Indoor and Outdoor Air Pollution | An Introduction

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Written By Jamila W.

Air pollution is one of the world’s most important public and environmental health issues. Though awareness and understanding of the causes of pollutants in indoor and outdoor air have increased in the last 50 years, the damaging effects of airborne contaminants in ambient air and indoor spaces have continued to drive rising levels of human disease and environmental damage. In this article, we take a closer look at:

  • What indoor and outdoor air pollution are
  • The main pollutants and their sources
  • The health effects of these forms of health pollution
  •  What can be done to mitigate the effects of air pollution

What Is Indoor Air Pollution?

indoor air pollution

Indoor air pollution (IAP) is the deterioration of indoor air quality (IAQ) due to the presence of elevated levels of gaseous, particulate, and biological pollutants. The Environmental Protection Agency defines indoor air quality as the quality of air inside buildings and structures. The presence of aerosolized, suspended, or emitted air pollutants degrade air quality and can affect the health and comfort of building occupants. 

IAP is an important public health issue

Indoor air pollution is a challenging environmental health issue, as many noxious air pollutants are colorless and odorless. Contained spaces cause these pollutants to build up and become concentrated inside rooms. This means that concentrations of pollutants in indoor air can be up to 10 times more concentrated than outside air. As people spend the majority of their time indoors, they experience prolonged exposure to these harmful substances.

Indoor air pollution comes from diverse sources

Left unchecked, indoor air quality affects the general indoor environment quality (IEQ). Indoor air pollutants can come from a variety of natural and synthetic sources within and outside a building. Indoor air pollution is heterogenous, with the strength of persistence of various pollutants changing over time. 

What Is Outdoor Air Pollution?

air pollution

Outdoor air pollution, atmospheric pollution, ambient air pollution, or air pollution is the persistent contamination of air by pollutants that harm humans and the environment. Rather than being confined to an indoor space, concentrations of these pollutants build up in the atmosphere causing wide-ranging effects. 

Natural and man-made pollution of ambient air

Outdoor air pollution is caused by naturally occurring and synthetic pollutants that are largely released because of human (anthropogenic) activity. Natural phenomena like volcanic eruptions and wildfires are also sources of outdoor air pollution. The pollutants involved include glasses, particulates, and microbial agents. 

An Environmental Emergency

Multiple substances now persist at unnatural concentrations in outdoor air. This impacts atmospheric composition with the potential to affect climate by trapping heat in the atmosphere. Though the types and concentrations of air contaminants may vary from place to place, the impact of outdoor air pollution on the environment is global. 


Every nation on Earth is feeling the effects of the general deterioration in outdoor air quality making it a global issue.  The accelerated urbanization and industrialization of many low- and middle-income countries have also led to disproportionate morbidity and mortality from air pollution in those countries, challenging their economic development.  

Key Sources of Indoor Air Pollution

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Indoor air pollution consists of a range of internally and externally generated air pollutants that build up in room air. Pollutants can enter internal spaces in buildings through ventilation points, or gaps in the fabric of the building. Air contaminants are also released from appliances, building materials, furnishing, and human activity like smoking. Here is a list of the main indoor air pollutants and their sources:

Indoor air pollutantSource
Nitrogen dioxide (NO21. Atmospheric formation from Nitric oxide (NO)
2. Fuel combustion: gasoline, diesel, wood, natural gas
3. Space heaters, stoves, furnaces
Radon1. Radioactive gas release through uranium degradation in rocks and soils
2. Entry into buildings through gaps in their structure
Particulate Matter (PM1, PM2.5, PM10)1. Combustion
2. Cooking
3. Smoking 
4. Road traffic
5. Clothing
6. Carpets
7. Soft furnishing
8. Burning candles 
Volatile Organic Compounds (VOCs)1. Building materials
2. Paints and adhesives
3. Sealants
4. Furnishings and interior decor
5. Cleaning chemicals
5. Air fresheners
6. Perfumes and toiletries
Formaldehyde (HCHO/CH2O)1. Secondhand smoke
2. Household chemicals
3. Building materials
4. Gas stoves
5. Dry cleaning
6. Kerosene heaters
Phthalates1. Plastic bags
2. Children’s toys
3. PVC flooring 
Carbon monoxide (CO)1. Inefficient combustion of fuel-burning appliances
2. Blocked exhaust flues
3. Secondhand smoke
Mold1. Damp, poorly ventilated environments
2. Indoor spaces with high humidity
Bacteria and viruses1. Overcrowded indoor environments
Ozone1. Emissions from electronic devices including photocopiers, and printers. 
2. Ozone-based air sanitizers and disinfecting equipment
3. Ingress of ozone present in outdoor air

Important Sources of Outdoor Air Pollution

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There is a significant overlap in the pollutants present in indoor and outdoor air pollution. However, the sources of outdoor pollutants are much more diverse with a significant contribution from anthropogenic sources. Here is a list of the main outdoor air pollutants and their sources:

Indoor air pollutantSource
Nitrogen dioxide (NO21. Natural formation from lightning, volcanic activity, and bacterial respiration (nitrogen fixing)
2. Human activity including internal combustion engines, traffic, smoking, grain silos, industry
Sulfur dioxide (NO21. Natural emissions from volcanic activity
2. Human activity including the combustion of fossil fuels (coal in particular), vehicle exhaust, industrial activity
Particulate Matter (PM2.5, PM10)1. Construction
2. Road traffic
3. Dirt roads
4. Wildfires
5. Volcanic activity 
6. Reactions between chemicals including nitrogen and sulfur dioxide
7. Industry exhaust
8. Waste to Energy plants
9. Power stations
Volatile Organic Compounds (VOCs)1. Burning gasoline, diesel, coal, and natural gas
2. Vehicle exhaust
3. Waste incineration
4. Industrial activity 
Lead and heavy metals 1. Gasoline
2. Cigarette smoke
3. Industry
4. Building materials
5. Gas stoves
5. Kerosene heaters
Toxic Organic Micropollutants1. Pesticides
2. Herbicides
3. Fertilizers
4. Industry
Carbon monoxide (CO)1. Fossil fuel combustion
2. Vehicular traffic
Ozone1. Vehicle emissions
2. Industrial emissions
3. Natural formation in the stratosphere
Microorganisms1. Algal bloom
2. Bioaerosols
Methane1. The oil and gas industry
2. Animal husbandry
3. Wastewater treatment
4. Landfills

Indoor vs Outdoor Air Pollution

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Though indoor and outdoor air pollution are both concerned with air contaminants, there are significant differences between these two forms of pollution. Outdoor air pollution is by far the more extensive problem, affecting the entire world due to the sustained emission of pollutants from the burning of fossil fuels.

Outdoor air pollution contributes to indoor air pollution through the passive or active movement of pollutants into outdoor air. Once pollutants enter an indoor space, reduced airflow, inadequate ventilation, humidity, and high temperatures increase their indoor concentrations, with levels that are routinely several times higher than ambient air

Air quality inside buildings is affected by outdoor air quality, but in addition, indoor environments can be contaminated by a wide range of gasses that are emitted from the building structure and materials, decor, furnishing, and appliances. Pollutant emissions are now thought to be the cause of sick building syndrome where certain indoor environments trigger a variety of non-specific health symptoms. Low ventilation, high temperatures, and humidity can decrease indoor air quality and potentiate the effect of pollutants. Indoor air pollution is as important as outdoor air pollution because of the amount of time the average person spends indoors (90%). Sustained exposure to indoor air pollutants therefore significantly raises the risk of a variety of health problems, reflected in the significant morbidity and mortality attributed.

Health Effects of Indoor and Outdoor Air Pollution

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Indoor air quality is increasingly recognized as a global public health problem. According to the World Health Organization (WHO), household air pollution was a contributing factor in the premature deaths of more than 4.2 million people worldwide in 2019. In addition over 270,000 infants and young children die from respiratory diseases that have been worsened by poor indoor air quality. 

Developing nations’ health burden from air pollution

The health hazards of indoor air pollution disproportionately affect people in low-income countries who are reliant on the most polluting solid fuel sources and use inefficient equipment for heating and cooking. In wealthier countries, building insulation and materials in modern properties are a key source of emissions of health-harming gasses. 

Varied health effects of outdoor air pollution

Outdoor air pollution also has wide-ranging short and long-term effects. People who live in regions that have poor ambient air quality report a diminishment of their quality of life and well-being in addition to the numerous health risks associated with gasses and particulate air pollution. Urban areas are particularly affected by many pollutants emitted by vehicles on congested roadways. 

Here are some of the main effects of indoor and outdoor air pollution: 

  • Respiratory disease: IAP has irritant acute respiratory effects like chest tightness, wheezing, and coughing. Longer-term adverse health effects due to sustained exposure to fine particulates include chronic lung diseases like asthma, bronchitis, Chronic Obstructive Pulmonary Disease (COPD), pneumonia, and lung cancer.
  • Cardiovascular effects: The EPA reports that sustained exposure to air pollution is leading to rising hospital admissions due to cardiovascular problems that include acute coronary syndrome, hypertension, and strokes. Cardiovascular and respiratory disease are linked, with breathing problems worsening the symptoms of heart disease.
  • Neurological and psychiatric effects: The effects of inhaled air pollutants on the central nervous system have only recently been recognized. It is now acknowledged that living in environments that have high levels of indoor or outdoor air pollution increases the risk of developing neurological and psychiatric conditions that include dementia, depression, anxiety, schizophrenia, and attention deficit hyperactivity disorder (ADHD).
  • Dermatological effects: Many organic, inorganic, and biological air pollutants are irritants that can produce allergies in exposed individuals. Indoor and outdoor air pollution aggravates skin conditions like eczema, psoriasis, and acne. 
  • Endocrine effects: A specific class of air contaminants known as endocrine-disrupting chemicals (EDCs) can disrupt hormonal signaling in the body. Substances like brominated and organophosphate flame retardants, polycyclic aromatic hydrocarbons, cadmium, lead, and polychlorinated biphenyls can affect the growth, development, and fertility of humans who are exposed to them. 

Prevention and Control of Indoor and Outdoor Air Pollution

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Preventing and controlling air pollution is a priority for every country in the world. Without concerted action to tackle indoor and outdoor air pollutants their pervasive health and environmental impacts will persist, harming all living things. Approaches to tackling both forms of air pollution require:

Here are some of the key strategies and methods for reducing outdoor and indoor contaminants:

Indoor Air Pollution Prevention and Control

Mitigation strategies for indoor environmental pollutants start with air quality testing and monitoring of levels of common indoor air pollutants. Then the source of pollutants or gaseous emissions can be identified and removed or sealed to improve air quality. Strategies for controlling indoor air pollution include: 

Increasing natural and mechanical ventilation: This increases airflow and dilutes indoor pollutants with fresh air drawn in from the outdoors. 

Reducing humidity and temperatures: High humidity and temperature stop air movement allowing pollutants to accumulate in room air.

Regular cleaning: Cleaning of floors, carpets, and upholstery can help remove dust and other pollutants from indoor air. Air conditioning ducts should also be regularly cleaned.

Reducing the use of household chemicals: Using natural products such as vinegar and baking soda for cleaning can help reduce the use of harmful chemicals that can contribute to indoor air pollution.

Choosing low-VOC paints and interior decor: Recognizing the negative effects of VOCs, paint, adhesive, and interior decor manufacturers are increasingly producing low-VOC products that do not off-gas harmful volatile agents.

Upgrading HVAC filters: Building managers can improve indoor air quality by installing sealed MERV 13 or HEPA air conditioning filters that can strip small particle pollution from the air that enters or moves through a building. 

Using an air cleaner: Air cleaners are powered devices that move indoor air through a series of filters, stripping the air of particulates, biological agents, and allergens. 

Outdoor Air Pollution Prevention and Control

Strategies for reducing outdoor air pollution target emissions from industry, vehicle traffic, and the burning of fossil fuels for energy. Over the last 20 years, ambient air quality testing has helped municipal authorities understand the scale of the air pollution problem. Methods used to reduce exposure to outdoor air pollutants include: 

Implementing industrial air scrubbing: Exhaust air generated by industry can be passed through a variety of air cleaners and air scrubbers that filter and strip the air of dust, ash, and particulate pollutants. 

Reducing vehicle use: Reducing vehicle use can help reduce outdoor air pollution. Municipal authorities worldwide are promoting the use of public transportation, electric vehicles, carpooling, or biking to reduce emissions from vehicles.

Planting trees and vegetation: Planting trees and vegetation can help absorb pollutants and improve air quality.

Opting for clean energy sources: Many governments are promoting clean energy sources such as solar, wind, or hydropower. These renewable energy sources do not directly involve the combustion of wood, waste, or fossil fuels to generate energy. 

Proper waste management: Open-air burning of waste is a significant source of air pollution in the developing world. This practice can be stopped by supporting countries to invest in proper municipal waste management infrastructure.

In Conclusion

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Indoor and outdoor air pollution is a major public health issue. Tackling the sources of both types of air pollution can significantly improve human health and well-being. Being proactive in measuring indoor and outdoor air quality and implementing varied improvement strategies can help to bring this widespread environmental problem under control.