Indoor air quality (IAQ)

Introduction

On average, a European spends 90% of his or her life indoors^[1]. 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. The COVID-19 pandemic highlighted the importance of ensuring that at all times the indoor air we breathe is clean. In addition to a good indoor climate, the air should be free of substances that are harmful 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. 

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. 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)^[2]. The term is used in reference to reports of discomfort and health complaints 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^[3]. The symptoms generally disappear a few hours after the affected individual has left the building. Besides 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.

The staff working document from the EU Commission on supporting Indoor Air Quality^[4] describes good indoor air quality as: indoor air that meets the necessary parameters to ensure that activities within the indoor space concerned can be carried out in a safe and comfortable manner. In general, this means that indoor air is at comfortable temperature and humidity levels, that there are no uncomfortable CO₂ levels, that the air is free of significantly disturbing odours and that the level of harmful chemicals, dust, air pollutants such as particulate matter, microbes or other pathogens and allergens meets minimum standards.

Sources of pollutants

A number of different factors influence indoor air quality. Firstly, vapours from 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 perform (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. As well as the design of the building enclosure and the layout of the individual rooms, the construction materials used, the interior furnishings and the technical equipment are also of crucial importance. Vapours from materials (e.g. volatile organic compounds, formaldehyde) can affect air quality, especially in new buildings and as a result of renovations.

Enhancing the energy efficiency of buildings is a crucial component of the EU's strategy to achieve its energy and climate objectives^[5]. However, this focus on efficiency often involves sealing buildings more tightly and reducing ventilation, which can inadvertently lead to health issues for occupants. Therefore, while aiming for energy efficiency, it is important to balance these measures with the need for proper air circulation to ensure the health and well-being of those who live and work in these spaces. 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 emissions

Human beings are among the primary sources of indoor air pollutants. In addition to respiration, which releases large amounts of carbon dioxide into the indoor air, the human body emits various odorous vapours, both naturally occurring and of from external sources such as perfumes. These emissions are exacerbated by perspiration due to high temperatures or physical exertion. Other contributors to indoor air pollution include the release of intestinal gas, as well as bacteria and viruses expelled during sneezing which became especially relevant during the COVID-19 pandemic due to increased viral transmission indoors. Additionally, the shedding of skin cells and hair, which contribute to  dust, is often overlooked but also negatively impacts indoor air quality.

Equipment

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.

Cleaning

Residues from cleaning products can contaminate indoor air over a long period of time if their ingredients evaporate or gas out. 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^[6].

Combustion

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. Smoking in the workplaces is now banned in many countries. Where workers do smoke in the workplace, the dominant indoor air pollutants are those from cigarette smoke. Exposure to second-hand smoke is hazardous to health. Second-hand smoke contains the same chemicals as active smoke. Many of these have been classified as toxic or even carcinogenic to humans by the International Agency for Research on Cancer (IARC)^[7]. 

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.

External sources

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.

Most common chemical and biological IAQ-related hazards

Carbon dioxide

Carbon dioxide is a natural component of the ambient air, with an average concentration of 400 ppm. CO₂ is primarily a byproduct of human respiration and is commonly used as an indicator of ventilation quality in indoor environments. Elevated CO₂ levels (above 1,000 ppm) are an indication of poor indoor air quality and may cause general feelings of unwellness such as fatigue, headaches and loss of concentration.

Carbon monoxide

Carbon monoxide is produced by the incomplete combustion of organic material. In addition to gas-fired cookers and heaters with poor or no air extraction, smoking is the main 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. Symptoms of CO poisoning include headache, dizziness, fatigue, disorientation, memory loss and coma^[8]. The WHO proposes the concentration limit for 24-hour of 4 mg/m³ for indoor areas^[9].

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. Besides 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.

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.

Formaldehyde

The main sources 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 body care 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).

Irritation of the upper respiratory tract is thought to be the main effect of inhaled formaldehyde. Chronic exposure to very high concentrations of formaldehyde may result in cytotoxic and carcinogenic effects in this tissue^[7]. The International Agency for Research on Cancer (IARC) has classified formaldehyde as 'carcinogenic to humans'^[10]. At EU level, the CLP Regulation classifies formaldehyde as a carcinogen, category 1B (presumed to have carcinogenic potential for humans)^[11]. An occupational exposure limit value for formaldehyde (0.37 mg/m³ , 8 hours time-weighted average) is  included in Directive 2004/37/EC on the protection of workers from the risks related to exposure to carcinogens, mutagens or reprotoxic substances at work ^[12]. The WHO guideline value for formaldehyde is 100 μg/m (0.1 mg/m³) based on 30 min averaging time^[8].

A range of options are available for substituting formaldehyde with non-hazardous, or less hazardous substances in paints, coatings, furniture, etc.^[13]. 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

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 is not only found in outdoor air. It can also penetrate the walls of a building’s basement. Minor differences in pressure, arising in particular during the period when the building is heated, may cause radon to rise from the basement to the floors above. Within the rooms of a building, the radon concentration depends on 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^[14].

Airborne particulate matter

Airborne particulate matter (PM) is a complex mixture of different solid or liquid particles with different chemical and physical properties that remain in the air for long periods of time due 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 decades, there has been growing concern about the potential health risks associated with the presence of ultrafine particles in indoor areas. 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^[15]. Particulate matter (PM) can be deposited in different parts of the respiratory system depending on its size. PM10 (<10 μm in diameter) can reach the oropharynx when inhaled. PM2.5 (<2.5 μm) can penetrate the defences of the upper airways and reach the alveoli, causing respiratory irritation. PM0.1 (<0.1 μm) can cross the alveolar-blood barrier and enter the bloodstream. Short-term exposure to PM2.5 has been shown to increase cardiovascular effects (ischaemic heart disease, stroke and heart failure), respiratory effects such as exacerbation of asthma and chronic obstructive pulmonary disease (COPD), and metabolic and neurological effects. Long-term exposure to PM2.5 is associated with specific causes of cardiovascular and respiratory mortality^[8].

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^[9].. The air quality guideline values for the 24-hour average are 45 µg/m³ for PM10 and 15 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.

Microbial pollutants

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.

The presence of biological agents in an indoor environment has clearly been linked to increased prevalence of respiratory symptoms, allergies and asthma, as well as disturbances of the immunological system. Studies suggest a higher risk of certain conditions, such as hypersensitivity pneumonitis, allergic alveolitis or allergic fungal sinusitis as a result of exposure to mould and damp.

The microbial quality of indoor air is typically measured in colony-forming units per cubic meter (CFU/m³) and although there is no generally accepted threshold value, the threshold often suggested is less than 500 CFU/m³ for total bacteria for indoor environments with normal activity. For mould (fungal spores), the generally accepted guideline is less than 500 CFU/m³ in non-contaminated environments. 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^[16]. The WHO recommends that ‘persistent dampness and microbial growth on interior surfaces and in building structures should be avoided or minimized’. The WHO does not provide a threshold value for microbial concentrations in indoor air but emphasises that microbial contamination in indoor environments can have significant health impacts, particularly for those with respiratory issues or weakened immune systems. The focus is on preventing dampness and mould growth, which can lead to elevated levels of airborne microbes^[17].

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 Occupational Safety and Health (OSH) Framework Directive 89/391/EEC^[18] and the chemical agents Directive 98/24/EC^[19] 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 or reprotoxic substances at work introduces stricter provisions, for example on substitution, and equally applies^[12]. 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^[20]. The Directive also the maintenance of ventilation systems and the removal of deposits and dirt that could create an immediate danger to the health of workers by polluting the atmosphere.

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^[21]. 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^[22]. As part of the European Green Deal and the goal of zero pollution, the Commission has published a proposal to revise the air quality directives. One of the aims of the proposal is to bring EU air quality standards more closely into line with WHO recommendations^[4].

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, 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. In 2005 a global update of these guidelines was published and in 2021 followed by another revision based on the latest scientific evidence. These WHO Air quality guidelines (AQG) serve as a global target for national, regional and city governments^[9].

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. An overview of indoor environmental quality guidelines is available in the IEQ database^[23].

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. In addition to 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.

Ventilation

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 indoor air. 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 Regulation 2023/988/EU^[24] ensures a high level of product safety for consumer products that are not covered by specific sector legislation (e.g. toys, cosmetics, machinery) and applies for instance for products such as office furniture. Furthermore, under the Construction Products Regulation on the placing on the market of construction products^[25], emissions or content of harmful substances can be declared, e.g. for VOC and formaldehyde emissions. Also, hazardous cleaning chemicals used to clean offices have to be labelled according to the CLP Regulation^[11]. If possible, they should be substituted by less harmful products.