General questions

What is Indoor Air Quality?

This refers to both the positive and negative impact that indoor air in a building has on human health or well-being.

What is good Indoor Air Quality?

This refers to air quality that does not contain excessive concentrations of gases or particles that adversely affect metabolism.

In normal conditions, poor indoor air quality should only cause people discomfort and irritation. In extreme conditions, poor indoor air quality may be fatal to all occupants of the environment.

It should be noted that the concentration of pollutants is essential, as an increase in concentration causes more harmful effects.

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-like symptoms, or similar conditions and discomfort. In the case of work environments, working days are lost and company productivity is also negatively affected.

More serious problems may occur (Building-related illnesses); in fact, there are serious diseases that are directly related to poor indoor environmental quality (Legionnaires’ 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 listed below:

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

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 allow pollutants to circulate from one area of a building to another, and ultimately, they may 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, Legionnaires’ disease, bacteria or biocides from adjacent cooling towers, etc.

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 the air flow eroding duct insulation and dirt build-up in the ducts due to lack of maintenance and/or filtration.
Poor management of indoor air pressures may 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 a bathroom has an air supply with a higher pressure than the extractor, then the air will flow from the bathroom into the adjacent areas. Ducts with loss of air flow located in false ceilings may cause air to travel from false ceilings into occupied areas.

“Airtight buildings” are constructed to reduce energy costs and depend on outdoor air to dilute pollutants that are transmitted from or are present inside the building. When the outdoor air supplied is insufficient, pollutants may build up and become a serious problem for the occupants of a building.

The most common causes of insufficient outdoor air supply are listed below, starting with the most regular:

  1. Thermostat fans in “auto” mode that only work when heating or air conditioning is in operation.
  2. Units turned off or have clocks set incorrectly, causing the system to be inoperative during building occupancy hours.
  3. Absence of outdoor air intakes or outdoor air intakes are completely closed.
  4. Buildings constructed according to obsolete standards with insufficient outdoor air supply (1970s’ oil crisis and exploring energy conservation).
  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.
  • Odours such as perfumes, kitchen, bathroom, etc. noticed upon entering 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). Thermostats are controlled by the occupants, who can turn them on or off and adjust the temperature as they wish. As indoor air is not changed, it becomes stagnant and stuffy.
How can indoor air quality problems relating to ventilation and air conditioning systems be solved?

Systems need to be turned on before people arrive at the building, or before peak occupancy hours.
It must be ensured that the outdoor air intakes are open and that there are no sources of pollutants nearby (e.g. air intakes located in a garage).

Use adjustable thermostats that enable occupants to regulate the temperature a few degrees up or down but which do not have many more settings.
Check the interior of the systems frequently, have a maintenance programme in place and monitor and control the build-up of water in the cooling tower drain trays.

Do not allow water to enter the systems and ensure that there is no stagnant water inside them. If this were to occur, dry the systems quickly.

If possible, all duct fibreglass that is exposed to air flow should be removed or replaced with insulation that does not contain fibreglass. It would be a good idea to at least 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 every year and the ducts, every 5 to 10 years. The best possible filtration that can be operated by the system must be used.

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

Cleaning of ducts

Dirt is built up in ventilation and air conditioning systems over time and these must be cleaned regularly in order to be efficient and have a long life.

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 used for cleaning ducts. If duct cleaning is carried out properly, it 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 inside a building important?

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

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

Numerous studies have shown that the cleaner the air, the healthier it is. In fact, whenever dust levels rise in the air, there is a measurable increase in asthma and allergy attacks, A&E visits and deaths. This effect is quantifiable even at very low levels.

Ways to keep indoor air in a building clean.

There are three basic ways to keep air clean:

  • Preventing the entry of outdoor pollutants.
  • Filtering the pollutants that are present.
  • Cleaning in order to eliminate pollutants from the surfaces on which they are found.

If we do not allow outdoor air to enter the building, we will not allow outdoor pollutants to enter either. This is not practical as outdoor air is necessary in order for indoor air to be changed. If the building is close to major access roads, outdoor air supply can at least be cut off during rush hour.

By using low-emission compound materials in the building structure and furniture, the level of pollutants in these areas will drop.

Filtration in the form of filters or purifiers in the ventilation system or stand-alone filtration systems may be suitable for removing particles. A special type of filter must be used to remove gaseous pollutants.

No filtration system will be effective if high amounts of outdoor pollutants are allowed to enter the building, if indoor surfaces are excessively dirty, or if there are large sources of pollution inside the building.
Regular and 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 (with vacuum cleaners) or high-efficiency filtration (HEPA) is carried out; this 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 damp for too long, this causes the growth of mould and, if not cleaned carefully, toxic soap deposits will remain on the carpeting.

Types of filters

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

Typical blanket filters, with a maximum thickness of 5 centimetres, are very ineffective as they only remove larger particles that are harmless to health. They are used more to protect office equipment than occupants.

Electrostatic filters are advertised as “highly efficient” in removing allergens and dust from the air. This is just a passing trend. When clean, they are somewhat more efficient than blanket filters, but they do get dirty very quickly, losing a great deal of efficiency. Their main advantage is that they are washable and do not require new filters to be bought. The main drawback is that they have to be washed continuously. Advertisements that promote “95%” efficiency are misleading as this refers to the weight percentage of dust removed and this corresponds to the heavier particles. Electrostatic filters are technically referred to as “low-efficiency impact filters”.

Pleated filters are more efficient; they are of medium efficiency and remove a greater amount of smaller particles, which are the most harmful to health. These filters vary in thickness: 15 cm, 20 cm or 30 cm, with the latter being the most effective. The problem is that some systems do not have a high enough flow for the air to pass through the filters, or enough space for the filters to be placed inside.

HEPA (High-Efficiency Particulate Air) filters were originally developed for the nuclear industry. They remove 99.97% of particles (even very small ones) and require extra drive power as they produce a large pressure drop. Virtually no unwanted matter passes through these filters.

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

More about filters and air purifiers

It is highly recommendable to install filters in central air conditioning and heating systems. But even so, these filters will be useless 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 a high-efficiency stand-alone purifier may not prove to be a totally effective measure, since it must process a high enough volume of air. A measurement system is used to gauge the required air volume, which is expressed as a number of changes per hour. This represents 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 to be changed between 5 and 7 times an hour in offices where there are smokers.

All central filter systems and stand-alone purifiers will be ineffective when doors or windows are left open, as they draw in more air than the system can handle. They will also be ineffective if the interior is very 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 refer to a wide range of chemical compounds containing carbon atoms that may be gases or liquids, with the latter tending to evaporate easily at normal temperatures. Petrol and solvents are typical examples of these compounds.

What kinds of problems do they cause and how often?

VOCs evaporate easily and they may be inhaled given that they are airborne. Many VOCs are common and also toxic. Any substance that requires drying will almost always emit VOCs.

Many new materials emit relatively high levels of VOCs. With time, a drastic drop is seen in emission levels over a space of days, weeks, or even months. This phenomenon is what gives new cars, new furniture, etc. their “new” smell.

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? Multiple Chemical Sensitivity is also a serious problem.

Rugs, carpets and Indoor Air Quality

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

This began with an incident at the Environmental Protection Agency (EPA) headquarters in Washington, D.C., where a number of employees fell ill following office renovations, which included the replacement of rugs and carpets.

Following this incident, a laboratory put a piece of carpet inside a chamber with mice, and this resulted in the mice dying.

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

However, it has been proven that carpets and rugs, and the glues and adhesives used to stick them to the floor emit VOCs and these levels must be minimised. To ensure a reduction in VOC levels, it is a good idea to store rugs and/or carpets in a warehouse in the weeks prior to their installation, and use non-toxic glues and adhesives with low emission levels.

Aside from these factors, it is certainly clear that rugs and carpets store particles and other pollutants.


Formaldehydes are used in the composition of insulators, which are of value in both construction and furniture manufacturing.

Since its toxic properties have been discovered, it is now used much less in the composition of insulators.

When exposed to formaldehydes, people experience several types of adverse reactions and this mainly affects their breathing, 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 must 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 to outside.

In new constructions or after renovations, the amount of outdoor air that flows inside must be higher in order to offset the increase in levels of VOC. One authority in this field actually recommends 100% outdoor air, without any recirculation whatsoever, in the first 6 months.

Bioaerosols and problems with water

What are bioaerosols?

Bioaerosols are airborne particles that are or have been alive, or are the product of a living thing. The following are examples of bioaerosols:

  • Viruses
  • Bacteria
  • Fungal spores
  • Algae
  • Protozoa
  • Pollen
  • Dust mites
  • Flakes of skin / hair (animal or human)

The presence of bioaerosols in the air causes respiratory problems, dryness in the throat, itchy eyes, etc.

How does humidity contribute to air quality problems?

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

Although it is normal for air to have mites, fungi, bacteria, etc., in conditions of excessive humidity, these concentrations may reach extremely high levels, particularly inside buildings.

Dust Mites

Dust mites are tiny creatures that feed on flakes of skin. They live in materials such as carpets, sofas and mattresses, where there is a build-up of these flakes. Mite faeces are highly allergenic, and the majority of people are actually 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 constantly floating in the air. When something dies, mould spores settle on the body 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. People may be affected after touching or inhaling airborne spores. The effects of toxic mould on humans vary from very mild to very severe, and may even damage the liver or the immune system.

Mould can only infect people whose defences are low or who have a superficial infection on their nails or skin.


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

A high percentage of bacteria is toxic, as the cell walls of these microorganisms contain a substance called endotoxin and adverse health effects may be caused when a high enough level of this is inhaled.

Legionnaire’s disease

What is Legionnaires’ disease?

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

How does it spread?

Low concentrations of the bacteria that cause Legionnaires’ disease are normally found in drinking water tanks. The population can increase dramatically when in the right conditions (warm water temperature of between 32 and 54ºC, in addition to the presence of nutrients). If contaminated water droplets are inhaled by susceptible individuals (whose defences are low), 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 glass strands with a very small diameter and is normally used as an insulator in walls, ceilings and ventilation systems of buildings. It is part of a broader range of materials called man-made mineral fibres.

Is fibreglass carcinogenic?

That question is still open to 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 these substances do share certain similarities. It appears that cancer rates are higher among people who work in fibreglass processing plants.

Medically-recognised effects of fibreglass

Fibreglass has been medically recognised as an irritant to the skin, eyes, and upper respiratory tract. Hives, headaches, conjunctivitis are some of the most common effects suffered by people 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 said that there are problems?

When several complaints are made about itchy eyes, fibreglass becomes one of the main factors suspected of causing this discomfort. If there is residue 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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