are produced in indoor areas in combustion processes (fireplaces, gas-fired heaters or cookers), by candles, and in particular by smoking. In addition, diesel-engine particulate emissions in the outdoor air may for example enter the indoor air. Laser printers, photocopiers and multifunction computer peripherals have also repeatedly been considered a source of particulate matter containing toner particles and other constituents in the indoor air.
To enable the concentrations of airborne particulates to be assessed, the WHO has published guideline values applicable throughout the world for indoor and outdoor air. The air quality guideline values for the 24-hour average are 50 µg/m3 for PM10 and 25 µg/m3 for PM2.5. It was not possible to determine a threshold concentration below which the particulate concentration in the environment had no impact upon human health. If possible, the guideline values should not therefore be exceeded.
A link has been demonstrated between exposure to fungi, bacteria, viruses, mites, etc. on the one hand and diseases of the airways, allergies, asthma and immune reactions on the other. Microbial pollutants arise in numerous ways. Human beings themselves give off microorganisms to the environment, for example when exhaling or by skin flaking. Even plants are not as harmless as they seem; microorganisms, particularly fungi, frequently colonise the plant substrate. Pollen and spores may also be transferred into the indoor areas from the outdoor air.
Microbial pollutants cause problems primarily in conjunction with indoor damp and unsatisfactory ventilation. Damp causes the growth of mould, fungi and bacteria on virtually all indoor materials. As a result, spores, cells and VOCs are released in greater quantities. At the same time, decomposition processes of the materials are triggered which in turn cause substances to be given off into the indoor air.
Since it has not yet been possible to quantify the mutual effects of humidity and microbial load and their impact upon human health, no guideline or threshold values have yet been set out for colonisation by microorganisms. With regards to airborne levels, the Joint Research Centre of the European Commission established that levels above 500 Colony Forming Unit per Cubic Meter (CFU/m3) are an intermediate contamination in non-industrial indoor workplaces, and above 2,000 CFU/m3 high contamination. In addition to determining the number of airborne CFU/m3, the identification of predominant taxa, or at least fungi, is recommended to evaluate properly the hazard to workers. Intervention and remediation are considered necessary when levels are above 500 CFU/m3 and occupants complain of non-specific symptoms. The WHO recommends that ‘persistent dampness and microbial growth on interior surfaces and in building structures should be avoided or minimized’.
Legislation and guidelines
European Union directives
In contrast to the outdoor air, very few specific provisions exist to date governing the quality of indoor air in non-industrial premises. Although not specific to IAQ, the “Framework” Directive 89/391/EEC and the chemical agent Directive 98/24/EC apply, meaning that the potential risks of poor IAQ have to be considered in the workplace risks assessment and that the hierarchy of control measures apply (giving the priority to elimination of the risk, followed by substitution, etc). Directive 2004/37 on carcinogens and mutagens introduces stricter provisions, for example on substitution, and equally apply. In the case of IAQ in office workplaces, elimination of the risk means that only construction material, furnishing material and equipment, office work equipment (e.g. printers) not releasing harmful substances into the atmosphere is purchased. EU Directive 89/654/EEC requires that sufficient fresh air be available taking into account the working methods used and the physical demands placed on the workers. By extension, this means that the indoor air quality must equal to that of clean outdoor air. In the absence of other values for the assessment of the indoor air quality, EU directive 2008/50/EC on ambient air quality and cleaner air for Europe, which was adopted on 21 May 2008 in order to protect human health and the environment as a whole, can be applied. This directive sets out limit values, target values, and information and alert thresholds for selected atmospheric pollutants. Directive 2004/107/EC governs arsenic, cadmium, mercury, nickel, and polycyclic aromatic hydrocarbons in the atmosphere.
Where the breathing air contains hazardous substances, occupational exposure limits (OELs), when available for the substances concerned, are generally used to assess a possible risk to human health. However, in offices the levels of hazardous substances found in the ambient air are generally much lower than the OELs. Therefore, the guidelines set for living areas, which are subject to different criteria owing to the presence of groups of persons at particular risk (such as children) are generally used as orientation. In these cases, the outdoor air serves as a better guideline for the air quality.
Guidelines of the World Health Organization
In 1987, a working group at the World Health Organization (WHO) drew up air quality guidelines for Europe for the first time, in order to protect the health of the population against the effects of atmospheric pollutants. These guidelines apply to both the outdoor and the indoor air. For the individual substances, guideline values were defined below which the health of the population is not significantly at risk, either at all or at least for a defined period of time. Following revision of the guideline values in 2001, guideline values applicable throughout the world were issued in addition in 2006 for dust, ozone, sulfur dioxide and nitrogen dioxide.
In 2009, the WHO published guidelines for the first time for indoor air quality for the purpose of protecting the public against risks to health caused by damp and the associated growth of microorganisms. These guidelines were supplemented in 2010 by further guidelines for certain chemicals frequently encountered in the indoor air. The substances in question are benzene, carbon monoxide, formaldehyde, naphthalene, nitrogen dioxide, polycyclic aromatic hydrocarbons, radon, tri-/tetrachloroethylene.
National legislation and guidelines
Some European countries, as well as Canada, the USA and Australia, have their own individual regulations and recommendations for the assessment of indoor air quality. These regulations and recommendations also differ in their legally binding status. In addition to values based upon epidemiological and toxicological findings, use is frequently made of comparative values based upon the statistical evaluation of field studies.
Prevention and protective measures
Health complaints at indoor workplaces can have a wide range of causes; resolving them is frequently difficult. The subjective assumptions made by the affected individuals regarding the causes are often incorrect. They may lead to expensive air quality measurements, even though the problem may for example be inadequate lighting. Beside sources of hazardous substance emissions, issues such as inadequate ventilation or a poor indoor climate must be considered. Attention must however also be paid to psychosocial and ergonomic aspects, which are in many cases regarded as 'hidden' causes of complaints.
The quality of the breathing air at indoor workplaces is influenced by the room ventilation. An essential distinction is drawn between two types of room ventilation: firstly, natural ventilation, in which the air is exchanged through open windows and doors, and cracks; and secondly, forced ventilation, in which air is supplied and discharged selectively. With natural ventilation, the air exchange rate is dependent entirely upon the weather conditions, the temperature situation within the building and the outdoor temperature. Controlled ventilation is not therefore possible with such systems. By contrast, forced ventilation enables a controlled room air situation to be maintained independently of the weather and the conditions within the building.
Systems which are properly planned and regularly maintained have a positive impact upon the climate and the concentration of pollutants in the air of indoor areas. Conversely, systems that are maintained poorly or not at all may give rise to unpleasant odours and moulds in indoor areas, and humidifiers may increase the development of biological agents.
Prevention at the source and product labelling
In order to minimise the amount of substances entering the indoor air, only materials not releasing, or with the lowest possible release of harmful substances into the indoor air should be used from the outset during new construction, conversion and redevelopment work. The same applies to the selection of suitable furniture and work equipment, and to the use of cleaning and hygiene products.
Throughout the European Union, the General Product Safety Directive 2001/95/EC ensures a high level of product safety for consumer products that are not covered by specific sector legislation (e.g. toys, cosmetics, machinery). Hazardous chemicals used, for instance, to clean offices have to be labelled according to the Regulation (EC) No 1272/2008 of the European Parliament and of the Council of 16 December 2008 on classification, labelling and packaging of substances and mixtures. If possible, they should be substituted by less harmful products.
Furthermore, ecolabels may assist in the selection of suitable products. A range of such labels, varying in their quality and relevance, are now in use on the market. The world's oldest official ecolabel is Germany's 'Blue Angel'. Based upon scientific findings, a wide range of criteria have been developed by neutral, state bodies according to the product group for the issuing of the label. In 1992, the European 'flower' ecolabel was awarded for the first time. These two ecolabels serve to identify products in a wide range of categories which have a low impact upon the environment and which, when used properly, have little or no detrimental effect upon human health.
- WHO – World Health Organization, ‘Indoor air pollutants: exposure and health effects’, EURO Reports and Studies Vol. 78, 1983. Available at: 
- EU-OSHA – European Agency for Safety and Health at Work, ‘The occupational safety and health of cleaning workers’, Luxembourg, 2009. Available at: 
- German Ad-hoc working group of members of the Federal Environment Agency’s Indoor Air Hygiene Commission (IRK) and the permanent working group of the Highest State Health Authorities (AOLG), Health-related evaluation of carbon dioxide in indoor air , Bundesgesundheitsbl Gesundheitsforsch Gesundheitsschutz, no 51, 2008, pp 1358-69. Available at: 
- WHO – World Health Organization, ‘Guidelines for indoor air quality: selected pollutants’, Copenhagen, 2010. Available at: 
- Commission of the European Communities, ‘Indoor Air Quality & its Impact on Man’, Report No. 11: Guidelines for Ventilation Requirements in Buildings, EUR 14449 EN, 1992. Available at: 
- IARC – International Agency for Research on Cancer, ‘Formaldehyde, 2-Butoxyethanol and 1-tert-Butoxypropan-2-ol’, Monographs on the Evaluation of Carcinogenic Risks to Humans, Vol. 88, 2006. Available at: 
- Regulation (EC) No 1272/2008 of the European Parliament and of the Council of 16 December 2008 on classification, labelling and packaging of substances and mixtures. Available at: 
- Afsset - Agence française de sécurité sanitaire de l’environnement et du travail, Risques sanitaires liés à la présence de formaldéhyde, 2009. Available at: 
- WHO – World Health Organization, ‘Handbook on indoor radon: a public health perspective’, Geneva, 2009a. Available at: 
- Morawska, L., He, C.; Johnson, G., Jayaratne, R., Salthammer, T., Wang, H., Uhde, E., Bostrom, T., Modini, R., Ayoko, G., McGarry, P., Wensing, M., ‘An investigation into the characteristics and formation mechanisms of particles originating from the operation of laser printers’, Environmental science and technology 43, Nr.4, 2009, pp. 1015-22.
- WHO – World Health Organization, ‘Air Quality Guidelines for particulate matter, ozone, nitrogen dioxide and sulfur dioxide’, Geneva, 2006. Available at: 
- EC – Commission of the European Communities, ‘Biological particles in indoor environments’, Report 12, 1993. Available at: 
- WHO – World Health Organization, ‘Guidelines for Indoor Air Quality: Dampness and Mould’, Copenhagen, 2009b. Available at: 
- Council Directive 89/391/EEC of 12 June 1989 on the introduction of measures to encourage improvements in the safety and health of workers at work. Available at: 
- Council Directive 98/24/EC of 7 April 1998 on the protection of the health and safety of workers from the risks related to chemical agents at work. Available at: 
- Directive 2004/37/EC of the European Parliament and of the Council of 29 April 2004 on the protection of workers from the risks related to exposure to carcinogens or mutagens at work. Available at: 
- Council Directive 89/654/EEC of 30 November 1989 concerning the minimum safety and health requirements for the workplace. Available at: 
- Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on ambient air quality and cleaner air for Europe. Available at: 
- Directive 2004/107/EC of the European Parliament and of the Council of 15 December 2004 relating to arsenic, cadmium, mercury, nickel and polycyclic aromatic hydrocarbons in ambient air Available at: 
- WHO – World Health Organization, ‘Air Quality Guidelines for Europe’, Copenhagen, 1987.
- WHO – World Health Organization, ‘Air Quality Guidelines for Europe’, 2nd edition, Copenhagen, 2001. Available at: 
- Directive 2001/95/EC of the European Parliament and of the Council of 3 December 2001 on general product safety. Available at: 
Links for further reading
Air infiltration and ventilation centre – AIVC. European Collaborative Action on Urban Air, Indoor Environment and Human Exposure – Publications about miscellaneous themes (e.g. SBS, TVOC). Retrieved 12 April 2011, from: 
EU-OSHA – European Agency for Safety and Health at Work, ‘Expert forecast on emerging biological risks related to occupational safety and health, Luxemburg, 2007. Available at: 
European Commission Environment. Air quality. Retrieved 12 April 2011, from: 
European Commission Environment. Ecolabels. Retrieved 12 April 2011, from: 
European Commission, Directorate-General for Health and Consumers, Public Health. Tobacco. Retrieved 24 October 2011, from: 
Smokefree Partnership. Smoke-free legislation in the EU. Retrieved 24 October 2011, from: 
The Blue Angel. Retrieved 12 April 2011, from:  WHO – World Health Organization – Regional office for Europe. Air quality. Retrieved 12 April 2011, from: http://www.euro.who.int/en/what-we-do/health-topics/environmental-health/air-quality