General questions

What is Indoor Air Quality?

This refers to the impact, good or bad, that indoor air in a building has on human health or well-being.

Are Indoor Air Quality problems very common?

According to the World Health Organisation (WHO), approximately 30% of all buildings have indoor air quality problems.

Are Indoor Air Quality problems serious?

Most Indoor Air Quality problems are fairly minor. This means that occupants may experience cold or flu symptoms, or similar conditions and discomfort. In the case of workplace environments, working hours are lost and company productivity is also negatively affected.

More serious problems may occur (Building-related illnesses); in fact, there are serious diseases that relate directly to poor indoor environmental quality (Legionnaire’s disease, certain types of cancers, etc.).

Types of pollutants that may be inside buildings

Certain pollutants are found in the form of gases, classified as toxic compounds. The main ones are the following:

  • Combustion products (carbon monoxide, nitrogen dioxide…)
  • Volatile Organic Compounds (formaldehydes, solvents, perfumes…)
  • Volatile Semi-Organic Compounds (pesticides…)
  • Other pollutants are found in the form of particles; the main ones are:
  • Bioaerosols (fungal spores, pollen, viruses, bacteria…)
  • Construction and furniture material particles (fibreglass, asbestos…)

Ventilation and Air Conditioning Systems

How do ventilation and air conditioning systems affect indoor air quality?

Ventilation and air conditioning systems can introduce outdoor pollutants; in themselves, they can become a source of pollutants. Through their ducts they cause pollutants to circulate from one area of the building to another, and ultimately, they can be ineffective in removing or diluting pollutants from a building.

The combination of all these factors gives rise to one of the main reasons for indoor air quality problems: Unsuitable design, installation, operation or maintenance of ventilation systems.
The most common outdoor pollutants are the following: SMOG, fungal spores and pollen (depending on the time of year), vehicle exhaust emissions and the incorrect installation of air intakes, Legionnaire’s disease, bacteria or biocides from adjacent cooling towers…

The most common pollutants generated by ventilation and air conditioning systems are fungal and bacterial spores, due to the water used by some of these systems. Fibreglass is also produced by erosion of duct insulation by the air flow, and dirt accumulated in the ducts due to lack of maintenance and/or filtration.
Poorly managed indoor air pressures can cause pollutants to be transmitted between different areas of a building. For example, in bathrooms there are usually extractors to eliminate fungi and bacteria, and if that bathroom has an air supply at greater pressure than the extractor, then the air will flow from the bathroom into the adjacent areas. Ducts with airflow loss located in the false ceilings can result in air flowing out from the false ceiling into occupied areas.

“Airtight buildings” built to save energy costs depend on outdoor air to dilute pollutants originating from or present inside the building. When the outdoor air provided is insufficient, pollutants can accumulate to the point of becoming a serious problem for the occupants of a building.

The most common causes of insufficient outdoor air supply are, according to frequency, the following:

  1. Thermostat fans in “auto” mode that only work when the heating or air conditioning is in operation.
  2. Units turned off or with clocks set incorrectly that cause the system to be inoperative during building occupancy hours.
  3. Absence of outdoor air intakes or completely closed outdoor air intakes.
  4. Buildings built according to obsolete patterns with insufficient outdoor air supply (1970s with the oil crisis and the search for energy savings).
  5. Use of economisers to reduce energy consumption in the systems.
When do problems emerge due to ventilation and air conditioning systems?

The following indicators are good signs for determining if there are indoor air quality problems due to ventilation and air conditioning systems:

  • Odour coming from fans.
  • Fibreglass or dirt residues on tables located under the air ducts.
  • Presence of odours such as perfumes, kitchen, bathroom… as soon as you access the building.
  • People start to sneeze or have itchy noses when the air conditioning system is turned on. These systems are typically over 10 years old and have never been cleaned (unless their filtration is exceptionally good). The thermostats are managed by the occupants, who can turn it on or off at will and adjust the temperature as they wish. This causes stagnant, stuffy indoor air because it is not renewed.
How do you solve indoor air quality problems relating to ventilation and air conditioning systems?

Systems need to be turned on before people arrive in the building, or before peak occupancy hours.
You have to make sure that the outdoor air intakes are open and that there are no sources of pollutants nearby (for example air intakes located in a garage).

Use adjustable thermostats that enable occupants to adjust the temperature a few degrees up or down but don’t have many more settings.
Check the interior of the systems frequently, have a maintenance programme in place and monitor and control the accumulation of water in the cooling tower drain trays.

Do not allow water to enter the systems or have stagnant water inside them and when this occurs, dry it quickly.

If possible, all duct fibreglass that is exposed to airflow should be removed or replaced with insulation that does not contain fibreglass. At the very least, it would be good to coat the ducts with an antimicrobial hard surface suitable for this purpose. Any microbial contamination must be removed as soon as it appears.
Air conditioning splits must be cleaned annually and the ducts, every 5 to 10 years. The best possible filtration that the system can handle should be used.

Make up for the deficiencies of the central ventilation and air conditioning system with stand-alone filtration systems.

Cleaning of ducts

Ventilation and air conditioning systems accumulate dirt over time and require regular cleaning for their own efficiency and longevity.

The cleaning of systems also affects indoor air quality, although this is a controversial issue as no studies have been conducted to scientifically verify the true impact of duct and system cleaning on indoor air quality.
There are several different methods of cleaning ducts. Duct cleaning carried out correctly does not need to be repeated for another 5 to 15 years, depending on factors such as dirt in the environment and the quality of filtration available.

Cleaning, filters and stand-alone filtration systems

Why is hygiene important inside a building?

Pollutants that affect indoor air quality are present in the form of gases or particles. Particles accumulate in the form of dust on surfaces. Dust inside buildings typically contains bacteria, various types of fungal spores, fibreglass, and other toxic, allergenic, or irritating substances. The more dust, the greater the amount of these substances.

Occupant activity causes the dust to move around and it becomes suspended, and therefore, susceptible to being inhaled by occupants, accumulating on their skin or re-settling on the surface of the furniture until the next time it is moved.

Numerous studies have shown that the cleaner the air, the healthier it is. In fact, for every increase in dust in the air, there is a measurable increase in asthma and allergy attacks, emergency room visits and deaths. This effect is quantifiable even at very low levels.

Ways to keep indoor air in a building clean.

There are basically three ways to keep the air clean:

  • Preventing the entry of outdoor pollutants.
  • Filtering the pollutants that are present.
  • Eliminating them from the surfaces on which they are deposited by cleaning.

If we do not introduce outdoor air into the building, we will not introduce outdoor pollutants that are present either. This is not practical as outdoor air is necessary for indoor air renewal. If the building is close to major access roads, outdoor air supply can at least be cut off during peak traffic hours.

The use of low-emission compound materials in the building structure and furniture will result in a reduction of pollutants in these areas.

Filtration in the form of filters or purifiers in the ventilation system, or stand-alone filtration systems may be suitable systems for removing particles. The removal of gaseous pollutants requires a special type of filter.

No filtration system will be effective if high amounts of outdoor pollutants are introduced into the building, if indoor surfaces are excessively dirty, or if there are large sources of pollution inside the building.

Regular, appropriate cleaning of the inside of buildings is a good way to significantly reduce airborne particles.

The use of carpets or rugs is not advisable, even if vacuum cleaning (vacuum cleaners) or high efficiency filtration (HEPA) are carried out; it is a losing battle due to their capacity for storing harmful substances.

In addition, cleaning carpets and rugs often causes more problems than it solves as, if they remain wet too long, this causes the growth of mould and, if not carried out carefully, toxic soap deposits will remain on the carpeting.


Types of filters

The main types of filters are usually located in central heating and air conditioning systems, in stand-alone filtration systems and even in vacuum cleaners.

Typical blanket filters, 5 centimetres thick or less, are very ineffective as they only remove larger particles that are harmless to health. They are used more to protect office equipment than the occupants.

Electrostatic filters are advertised as “highly efficient” in removing allergens and dust from the air. This is just a passing trend. They are somewhat more efficient when they are clean than blanket filters, but they do get dirty very quickly, losing much of their efficiency. Their main advantage is that they are washable and you don’t have to buy new filters. The main drawback is that you have to wash them all the time. Advertisements that promote “95%” efficiency are misleading as this refers to the weight percentage of dust removed and this corresponds to the heavier particles. These filters are technically referred to as “low efficiency impact filters”.

Pleated filters are more effective; they are of medium efficiency and remove a greater amount of the smaller particles, which are the most harmful to health. They come in various thicknesses, 15, 20 or 30 cm, which are the most effective. The problem is that some systems do not have enough flow for the air to pass through the filters, or enough space to position the filters inside.

HEPA (High Efficiency Particulate Air) filters were originally developed for the nuclear industry. They remove 99.97% of particles, even the really small ones; they require extra drive power as they produce a large pressure loss. Virtually no ‘undesired’ matter passes through these filters.

All the filters mentioned above remove airborne particles. The most common filter to remove gases is the Activated Carbon filter. These filters remove certain, but not all, gaseous airborne pollutants. There are also Activated Carbon, Potassium Permanganate and Zeolite (CPZ) filters that remove more gases. These filters must have enough CPZ to be effective, depending on the volume of air the system is capable of moving

More about filters and air purifiers

Filters in central air conditioning and heating systems are highly recommended. But even so, they will do nothing if the fan is not running, and given that a lot of people only start up the system occasionally, stand-alone purification systems are the best option for filtration.

Even using a high efficiency stand-alone purifier, this measure may not be totally effective, since it must process a sufficient volume of air. To gauge this required air volume, there is a measurement system expressed as a number of renewals per hour. This being the number of times that the purifier can clean the air in a room. The ASHRAE (American Society of Heating Refrigerating and Air Conditioning Engineers) recommends air renewal between 5 and 7 times an hour in offices where there are smokers.

No central filter system or stand-alone purifier will be effective when doors or windows are open as they draw in more air than the system can handle. They will also not be effective if the interior is extremely dirty or has a source of pollutants, such as a problem of active mould growth.

Volatile organic compounds, rugs, carpets, renovations

What are Volatile Organic Compounds (VOCs)?

VOCs are a wide range of chemical compounds that contain carbon atoms and that can be gases, or if they are liquid, tend to evaporate easily at normal temperatures. Gasoline and solvents are typical examples

What kinds of problems do they cause and how often?

VOCs evaporate easily, therefore, they can be inhaled given that they are airborne. Many VOCs are toxic in addition to being common. Any substance that requires drying will almost always emit VOCs.

Many new materials emit relatively high levels of VOCs. As time passes, days, weeks, or even months, emission levels drop drastically. This phenomenon is the source of the so-called “new” smell in new cars and new furniture…

It is rare for serious health problems to arise from exposure to VOCs, except in unusual circumstances. Insignificant or minor effects are very common. Who hasn’t gotten dizzy after inhaling a chemical compound? There is also the serious problem of Multiple Chemical Sensitivity.

Rugs, carpets and Indoor Air Quality

In recent years there has been certain controversy over rugs and carpets as a source of indoor air quality problems.

It started with an incident at the Environmental Protection Agency (EPA) headquarters in Washington, D.C., where a number of employees fell ill after office renovations, including rug and carpet replacement.

Following this incident, a laboratory placed mice inside a chamber with a piece of carpet, resulting in the death of the mice.

Studies are still ongoing and this is an area of deep disagreement. One side says that 25% of a new carpet is toxic, the other says that no toxic effects have been discovered from carpets and rugs.

What is certain is that carpets and rugs and the glues and adhesives used to glue them to the floor emit VOCs and their levels should be minimised. To achieve this, it is a good idea to store rugs and/or carpets in a warehouse during the weeks prior to their installation, and use non-toxic glues and adhesives with low emission levels.

Aside from these factors, what is evident is that rugs and carpets store particles and other pollutants.


Formaldehyde is used in the composition of insulators used both in construction and in furniture manufacture.

At present, since its toxic properties have become known, it is used much less in the composition of insulators.

People exposed to formaldehydes experience several types of adverse reactions, mainly respiratory effects, even when exposed to extremely low levels of these compounds.

Constructions and renovations

Most problems relating to Volatile Organic Compounds are caused by new construction and renovation works. To avoid these problems, in addition to using materials with low emission levels, ventilation must be adapted during the course of the works and after the new construction or renovation works. During renovations, the areas in which work is being carried out should remain isolated so that air does not flow from these areas into other sections. This is achieved by shutting off air returns and venting air from construction areas.

In new constructions or after renovations, the amount of outdoor air coming in must be higher in order to compensate for the higher levels of VOC, in fact, an authority in the field recommends 100% outdoor air, without any recirculation, during the first 6 months

Bioaerosols and problems with water

What are bioaerosols?

Bioaerosols are airborne particles that are or were living, or are the product of something that is alive. Examples of bioaerosols are:

  • Virus
  • Bacteria
  • Fungal Spores
  • Algae
  • Protozoa
  • Pollen
  • Dust mites
  • Skin/hair scales (animal or human)

The presence of bioaerosols in the air causes respiratory problems, dry throat, itchy eyes…

How does humidity contribute to air quality problems?

Many indoor air quality problems begin with leaks, flooding, or excessive humidity. When this happens, there is an explosion in the population of mites, fungi, bacteria and other microorganisms.

Although the presence of mites, fungi, bacteria and so on in the air is normal. However, under conditions of excessive humidity the concentrations can be excessively high, especially inside buildings.

Dust Mites

Dust mites are tiny creatures that feed on the flakes of your skin. They live in materials such as carpets, sofas and mattresses, where there is a build-up of such flakes. Mite faeces are highly allergenic, in fact most people are more allergic to faeces than to any other substance.

Mites need a certain level of humidity to reproduce; mite infestations occur at relative humidity levels in excess of 45%.


Mould is a natural recycling mechanism. Microscopic spores (mould reproductive cells) are always floating in the air. When something dies, mould spores settle on the corpse to germinate, and the mould consumes and recycles its organic compounds.

Many types of mould produce toxins when subjected to certain conditions. The toxins are normally concentrated in the spores. Airborne spores can affect people by contact or by inhalation. The effects of toxic mould on humans vary from very mild to very severe, and can even damage the liver or the immune system.

Mould cannot infect people unless they have low defences or have a superficial infection on their nails or skin.


They develop abundantly in high levels of humidity. They normally grow in the environment, not in the human body. This is due to the difference in temperature.

A high percentage of bacteria are toxic, as they contain a substance in the cell walls called endotoxin that when inhaled in sufficient quantity can cause adverse health reactions.

Legionnaire’s disease

What is Legionnaire’s disease?

Legionnaire’s disease is a type of pneumonia caused by a bacterium called Legionella Pneumophilia. It has a penetration rate of 5%, that is, only 5% of those exposed to the bacteria contract the disease, and a mortality rate of 15%, namely, 15% of those who have contracted the disease die.

How does it spread?

The bacteria that cause Legionnaire’s disease are normally found in low concentrations in drinking water tanks. If the right conditions are met (warm water temperature, between 32 and 54ºC, in addition to the presence of nutrients), the population can increase dramatically. If contaminated water droplets are inhaled by susceptible individuals (people with low defences), they can contract the disease.

The most common sources of infection are cooling towers and shower heads.

Fibreglass and other man-made fibres

Fibreglass and other man-made fibres

Fibreglass is made up of tiny strands of glass with a very small diameter. It is normally used as an insulator in walls, ceilings and ventilation systems of buildings. They can be found within a broader range of materials called man-made mineral fibres.

Is fibreglass carcinogenic?

That question remains an open debate. The American and German governments have classified fibreglass as a potentially carcinogenic element. In any case, more in-depth research is necessary, as it is not the same as asbestos, although they share certain similarities. It appears that workers who work in fibreglass processing plants suffer from higher rates of cancer.

Medically recognised effects of fibreglass

Fibreglass has been medically recognised as an irritant to the skin, eyes, and upper respiratory tract. Hives, headaches, conjunctivitis and so on are the most common effects when exposed to high concentrations.


La fibra de vidrio se encuentra dentro de los edificios al ser utilizada como aislante. Altas concentraciones pueden pasar a formar parte del aire interior cuando se lleven a cabo remodelaciones. Las placas del techo normalmente contienen fibra de vidrio al igual que los conductos de la climatización. La fibra de vidrio contenida en los conductos está sometida a la erosión del flujo de aire de los mismos que provoca que se desprenda y pase a estar en suspensión en el aire interior.

When can it be stated that there are problems?

When there are several complaints about itchy eyes, fibreglass becomes one of the main suspects of being the cause of this discomfort. If there are remnants in the air conditioning fans or on tables near the air conditioning duct outlets, this means there are problems caused by fibreglass.

Sick building syndrome, building-related diseases, and multiple chemical sensitivity


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