Causes and Sources
Indoor Air Pollutants

Causes of Indoor Air Pollutants

Indoor air pollutants that release pollutants (such as gases and particulates) into the ambient air are the major sources of IAQ problems. Improper ventilation or deficiencies in the ventilation system employed is another main source of the Indoor air pollutants. The deficiency in ventilation system degrades the indoor air quality by reducing the dilution of the indoor air pollutants and by also not allowing the polluted indoor air outside the house.

Indoor air quality problems arise due to the following causes:

ˇ     Pollutant sources

ˇ     Deficiencies in ventilation system

ˇ     Outdoor air pollution

ˇ     Overcrowding

ˇ     High temperature and

ˇ     High humidity

Air pollutants and ventilation both play a very important part in determining indoor air quality. The pollutants come from a variety of sources as indicated in Table 5, and emission from these sources may be weak or strong. The situation can be regarded as a contest between the pollutants and the ventilation system. If a source is weak and the ventilation system is performing as designed, air quality will be good; however, if a strong source is present, then the ventilation system may be operating at the maximum efficiency but still the air quality may be bad. Other factors that aggravate this situation are temperature, humidity and microbiological contamination. A brief description of general categories of major indoor air quality problems is as follows.

Inadequate Ventilation

In the context of air quality, this can be defined as insufficient air to remove pollutants that are degrading the quality of air.

Causes of inadequate ventilation include:

ˇ     Early shutdown and late startup of ventilation system


ˇ     Insufficient fresh air (outdoor) entering the ventilation system

ˇ     Poor air distribution by the ventilation system with in the building

ˇ     Limited air mixing in occupied areas

ˇ     Clogged filters

The first two causes are often a direct result of overzealous energy saving procedures. The remaining causes can be due to:  

     Poor designing and incorrect installing of components
     Operating the system differently from its design
     Incorrect balance
     Component malfunction
     Occupant intervention
     Lack of maintenance etc

Temperature and Humidity extremes

These factors can affect the emission rates of some pollutants, such as formaldehyde. However, the way the extremes affect occupant perceptions is probably more important. High humidity and high temperature cause people to feel lethargic and want more air movement. Low humidity induces coughing, dry throat and dry eyes. These are all symptoms of the "sick-building syndrome" mentioned above, and are thus usually blamed on poor air quality. An additional problem with low humidity is that it accentuates the sense of smell.

Other Physical Stressors

Noise from the mechanical systems or glare from lights can cause headaches and fatigue. Again, these are symptoms of the "sick building syndrome" and may be blamed on the air.

Sources of Indoor Air pollution  

There may be many sources of the air pollution indoors in any home. Some of the indoor air pollutant sources include:

    Building materials
    Combustion sources (gas stoves, kerosene stoves, smoking etc.)

     Asbestos as insulation
     Outside sources (e.g. pesticides, radon, outdoor pollution etc.)
     Household products and Personal care products
   Hobbies (such as welding, soldering etc.)

     Central heating and cooling devices
     Humidification devices





Building Materials

The main pollutants released due to the building materials are Formaldehyde, Asbestos and to a lesser extent Radon.


Formaldehyde is a colorless, organic chemical. It is used in an amazing range of products - everything from lipstick and shampoo to kitchen cabinets and carpeting because it is an excellent preservative and bonding agent.

Formaldehyde,  a main pollutant is released from building materials. Building materials like pressed wood products (hardwood plywood wall paneling, particleboard, fiberboard) and furniture made with these pressed wood products. Urea-formaldehyde foam insulation (UFFI) a widely used insulation material is one of the major sources of formaldehyde.

Exposure to formaldehyde vapors can cause eye, nose and throat irritation, coughing, skin rashes, headaches, dizziness, nausea, vomiting and nosebleeds. It is suspected of being carcinogenic to animals and the US EPA ranks it to be a carcinogen to humans.


Radon is a colorless, odorless and noble radioactive gas that is formed by natural decay of radium. Radon releases isotopes after decay, which is of primary concern.

Radon a major pollutant can be released due to the building materials. Granite, clay, bricks, marble and sandstone etc. are the major sources of radon in the building materials.

Radon has no immediate effect on an individual but it is carcinogenic. Cancer deaths in the US due to radon per year are estimated to be around 7,000 to 30,000 (US EPA). Smokers are at high risk due to radon induced lung cancer.
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Combustion Sources

The main pollutants released due to the combustion sources (such as wood-burning stoves, fire places, furnaces and tobacco products) are Formaldehyde, Inorganic gaseous pollutants (such as CO,CO2,NOx) and Respirable suspended particulates etc. 

Inorganic Gaseous Pollutants

The major combustion products are CO2, CO, NO2, NO and SO2 along with formaldehyde, hydrocarbons and respirable particles.

The main sources of Inorganic gaseous pollutants are the combustion appliances like wood-burning stoves, fireplaces, furnaces, space heaters and tobacco products. The heaters are mainly divided as Unvented and Vented space heaters. Further, they are classified based on the type of fuel used as kerosene or gas heaters of each kind.

Kerosene heaters are further classified into convective, radiant, convective / radiant, two stage and wickless depending on the design of the burner used in the heaters. The emission rates of NO2, CO and SO2 were higher from old radiant heater than from the new one. CO was released from the heaters with blue flame. Emission rate of CO2 depends on the type of fuel used. The average emission rates from a vented gas heater were higher to that of unvented gas heater.

Wood-burning stoves, fireplaces and furnaces release NO, NO2, CO, CO2, SO2, respirable particles and HCHO (formaldehyde).

CO mixes with hemoglobin and myoglobin in blood to form carboxyhemoglobin and CO-myoglobin, which reduce the transformation of oxygen to the tissues effecting the brain, muscles and myocardium.

At high concentration NO2 is known to cause lung cancer. Both NO and NO2 active species combine with other air pollutants forming carcinogenic chemicals.

High SO2 concentration (in the range of 0.25 to 0.50 ppm) causes bronchoconstriction.

Respirable Particulates

Fine solid particles, mist, smoke, dust, fibers, fumes and aerosols are collectively called as particulates. Respirable suspended particulates include ETS, Asbestos and other fibers.


Secondhand Smoke also known as environmental tobacco Smoke (ETS) is the smoke coming out from the burning end of a cigarette, cigar or pipe and the smoke exhaled by the smokers.

ETS is a mixture of over 4,000 compounds, of which over 40 are considered to be carcinogenic (Group A carcinogen in humans by the US EPA) to humans and animals. Passive smokers (non-smokers) are people who are exposed to the smoke by the combustion of tobacco products by the smokers.

ETS causes various heath problems like respiratory irritation like cough, excess phlegm, and wheeze. It is suspected to be a carcinogen. ETS is particularly harmful to young children and infants whose parents smoke in their presence. Based on survey conducted by the US EPA there are approximately 3,000 lung cancer deaths (EPA publication viz. EPA-43-F-93-003) each year in nonsmoking adults and impairs the respiratory health of hundreds of thousands of children. According to a study by the American lung Association (ALA) there are about 37,000 heart disease deaths in non-smokers each year. (for more Info. Click Here)

EPA estimates that about 150,000 to 300,000 passive smokers have lower respiratory infections mainly children and infants below the age of 18 months resulting in 7,500 to 15,000 hospitalizations each year.

ETS has health effects in children like increased risk of lower respiratory tract infections, such as bronchitis and pneumonia, and ear infections, build-up of fluid in the middle ear, increased severity and frequency of asthma episodes, decreased lung function etc.

EPA also estimates that about 200,000 to 1000,000 asthmatic children have worsened their condition due to this Secondhand Smoke (SS). Exposure to SS or ETS causes eye, nose, and throat irritation. It can affect the cardiovascular system and some studies have linked exposure to secondhand smoke with the onset of chest pain irrespective of the age group (but more in children). For more information about the EPA publications on ETS Click Here.



Asbestos is a mineral fiber widely used in variety of building materials and as an insulating material and also as a fire-retardant.

There are many varieties of asbestos used for various purposes they are:

ˇ  Chrysotile or White asbestos

   Most common asbestos mineral, made from fibrous variety of the mineral serpentine, a sheet silicate mineral. It is recognized by its long, wavy, flexible fibers.  

Crocidolite: Made from fibrous variety of Riebeckite, a mineral in the Amphibole group. Crocidolite is purple, and the fibers are short and rigid. 

Amosite: Made from Amphibole (Cummingtonite-Grunerite series), amosite has short, rigid, tan or whitish colored fibers. Actinolite: Actinolite is a relatively common mineral in some metamorphic rocks. Actinolite is composed of microscopically fibrous crystals (asbestos) and is called byssolite. It is usually green or white with a vitreous luster. (Click Here) 

Anthophyllite: Anthophyllite is a polymorph found in some metamorphic and metasomatic rocks. It usually has various shades of brown such as yellow-brown, green-brown or brownish-gray, but also green, off-white or gray with a vitreous to dull or silky luster in fibrous forms. (Click Here) 

Tremolite: Tremolite is a common mineral found in some metamorphic rocks. It is usually white or gray but can be greenish, colorless, yellow and violet with vitreous or silky to dull luster. More than 95% of all asbestos in building materials is Crysotile.

Who are exposed to Asbestos?

Largest exposures to asbestos were found in workers who mined, manufactured or worked with large amounts of asbestos, in ways that fibers were released into the air (e.g. due to cutting asbestos containing materials) and the workers did not wear any protection to keep from breathing in fibers.

Today workers are required to wear respiratory protection if there are even small amounts of asbestos in the workplace.

Exposure to asbestos in building materials is only possible if the materials are damaged in a way that allows fibers to escape into the air.

Health risks associated with Asbestos?

Exposure to asbestos has no immediate health effect but it can cause long-term risk of chest and abdominal cancers and lung diseases. A person is exposed to asbestos has symptoms after 20 to 30 years after the exposure.

Workers exposed to large concentrations of asbestos fibers in air were found to develop higher rates of asbestosis, Mesothelioma, and lung cancer than the general population.

Asbestosis is an irritation of the lung caused by inhalation of asbestos fibers. It results in decreased lung function, which in turn may cause heart failure and death.

Mesothelioma is a cancer of the lung lining or stomach lining. 80% of diffuse mesothelioma (malignancy) is linked to asbestos exposure.

Lung cancer -- some groups of workers exposed to high asbestos concentrations in air showed higher rates of lung cancer than found in the general population. Workers who were exposed to high asbestos concentrations and who smoke cigarettes have greatly increased risk of lung cancer.

The US Consumer Product Safety Commission (CPSC) has proposed a ban on the usage of asbestos on July 21, 1977.

CPSC also issued a safety alert about some materials containing asbestos, these include:    Asbestos paper and millboard    Asbestos-cement sheet
   Dry-mix asbestos furnace or boiler cement
   Asbestos wood/coal stove door gaskets
   Asbestos laboratory gloves and pads
   Asbestos stove mats and iron rests
   Central hot-air furnace duct connectors containing asbestos
   Bulk asbestos fibers
   Hair dryers etc

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The main pollutants released due to overcrowding of humans or animals are Bioaerosols. These originate mainly from humans, animals, plants, soil, debris etc and these include bacteria, molds, mildew, viruses, animal dander and cat saliva, house dust mites, cockroaches, and pollen. These are an integral part of the ecological system.

Most of the bioaerosols are present outdoor and are induced indoor either by natural or mechanical intake of the ventilation systems. Humidifiers, air-conditioning systems, cooling towers, mechanical ventilation systems, air-distribution ducts and areas of water damage are the best breeding places for these bioaerosols.

Moist places are the best places for them to breed such as leather, wood, carpets, soaps, cloth fabrics and some pastes and adhesives.

Pets are sources of saliva and dander. Humidity makes them increase their colonies. Hence, low humidity should be maintained inside the house to restrain their growth. Recommended EPA humidity level is 30 % - 50 %.

Bioaerosols trigger some allergic reactions including hypersensitivity pneumonitis, allergic rhinitis, and some types of asthma. Infections such as influenza, measles, and chicken pox are transmitted through the air.

Symptoms related to these bioaerosols include sneezing, coughing, short of breath, fever, dizziness etc.

Some specific diseases related to particular microorganisms are:

Viruses: influenza, German measles, mumps, chicken pox and shingles etc.

Bacteria: pneumonia, legionnaire’s disease, Otitis media, diphtheria, bronchitis, tuberculosis, rheumatic fever etc.

Fungi: histoplasmosis, cocciodomycosis and blastomycosis.

Antigens: allergic hypersensitivity, allergic asthma, allergic rhinitis and allergic aspergillosis.


Outside Sources

The pollutants due to the outdoor sources such as polluted air from industrial area, soil surrounding the residence, drinking water are, Heavy metals, Radon etc.


Radon (Rn) is a gaseous radioactive element that has atomic number 86, atomic weight of 222, melting point of – 71ēC, boiling point of – 62ēC, and 18 radioactive isotopes. The radon decay is shown in the Figure 3.

Radon is a colorless, odorless and radioactive short-lived gas. Radon in particular is not harmful but after its decay it produces progenies, which are of major concern.

During the decay of radon it releases radioactive particles and progeny, Po-218 and Po-214 tend to retain in the lungs ultimately causing cancer.

Main sources of radon are from the soil beneath and surrounding the building structure, water supplies and to lesser extent from building materials and natural gas used in heating purposes (such as cooking, heaters etc).

Radon present in soil moves slowly onto the surface depending on the permeability of the soil, porosity, water content, temperature and pressure difference between soil and building structure.

Radon is also present in fly ash released from the coal-fired power plant.

Health Effects:

Radon can attach itself to the aerosols thus entering into the respiratory tract.

The emitted radioactive isotopes during the decay of radon can damage the bronchial cells and hence radon is considered to be carcinogenic.

Radon is considered to be the second most leading cause of cancer in humans after ETS. Damage caused by radon depends on the strength, source cell geometry, the surrounding tissue and nature and mass of the cells and their distribution.

Heavy Metals

Main sources of these heavy metals are their concentration found outside due to several activities like mining, smelting, automobile exhaust etc.

Heavy metals are namely Lead (Pb), Mercury (Hg), Cadmium (Cd), Chromium (Cr) etc.

Heavy metals are highly dangerous to humans especially to children as they play in soil and the concentration of heavy metals is more in the soil and they are exposed to them.

Lead (Pb)

Lead is considered to be the number one threat to the health of children in the United States by the US EPA.

Sources of lead are air, drinking water, food, contaminated soil, deteriorating paint, and dust. Lead before it was considered a threat to health, was used extensively in paints, gasoline and many other products.

Lead affects  almost all the parts of the human body. At high concentrations it can cause convulsions and even death and at low concentrations it adversely affects the brain, central nervous system, blood cells and kidneys. It affects  children when playing  in soil keeping their hands or keeping objects in the mouth having high concentrations of lead . Lead based paints are the main causes of lead poisoning commonly found in children.

Lead causes several irreparable damages in children.

Mercury (Hg), Arsenic (As), Nickel (Ni), Cadmium (Cd)

The main source of mercury is primarily due to the paints using mercury. Broken thermometers and the use of silver-mercury amalgam in the dentist’s chamber can also be a source of the indoor concentration of mercury.

Hoses painted with latex paints had very high initial concentration of mercury vapors, which gradually drop to 20 % - 25 % within three to four months of the initial application of the paint.

Tobacco smoke, wood smoke and some pesticides are the source of arsenic, mercury and other heavy metals. Hence, their concentration depends on the number of cigarettes smoked and the frequency of smoking.

Mercury is generally a sensory irritant. Mercury is easily absorbed by skin and lungs, which cause rashes, skin burns, excessive perspiration and partial loss of hearing and also damage the cells of the kidney.

Cadmium has even more serious health effects such as damage to capillaries in kidneys and they interact with nutrients in liver.

Almost all the heavy metals are believed to be carcinogenic.

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