Indoor Air Pollutants: Main Sources

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Written By Jamila W.

Indoor air pollution is a concerning environmental issue with a high global prevalence. Large amounts of time spent indoors expose building occupants to significant levels of pollutants in room air, leading to acute and chronic health effects. Knowing the common indoor air pollutants and their effects is the first step to reducing household air pollution.

In this article, we share eight common indoor air pollutants, their sources, permissible exposure limits, and their effects on human health. 

8 of The Leading Indoor Air Pollutants 


These eight common indoor air contaminants affect indoor air quality, necessitating routine measurement as part of an air quality assessment. Many of these pollutants have permissible exposure limits but some have no safe level of exposure. Here are the key classes of pollutants causing health risks in indoor air:

1. Biological Air Contaminants 

What are biological air contaminants?

Biological contaminants are biological agents and microorganisms that are suspended in indoor air that can be inhaled, causing poor indoor air quality and health problems. They include

  • Bacteria
  • Viruses
  • Mold spores
  • Pollen
  • Dust mites and their eggs
  • Pet dander
  • Dried rat urine
  • Pests

Sources of biological air contaminants

The sources of biological contaminants are diverse. They include people with bacterial or viral respiratory infections who spread aerosolized respiratory droplets through coughing and sneezing. Other sources are pest infestations, warm, humid conditions, and pets. 

How are biological contaminants detected in indoor air?

Bioaerosols like viruses, bacteria, and fungi are detected in indoor air using pumped sampling techniques that include filters, liquid, and bubble samplers.

Permissible Exposure Limits

The diversity of biological contaminants means that there are no specific guidelines for safe levels. During the COVID-19 pandemic, the Environmental Protection Agency issued guidance that highlighted the role of increased ventilation and air filtration in reducing droplet spread. 

Health effects 

The health effects of biologicals include respiratory irritation and allergies, respiratory infections, and the development of lung disease. 

2. Carbon Monoxide (CO)

What is Carbon monoxide?

Carbon monoxide is a colorless, odorless gas that is a by-product of combustion. CO is extremely dangerous because its gas molecules actively bind free oxygen to become carbon dioxide. If inhaled, carbon monoxide causes asphyxiation by diminishing the availability of oxygen in the blood for respiration. 

Sources of carbon monoxide

In buildings, incomplete combustion of natural gas by faulty gas stoves and furnaces increases CO levels. CO can accumulate in blocked flues of gas appliances and re-enter room air. Environmental tobacco smoke also contains CO. 

How is carbon monoxide detected in indoor air?

Air quality assessment technicians, facilities managers, and homeowners detect raised levels of carbon monoxide in indoor air by using electrochemical sensors that respond to elevated concentrations of CO. Natural gas combustion appliances should be checked and serviced annually.

Permissible Exposure Limits

The Occupational Safety and Health Administration (OSHA) specifies a permissible exposure limit for carbon monoxide of 55 milligrams of CO per cubic meter or 50 parts per million in eight hours. 

Health effects 

An undetected increase in carbon monoxide concentration can quickly prove lethal. Even at low levels, CO causes fatigue. As concentrations rise, symptoms of carbon monoxide poisoning include dizziness, confusion, chest pain, and flu symptoms. Sustained raised concentrations lead to loss of consciousness and death. 

3. Volatile Organic Chemicals (VOCs)

What are volatile organic chemicals?

Volatile organic chemicals are gasses emitted from a wide range of solid or liquid substances. They often have distinctive odors or fragrances and are found in a wide range of manufactured products. Examples of VOCs include

  • Formaldehyde
  • Benzene
  • Methylene chloride
  • Ethylene glycol 
  • Toluene
  • 1,3-butadiene

Sources of VOCs

VOCs are present in materials that are used in building fabrication and decor. Petrochemical products including fuel also contain these volatile agents. This enables concentrations of VOCs to build up in indoor air as they are off-gassed from paints, varnishes, lacquers, laminates, air fresheners, and synthetic furnishings. 

How are VOCs detected in indoor air?

There are several diffusion-based methods available for detecting VOCs including diffusion tubes, adsorbent tubes, and wearable badges. Colorimetric indicator sampling and Near-direct Infra-red Spectroscopy (NIRS) are also used.

Permissible Exposure Limits

The Occupational Safety and Health Administration (OSHA) specifies a permissible exposure limit for VOCs of 0.75 parts per million. At concentrations above 0.5 parts per million action should be taken to identify the source of emissions and reduce levels.

Health effects 

Inhaled VOCs cause symptoms that include:

  • Respiratory irritation 
  • Eye irritation 
  • Headaches
  • Skin allergies
  • Nausea
  • Fatigue

Long-term exposure to many VOCs is associated with liver and kidney damage, and cancer. 

4. Asbestos

What is asbestos?

Asbestos is a fibrous silicate mineral that releases fine airborne fibers that can penetrate the lungs, causing chronic inflammation and cancer. It is extremely dangerous to human health. 

Sources of asbestos

Asbestos products were widely manufactured until their health effects were recognized. The US banned the production and sale of new asbestos products in 1989. Potential sources of asbestos in indoor spaces include: 

  • Building insulation
  • Roof tiles 
  • Heat resistant materials
  • Ceiling tiles

How is asbestos detected in indoor air?

Asbestos is detected in indoor air using pumped sampling techniques that use a filter to trap fibers that can then be measured relative to the volume of the room being assessed.

Permissible Exposure Limits

Asbestos is still present in many older public buildings and workspaces. The permissible exposure limits for asbestos are set by the Occupational Safety and Health Administration (OSHA) rather than the Environmental Protection Agency. It recommends an exposure level of no more than 0.1 fibers per cubic centimeter of air in eight hours. 

Health effects 

Asbestos is responsible for asbestosis, a condition characterized by chronic inflammation and scarring of the lung. This progressive respiratory disease is life-shortening and can also be complicated by lung cancer and tumors of the lung lining (mesothelioma). 

5. Particulate Matter

What is Particulate Matter?

Particulate Matter (PM) are microscopic particles that are suspended in air. They can be solid or liquid and come from natural or man-made sources. PM particles are classified according to particle diameter:

  • PM10 particles have a diameter of 10 micrometers
  • PM2.5 particles have a diameter of 2.5 micrometers 
  • PM1 particles have a diameter of 1 micrometer

The small size of these particulates makes them easy to inhale, with deep penetration of the airways and acute and long-term health effects described below. 

Sources of particulate matter

Particulates are generated through a variety of activities including cooking, smoking, and heating. The use of solid biofuels in developing countries is a notable source of PMs and household air pollution. In addition, vehicular traffic, building construction, and atmospheric reactions between sulfur dioxide and nitrogen dioxide. Poor ventilation, humidity, and raised temperatures increase the indoor concentrations of PMs. 

How is particulate matter detected in indoor air?

Particulate matter of various diameters can be detected using:

  • Optical particle counting
  • Filters
  • Near-direct Infra-red Spectroscopy (NIRS)

Permissible Exposure Limits

There is no truly safe level of PMs. The Environmental Protection Agency specifies an air quality index standard for PM2.5 particles of 9.0 to 10.0 micrograms per cubic meter. 

Health effects 

Particulate matter has been classified by the World Health Organization as a Group 1 carcinogen, meaning that there is evidence that PM exposure causes cancer. Other serious health complications caused by PM include:

  • Increased susceptibility to respiratory infections 
  • Heart disease
  • Developmental delay
  • Chronic obstructive pulmonary disease
  • Low birth weight
  • Dementia
  • Diabetes
  • Lung disease
  • Premature death

6. Nitrogen Dioxide (NO2)

What is nitrogen dioxide?

Nitrogen dioxide is a noxious gas that causes serious health problems and even death if inhaled in high enough concentrations. Most NO2 is generated in the stratosphere, but several indoor activities can generate this atmospheric pollutant within homes. NO2  can easily build up in indoor air if ventilation is poor. 

Sources of nitrogen dioxide

In buildings, NO2 is generated by combustion activities like cooking, heating, lighting (from oil or kerosene lamps), and smoking. Unvented or poorly vented stoves and furnaces are at greater risk of releasing NO2 into room air. Nitrogen dioxide exposure is also a complication of house fires.

How is nitrogen dioxide detected in indoor air?

Nitrogen dioxide is routinely measured as part of air quality assessment using diffusion tubes, sorbent tubes, and gas analyzers. 

Permissible Exposure Limits

According to the Environmental Protection Agency, there are currently no air quality standards for levels of NO2 in indoor air. However, the national annual exposure limit for NO2 ambient air is 0.53 parts per million.

Health effects 

NO2 is a toxic and irritating substance. It irritates the eyes, nose, throat, and airways. Chronic low-level exposure to nitrogen dioxide causes health problems that include asthma, acute bronchitis, and a predisposition to respiratory infections. High-dose exposure is associated with serious lung injuries and pulmonary edema.

7. Radon (Rn)

What is radon?

Radon is a colorless, odorless radioactive gas that enters and accumulates in indoor spaces. It is a degradation product of uranium which is present in the underground rocks beneath buildings. It seeps into soils and then buildings where it can be breathed by building occupants raising their risk of cancer. 

Sources of radon

Radon emissions are present all over the world. Different types of bedrock vary in the amount of radon they emit. Granite, shale rock, and uranium-enriched phosphatic rocks are recognized sources of radon. The metamorphic rock of the Rocky Mountains contains high levels of uranium which releases radon into regional indoor air and water supplies. 

How is radon detected in indoor air?

Radon can be detected and measured in indoor environments using sorption badge sampling devices, activated charcoal detectors (ACDs), and other passive sampling devices.

Permissible Exposure Limits

There is no known concentration of radon in indoor environments that is recognized as safe. The Environmental Protection Agency advises that properties should undergo remedial work to limit indoor radon levels if levels are between 2 and 4 picocuries per liter

Health effects 

Radon exposure is a leading cause of lung cancer. According to World Health Organization estimates, radon is responsible for up to 14% of all lung cancers. Radon exposure has a synergistic relationship with smoking, significantly raising the lung cancer risk for smokers. 

8. Environmental Tobacco Smoke (ETS)

What is environmental tobacco smoke?

Environmental tobacco smoke is secondhand smoke. Smoking releases over 4000 combustion products, hazardous volatile agents, and harmful particulates into room air. ETS not only harms the smoker but also affects the health of passive smokers who are in their vicinity, especially if the exposure is repeated. 

Sources of environmental tobacco smoke

Environmental tobacco smoke is generated at all stages of burning tobacco. Cigarettes and pipes release these pollutants as they are smoked. Even after a smoker has finished smoking, they continue to exhale smoke and carry particulates on the clothing that can become suspended in indoor air causing household air pollution. 

How is environmental tobacco smoke detected in indoor air?

  • A comprehensive indoor air quality assessment measures environmental tobacco smoke using fixed-location monitoring survey instruments
  • Direct personal monitoring, using wearable pumps and filters 

Permissible Exposure Limits

There is no official guidance for safe exposure limits for environmental tobacco smoke. However, academic studies have shown that benzene, a common marker for ETS, is detected at raised levels of between 16 and 193 micrograms per cubic meter where active smoking is taking place. The widely accepted limit for benzene in indoor air is 5 micrograms per cubic meter. The World Health Organization strongly discourages indoor smoking to prevent household air pollution.

Health effects 

Environmental tobacco smoke causes acute eye, nose, and respiratory tract irritation. Sustained exposure to ETS (passive smoking) is associated with the development of asthma, chronic obstructive pulmonary disease, cardiovascular disease, and lung cancer. 

Control Indoor Pollution to Improve Air Quality

erson smeilling a flower

The serious health consequences of exposure to these indoor air pollutants mean that their levels need to be controlled to protect human health. There are several strategies for reducing household air pollution that is effective in buildings of any type:

  1. Identifying and eliminating air pollutant sources:  Removing the source of air pollution from indoor environments is the most effective way to improve indoor air quality. Sources of pollutants can be sealed to prevent emissions if they cannot be completely removed and fault appliances should be checked and repaired.
  2. Improving ventilation: Increasing natural and mechanical ventilation improves airflow and introduces outdoor air that can dilute concentrations of indoor air pollutants. 
  3. Reducing humidity: High humidity in buildings reduces air movement, allowing pollutants to concentrate in indoor spaces. Reducing sources of humidity like drying laundry indoors, cooking, and damp improve air quality and prevent mold growth.
  4. Regulating indoor temperature: Using a thermostat to regulate temperature prevents high indoor temperatures that potentiate the hazardous effects of air pollutants. 
  5. Using air cleaners and air purifiers: Air cleaners and purifiers contain high-efficiency HEPA and MERV 13+ filters that can strip particulate pollutants and biologicals from room air.
  6. Installing high-efficiency HVAC air filters: HEPA filters can also be installed in HVAC systems to strip outdoor air of pollutants before it is circulated within buildings.

In conclusion

window, nature

Room air is contaminated by a wide range of indoor pollutants that have extensive health effects. Indoor air quality assessments can detect and measure common indoor air pollutants so that harmful indoor air can be remedied.  Prevention and control of household air pollution exposure can be achieved through a combination of improving ventilation, using air cleaners and filters, reducing or eliminating sources of pollution, and regular maintenance of appliances. By taking these steps, property owners and building managers can ensure that indoor air is clean and healthy to breathe.