A Guide to Indoor Air Quality

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Written By Jamila W.
Air pollution

Indoor air quality is an international public health issue. Room air can be polluted with chemicals, exhaust fumes, and infectious biological agents that compromise human health just as much as outdoor air pollution. 

The United Nations estimates that indoor air pollution causes the death of over 3.2 million people each year, through the development of cardiovascular disease and diverse respiratory diseases. In response, the World Health Organization and other agencies have addressed this environmental health issue by devising Indoor Air Quality (IAQ) standards to minimize the levels of hazardous substances in indoor air. 

In this guide to indoor air quality,  we explain:

  • What indoor air quality is
  • The factors that affect the quality of indoor air
  • Why air quality in buildings matters
  • How indoor air quality is measured
  • The effects of indoor air pollution
  • How to improve room air quality

In addition, we’ll examine the role of air cleaners in maintaining optimal indoor air quality in a variety of settings. 

Understanding Indoor Air Quality

modern office

What is Indoor Air Quality?

The United States Environmental Protection Agency (EPA) defines indoor air quality (IAQ) as the qualitatively or quantitatively measured air quality within or in proximity to buildings or other structures that can be inhabited. IAQ is primarily concerned with the levels of certain particles and gasses in the air. A variety of indoor air pollutants can compromise IAQ, affecting the short and long-term health and well-being of building occupants who are exposed over long or repeated periods in an indoor environment. 

Common indoor air pollutants

The types of indoor air pollutants affecting air quality are determined by the building or indoor setting. An industrial facility will have a very different set of indoor pollutants, to a domestic kitchen, or boiler room. Common examples of organic and inorganic indoor air pollution include:

Secondhand smoke

Exhaled tobacco smoke from an active smoker contains gasses like carbon monoxide (CO) and fine particulates that affect anyone in the environment of the smoker. Inhalation of secondhand smoke increases an individual’s risk of respiratory diseases, including lung cancer.


Secondhand e-cig or vaping vapor is an aerosol that can be passively inhaled by anyone in its vicinity. Using e-cigarettes in enclosed spaces increases the levels of harmful fine and ultrafine particles in indoor air in a similar way to conventional tobacco cigarettes. The health effects of the nicotine that e-cigs contain are particularly severe in children and young people.


Inhaled asbestos fibers, released from degrading insulation and other materials produce specific health problems. This fibrous silicate material was extensively used in construction, building insulation, and fireproofing until the 1980s when its health effects were acknowledged. Long-term exposure to asbestos causes progressive respiratory diseases including the inflammatory lung disease asbestosis, and the development of mesothelioma tumors on the lining of the lung.

Particulates from the combustion of solid fuel

In the developing world, the inefficient combustion of solid fuel that is used for heating or cooking is a leading cause of household air pollution. This environmental health problem causes high morbidity and mortality through the inhalation of pollutants that include soot (carbon), nitrogen and sulfur dioxide, and CO.  The disease burden from burning wet wood, dung, charcoal, crop waste, and kerosene is borne by women and children who have the greatest exposure to high indoor pollutant levels. 

Radon gas

Radon is a colorless and odorless radioactive gas that is released by the breakdown of radium in rock formations and certain building materials. This cancer-causing gas can enter and accumulate in buildings to produce specific health problems such as the development of lung cancer. 

Mold spores

Warm, humid, and poorly ventilated indoor spaces promote the growth of mold. Spores that have been generated by this form of fungal growth can cause respiratory allergies and produce toxins that have health effects. 


Ozone is a harmful gas present in air pollution that can enter and accumulate in indoor spaces with inadequate ventilation. Ozone can increase indoor pollutant levels in urban areas, where ozone levels are often high in outdoor air. It is usually generated by photochemical reactions of nitrogen oxides (NOx) and volatile organic compounds (VOCs) in outdoor air. Excessive exposure to ozone is associated with cardiorespiratory disease and cancer.

Volatile Organic Compounds (VOCs)

These volatile chemical pollutants are released by a variety of materials and liquids including paints, building materials, household cleaning products, air fresheners, and furnishings. VOCs include formaldehyde and Polycyclic aromatic hydrocarbons (PAHs). Concentrations of VOCs build up in indoor settings where they can cause respiratory, sensory, and immune system effects.

Bacteria and viruses 

Bacteria and viruses that have health effects can become aerosolized through the exhalation, sneezing, and coughing of infected individuals. Airborne viruses and infectious microorganisms are a particular concern in densely populated indoor environments where communicable diseases can spread quickly. 

Sources of Indoor Air Pollution


A wide range of sources release particulate or gaseous indoor air pollution into room air. These sources of indoor air pollution are the main cause of poor air quality. They include:

  • An improperly adjusted gas stove, furnace, or fireplace
  • Cigarettes and other tobacco products 
  • Paint, plaster, and other building materials
  • Household cleaning products
  • Aging building materials
  • Poorly maintained outdoor vented fans
  • Flooring, furniture, and upholstery
  • Residential heating, ventilation, and air conditioning equipment
  • External sources of ozone, and radon
  • Individuals who are infected with a communicable respiratory infection

Factors Affecting Indoor Air Quality

building with aircon

How much pollution is emitted by a source

Sources of indoor air pollution vary in the type and level of their emissions. For example, the age, condition, and maintenance of a heating furnace or cooking stove may determine the levels of CO it generates.

The level of ventilation

Mechanical and natural ventilation introduces outdoor air into an enclosed space increasing air movement. This is vital for carrying indoor air pollutants and water vapor out of buildings to regulate the quality of internal air.[1] Too little outdoor air allows indoor air pollution to build up and natural ventilation may be inadequate to keep air circulating so that indoor pollutant levels fall.

Air Temperature differences

Air temperature differences affect the comfort and perception of air quality by building occupants. In addition, high temperatures partnered with humidity can promote the survival of airborne viruses and other infectious microorganisms.[2]


High humidity in indoor environments potentiates the absolute levels of these pollutants in indoor air by decreasing air circulation. Moist environments are also breeding grounds for mold, bacteria, and viruses.

Persistence of pollutants in indoor air

Gasses and particulates in indoor air vary in the amount of time that they remain in the air. Pollutants that persist in room air over long or repeated periods increase indoor pollutant levels, leading to greater exposure and risk to building occupants. 

Sustained release vs intermittent release

The release of air pollutants may be related to activities like smoking, cooking, or cleaning, or sustained through emissions from furnishings or building materials. 

How is Indoor Air Quality measured?

Objective measurement of IAQ is necessary to monitor levels of hazardous pollutants that can harm human health. By measuring IAQ, poor air quality can be remedied and optimized to protect health in any type of building or setting. 

A range of techniques and technologies are used to monitor the levels of air contaminants and indoor air conditions like humidity and temperature. Homeowners, landlords, and facilities managers can use air quality measurements to ensure that IAQ complies with regional or national guidance. [3]

Indoor Air Quality Sensors are used to measure IAQ

The primary method of measuring IAQ is the use of indoor air quality sensors. These sensors can detect and measure the levels of physical, chemical, and mechanical stimuli caused by the presence of contaminants in room air. 

Types of indoor air quality sensor

Each class of IAQ sensors detects a specific type of pollutant in indoor air. [4]

  • Gaseous sensors detect gaseous pollutants like:
    • Carbon monoxide
    • Radon
    • Ozone
    • VOCs
    • Formaldehyde
    • Methane
  • Particulate Matter sensors  are configured to measure levels of solid particles and liquid droplets that include:
    • Smoke
    • Dust
    • Pollen
    • Particulate matter (PM) 
    • Bioaerosols
  • Ionizing radiation sensors (for radiation sources and radon gas detection and measurement)

How do indoor air quality sensors work?

Targeted air sensor technology converts sensor stimuli produced by the presence of a contaminant into an electronic signal that can be used to quantify the average pollutant concentrations found in the sensor’s environment. 

In addition, IAQ readings can be coordinated with temperature and humidity levels for more detailed insight. Average pollutant concentrations found by smart IAQ sensors can be used to regulate the environmental conditions of buildings using software-based platforms like Building Management Systems (BMS) with automated adjustments in building ventilation or heating to optimize IAQ. 

Consumer air quality sensors

In recent years, IAQ sensor technology has reduced in size, and cost, and become accessible to the consumer markets. Low–cost air quality monitors usually detect, monitor and report on a limited number of air pollutants. Examples are the smoke and CO detectors that are required by law to be installed in public and rented properties. 

As public awareness of the importance of IAQ has increased, consumer air quality sensors have become more sophisticated, detecting a wider range of contaminants. Homeowners can use the IAQ readings generated by these devices to take action to improve IAQ in their environment. However, the consumer product safety commission and other regulators advise that these devices are not always accurate, especially if readings are isolated.

Sensor placement is critical for accurate IAQ monitoring

For accurate IAQ measurement,  multiple smart indoor air quality sensors need to be strategically positioned throughout a room or building. A single sensor is inadequate for reliable readings.  The EPA has produced guidance on the implementation of IAQ monitoring in large buildings. Key directives include the placement of at least one sensor for every 10,000 square feet of space. Other important aspects of setting up IAQ sensors include:

  • Placement of sensors in communal areas of a building
  • Locating sensors away from:
    • Floor fans
    • Heaters
    • Direct sunlight
    • Furnaces and HVAC equipment
    • Induction units
  • Wall-mounting sensors at varying heights rather than attachment to the floor or ceiling 
  • Keeping sensors at the height at which people are active in the building (standing, sitting, or sleeping)

IAQ standards 

The Environmental Protection Agency (EPA) and the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) have worked together to develop IAQ standards for public and commercial buildings. The EPA undertook extensive monitoring of IAQ in large buildings in conjunction with the Office of Radiation and Indoor Air, and the Office of Research and Development. 

The consensus opinion of the EPA and international environmental health bodies including the European Environment Agency (EEA) is that public and private buildings should have IAQ monitored with smart IAQ monitors. The Environmental Protection Agency has published directives specifying indoor pollutant levels for the most common pollutants of indoor air:

Indoor Air Pollutant Threshold Limit Value (TLV)
PM 2.525 μg/m3 in 24 hours 
PM 10150 µg/m3 in 24 hours
Carbon Monoxide (CO)35 ppm25 ppm per 8-hour workday
RadonNo safe level of exposure. Action to be taken if levels exceed 4 pCi/L
Formaldehyde0.1 ppm
Ethanol1,000 ppm
PAHs10 ppm
Benzene 0.1 ppm
Ozone 0.070 ppm per 8-hour period
NO2100 ppb
Methylene chloride250 ppm

Effects of Poor Indoor Air Quality


Government agencies prioritize the monitoring of IAQ, because of a wider understanding that sustained exposure to contaminated room air can pose health risks, and compromise the well-being, and productivity of building occupants. The study of indoor and outdoor air pollution quality has impacted knowledge and innovation in HVAC and Building ecology. In particular, there is an increased understanding that the internal environment of buildings is not necessarily stable over time and the internal conditions of buildings can lead to the release of pollutants that pose health risks to occupants (sick building syndrome).

Health effects of poor room air quality

Exposure to contaminated indoor air can have short, medium, and long-term effects. Many of the symptoms experienced by occupants of buildings with low IAQ are non-specific and may not be immediately connected to the building they live or work in. Health effects include:

  • Headaches
  • Flu-like symptoms 
  • Dizziness
  • Coughing
  • Eye, nose, throat, and skin irritation 
  • Fatigue
  • Allergies
  • Impaired cognition
  • Personality changes

The immediate effects of indoor air pollutants often subside when the affected person leaves the building. Symptoms may make it difficult to carry out a job while in a building with low IAQ. If harmful microorganisms are present, poor IAQ and inadequate ventilation, may lead to the spread of infections like Legionnaires disease,  Pontiac fever, TB, or COVID-19. Serious, longer-term health problems include the development of chronic diseases like asthma, eczema, and depression. Sustained exposure to carcinogenic substances may lead to the development of cancer, diabetes, respiratory diseases, cardiovascular disease, and dementia.

Poor IAQ causes health and comfort problems

Studies have shown that building occupants’ subjective experience of IAQ is also extremely important for promoting productivity and comfort indoors. Poor air quality and inadequate ventilation affect the ability of workers to think, concentrate, and complete tasks quickly. [5] Health and comfort problems related to room air are notable causes of employee absenteeism. Studies have shown that the implementation of measures to improve air quality can boost worker productivity by up to 11%. [6]. Enhanced natural ventilation in offices and other large buildings not only improves employee performance but also provides an economic uplift that is equivalent to $6500 per employee.

How to Improve IAQ

concrete building

IAQ monitoring is undertaken to improve air quality and comply with state and federal standards and guidance. The EPA specifies three key strategies for addressing indoor air quality problems:

1. Identify indoor air sources that emit pollutants

Once IAQ monitoring has been completed, facilities managers and property owners are in a position to improve air quality by identifying indoor air sources of pollutants and reducing or eliminating their emissions. This is often more cost-effective than increasing mechanical ventilation. Source control strategies include:

  • Repairing an improperly adjusted gas stove or furnace. 
  • Undertaking cleaning and maintenance of existing mechanical ventilation devices.
  • Switching from solid fuel to energy sources that have low emissions or are emissions-free like natural gas and electricity.
  • Sealing or enclosing building materials or areas that contain asbestos and other hazardous materials. 
  • Removing furnishings, paints, and materials that emit high levels of VOCs and other chemical pollutants.
  • Avoiding the use of air fresheners and chemical-laden household cleaning products

2. Increasing ventilation in buildings 

Improving building ventilation will increase air movement, carrying indoor air pollutants out of a room for improved IAQ. Building owners and managers can implement strategies to ensure that enough outdoor air enters indoor environments and that indoor air pollution is removed efficiently. Increased natural ventilation, outdoor air, and air circulation also add to the comfort of building occupants. Strategies for increasing building ventilation include:

  • Opening a window or door (natural ventilation).
  • Installation of outdoor vented fans or other mechanical ventilation devices in kitchens or bathrooms.
  • Upgrading the building’s heating and cooling systems to increase airflow (mechanical ventilation).
  • Running window air conditioning with an open vent to allow in enough outdoor air.
  • Installing HVAC MERV13 or HEPA filters to filter particulate air pollution. 
  • Considering the installation of an air-to-heat exchanger (heat pump) as an alternative to a  gas furnace or open fire.
  • Enhancing the infiltration of outdoor air into buildings by retrofitting vents and air bricks in the building structure.

Maintenance of building ventilation systems and devices ensures that outdoor air enters buildings and periods of inadequate ventilation are prevented.

3. Maintaining optimal humidity conditions

Building occupants can also take steps to improve indoor humidity conditions by ensuring that there is adequate ventilation when undertaking activities that release a lot of moisture into the room air, like showering, drying laundry, or cooking. Too little outdoor air will lead to raised humidity levels as moisture cannot escape.

4. Using an air cleaner 

Air cleaners, or air purifiers are devices that suck up room air and filter any contaminants in it, recirculating cleaner air. Air purifiers target particulate indoor air pollutants that can be trapped by filters rather than gaseous contaminants. By trapping and removing particles from the air, air cleaners protect building occupants from adverse health effects. These devices were originally used in industrial settings to clean exhaust air before being released into the atmosphere, however, the technology has been refined and scaled to be accessible to consumers.[7]

Types of air cleaners

air conditioner

Air cleaners are available in several sizes, spanning table-top devices to complex, whole-house air handling systems for conditioned outdoor air. Air cleaning technologies used by air purifiers and cleaners include:

Air filtration using MERV 13 or HEPA filters

Forcing room air through a filter (mechanical ventilation) is the most common method of air cleansing. Filters are used in standalone air cleaners or integrated into heating and cooling systems with an airtight seal. MERV 13+ and HEPA filters are considered high-efficiency filters, with HEPA filters arresting over 99.95% of particles with a diameter of 0.3 μm.

Biocide-treated air filters

These special air filters are coated with antiviral and antimicrobial agents that help to arrest the spread of airborne pathogens. They also can be installed in sealed air handling systems to sanitize outdoor air that enters sensitive environments like hospitals.


Air cleaners that use ionizing technology contain charged surfaces that release ions into the indoor space. These charged particles attract and attach to indoor air pollutants and draw them towards a charged collector plate, removing them from the air.

Thermodynamic sterilization (TSS)

TSS devices target airborne pathogens, allergens, and spores. These air cleaners contain a heated ceramic core that incinerates any microbiological particles that come into contact with it. Convection is used to move room air over the heated core.

Ultraviolet germicidal irradiation (UVGI)

Room air can also be sterilized by being passed over UV lamps that kill airborne microorganisms. This type of air cleaner requires an efficient filter to remove dead organisms from the circulating air.

Activated carbon

This porous organic material uses adsorption to remove indoor air pollutants from room air. 

Photocatalytic oxidation (PCO)

PCO is an emerging HVAC technology that oxidizes and degrades organic indoor air pollutants, including bacteria, viruses, and fungi. PCO destroys microorganisms and ventilates properties with conditioned outdoor air using UV light and a titanium dioxide (TiO2) catalyst.

Each air cleaner will have a manufacturer-specified percentage efficiency rate that expresses how well particles are filtered from room air. Air cleaners also vary in the amount of air that they can draw through their filter (air exchange rate), usually measured in cubic feet per minute. The best air cleaners will be efficient collectors with a high air exchange rate. Any mismatch between these two factors will impair the effectiveness of the cleaner. 

Air cleaners may also struggle to provide a meaningful improvement in air quality if there is the persistent presence of a strong pollutant source. A high air exchange rate cannot overcome the sustained release of indoor pollutants. They cannot reliably reduce the levels of radon and any radioactive decay products present.

Do houseplants improve indoor air quality? 


Houseplants are often suggested as being beneficial for reducing VOCs and other chemicals. However, the EPA has stated that there is no substantive evidence that houseplants can remove significant amounts of indoor air pollutants or improve indoor air quality. 

In conclusion

Photo by Pixabay: https://www.pexels.com/photo/cloudy-sky-531756/

IAQ is an important factor in determining the long-term health and well-being of building occupants. The health effects of poor air quality are a global environmental health issue that is as important as outdoor air pollution and costs billions of dollars in lost productivity and illness.

A variety of government and professional bodies have specified standards for IAQ that can be monitored with a range of sensor-based technologies. IAQ monitoring can be used to guide interventions that remedy IAQ problems. A range of strategies including natural ventilation, mechanical ventilation, and air-cleaning devices are available to address indoor air quality problems and improve environmental health.