Indoor air quality (IAQ) refers to the condition of the air inside buildings and structures. Poor IAQ caused by indoor air pollution can harm the health, comfort, and well-being of people who live or work in those spaces. It is connected to health problems such as sick building syndrome, breathing difficulties, lower work or school performance, and trouble learning. Common indoor air pollutants include smoke from tobacco, harmful gases from burning fuels indoors, radon, mold and other allergens, carbon monoxide, chemicals from products like paints and cleaners, bacteria such as legionella, asbestos fibers, carbon dioxide, ozone, and tiny particles in the air.
To improve IAQ, methods such as removing sources of pollution, using filters, and increasing air movement to spread out harmful substances are most effective. While air movement is important for good IAQ, it may not always be enough. If outdoor air is polluted, other tools like filters may also be needed.
IAQ is studied by collecting air samples, checking how people are exposed to pollutants, examining building materials, and using computers to model how air moves inside buildings. IAQ is part of indoor environmental quality (IEQ), which includes other factors that affect how people feel physically and mentally indoors, such as lighting, sound levels, how comfortable the temperature is, and the quality of vision.
Indoor air pollution is a serious health risk in developing countries and is often called "household air pollution" in this context. It is mainly caused by burning fuels like wood, charcoal, animal waste, and plant materials for cooking and heating in homes without proper ventilation. This affects millions of people, especially women and children. About three billion people in developing countries are impacted. The World Health Organization (WHO) says that pollution from cooking indoors causes 3.8 million deaths each year. A study called the Global Burden of Disease estimated that 1.6 million deaths occurred in 2017 due to this issue.
Definition
For health reasons, it is important to breathe air that is clean and free from harmful chemicals as much as possible. People spend about 90% of their lives indoors, and in some areas, the air inside buildings can be more polluted than the air outside.
Many things can cause high levels of pollution indoors. These include pollutants entering from outside, chemicals released by furniture and carpets, activities like cooking, cleaning, or smoking, and the use of office equipment. Indoor air quality is also affected by factors such as temperature, humidity, airflow, and the chemical makeup of the air. Pollutants can enter buildings through open doors or windows. Air conditioning and ventilation systems that are not properly maintained can grow mold and bacteria, which spread through the air and may cause breathing problems or allergies.
Experts who study indoor air quality often disagree about how to define what makes indoor air "acceptable" in terms of safety and health.
Health effects
Indoor air quality (IAQ) is important for human health because people spend most of their time indoors. On average, Americans and Europeans spend about 90% of their time inside buildings.
The World Health Organization (WHO) says that about 3.2 million people die too early each year from illnesses caused by indoor air pollution, especially from cooking indoors. More than 237,000 of these deaths involve children under 5 years old. These deaths include about one-eighth of all global deaths from heart disease, stroke, and lung cancer. In 2019, the WHO estimated that poor indoor air quality caused the loss of 86 million healthy years of life.
Studies in the UK and Europe show that being exposed to indoor air pollutants, chemicals, and biological contamination can irritate the upper airway, make asthma worse or cause it to happen, and may even lead to cancer.
Poor indoor air quality can cause sick building syndrome. Symptoms include burning eyes, a scratchy throat, a blocked nose, and headaches.
Common pollutants
Indoor combustion, like cooking or heating, is a major source of indoor air pollution. It can cause serious health problems and early deaths. Fires from hydrocarbons, such as oil or gas, also create air pollution. Pollution comes from burning both natural materials like wood and man-made fuels like coal or gas. Some fuels are more harmful than others.
Indoor fires can release black carbon particles, nitrogen oxides, sulfur oxides, and mercury compounds. About 3 billion people cook using open fires or simple stoves. Common cooking fuels include coal, wood, animal waste, and plant materials. Indoor air quality is especially poor in low- and middle-income countries where these practices are common.
Cooking with natural gas (also called fossil gas or methane) can lower indoor air quality. Burning gas produces nitrogen dioxide and carbon monoxide. These gases can increase nitrogen dioxide levels in homes, which may cause breathing problems and illnesses.
Carbon monoxide (CO) is one of the most dangerous indoor air pollutants. It is a colorless, odorless gas that forms when fuels burn incompletely. CO can come from tobacco smoke or faulty stoves, heaters, or blocked chimneys. In developed countries, CO often comes from broken or poorly maintained gas or oil appliances. In low- and middle-income countries, burning biomass fuels and cigarette smoke are major sources of CO.
Exposure to CO can cause short-term or long-term health issues. It can lead to headaches, dizziness, nausea, and breathing problems. Severe exposure may cause unconsciousness, coma, or death. Long-term exposure to small amounts of CO can cause tiredness, headaches, and health problems in the brain, heart, and lungs.
The World Health Organization (WHO) recommends that indoor CO levels should not exceed 4 mg/m³ over 24 hours. For short-term exposure, levels should not go above 10 mg/m³ in 8 hours, 35 mg/m³ in 1 hour, or 100 mg/m³ in 15 minutes.
Secondhand smoke is smoke from tobacco that affects people who are not smoking. It includes smoke exhaled by the smoker (15%) and smoke from the burning end of a cigarette (85%). Secondhand smoke contains over 7,000 chemicals, many of which are harmful. It includes gases and tiny particles that can damage the lungs and cause conditions like asthma, pneumonia, lung cancer, and sudden infant death.
Thirdhand smoke refers to chemicals left behind on surfaces or objects after smoking. These chemicals can react with other substances in the air to form new harmful chemicals. The only way to reduce secondhand smoke indoors is to stop smoking inside. Using e-cigarettes indoors also increases indoor particle levels.
Indoor particulate matter, or tiny particles, can come from cooking, cleaning, or outdoor air. These particles range in size from very small (nanometers) to larger (micrometers). Cooking methods like frying or cooking meat produce more particles than boiling or grilling. For example, making a Thanksgiving dinner can create very high particle levels, sometimes over 300 μg/m³.
These particles can travel deep into the lungs and even into the brain, causing health issues like heart disease, lung disease, cancer, and early births.
Volatile organic compounds (VOCs) are chemicals that can harm health. Many indoor products, such as paints, cleaning supplies, and furniture, release VOCs. VOCs are often higher indoors than outdoors. Examples of harmful VOCs include benzene, formaldehyde, and trichloroethylene.
VOCs can come from building materials, cleaning products, and even hot water from chlorinated drinking water. Cooking and cleaning can also release VOCs. For example, heating oil can release aldehydes, and spices can release terpenes. VOCs can react with other chemicals in the air to form new harmful substances, like formaldehyde.
Health effects of VOCs include eye, nose, and throat irritation, headaches, nausea, and damage to the liver, kidneys, and nervous system.
Testing materials used indoors, like paints and carpets, helps identify VOCs. Some materials trap VOCs and slowly release them over time. These delayed emissions can cause long-term exposure.
Many countries and organizations have rules to reduce VOCs in products. Programs like EMICODE, Blue Angel, and California’s CDPH standards help ensure products emit fewer VOCs. Because of these efforts, more low-emission products are now available.
At least 18 types of microbial VOCs (MVOCs) are found in indoor environments.
IAQ and climate change
Indoor air quality is closely connected to outdoor air quality. The Intergovernmental Panel on Climate Change (IPCC) has different predictions about future climate changes. Climate change can increase outdoor air pollutants, such as ozone and tiny particles, for example, through emissions from wildfires caused by extreme heat and drought. Scientists have made predictions about how indoor air pollutants might change. They have also used models to see how future climate scenarios could affect indoor air quality and factors like humidity and temperature.
Meeting the net-zero goal needs major changes in how new and upgraded buildings perform. However, more energy-efficient homes can trap pollutants inside, whether they come from inside or outside, which may increase people's exposure to them.
Indoor air quality standards and monitoring
For work-related exposure, there are safety rules that apply to many chemicals and are meant for healthy adults who are exposed over time in workplaces, such as factories. These rules are created by groups like the Occupational Safety and Health Administration (OSHA), the National Institute for Occupational Safety and Health (NIOSH), and the UK Health and Safety Executive (HSE).
There is no global agreement on standards for indoor air quality or health-based guidelines. However, some countries and health organizations have their own rules. For example, the World Health Organization (WHO) has created worldwide air quality guidelines for the general public that apply to both outdoor and indoor air. The WHO also has specific guidelines for certain chemicals. In the UK, the Health Security Agency has guidelines for some indoor air pollutants called volatile organic compounds (VOCs). The Scientific and Technical Committee (STC34) of the International Society of Indoor Air Quality and Climate (ISIAQ) has made a public database that collects indoor air quality guidelines from around the world. This database focuses on indoor air quality but also includes rules and standards related to ventilation, comfort, sound, and lighting.
Indoor air pollutants can harm human health, so it is important to use systems that check indoor air quality in real time. These systems can improve air quality, detect leaks or spills in work areas, and help buildings use energy more efficiently by sending feedback to heating, ventilation, and air conditioning (HVAC) systems. Studies have shown that poor indoor air quality can reduce worker performance and productivity in office settings.
Combining Internet of Things (IoT) technology with real-time indoor air quality monitoring systems has become very popular. This allows actions to be taken quickly based on data from sensors, which helps improve indoor air quality.
Improvement measures
Indoor air quality can be improved, kept good, or kept at a high level during the planning of new buildings or through changes in existing buildings. A set of steps has been suggested by the Institute of Air Quality Management. These steps include removing sources of pollution, reducing pollution from any remaining sources, blocking paths between pollution sources and people, protecting people from pollution, and moving people away from areas with poor air quality.
A report from the Institute for Occupational Safety and Health of the German Social Accident Insurance can help examine health problems that happen in indoor workplaces and find practical solutions.
Designing buildings in an environmentally friendly way includes considering heating, ventilation, and air-conditioning (HVAC) systems for homes and businesses. One important topic is making sure indoor air quality is good during the planning and building of structures.
One method to save energy while keeping air quality good is demand-controlled ventilation. Instead of using a fixed rate for air replacement, sensors that measure carbon dioxide levels adjust the ventilation rate based on the number of people in the building.
A way to check if indoor air is healthy is by how often fresh air from outside is brought into a space. In the UK, classrooms must have 2.5 changes of outdoor air per hour. In spaces like gyms, dining areas, and physiotherapy rooms, ventilation must be enough to keep carbon dioxide levels below 1,500 ppm. In the US, classroom ventilation depends on the amount of outdoor air per person and per square foot of floor space, not on air changes per hour. Since indoor carbon dioxide comes from people and outdoor air, the amount of outdoor air per person is shown by the difference between indoor and outdoor carbon dioxide levels. A value of 615 ppm above outdoor levels means about 15 cubic feet of outdoor air per adult doing office work, assuming outdoor air has over 400 ppm (the global average as of 2023). In classrooms, the ASHRAE standard 62.1 usually results in about 3 air changes per hour, depending on how many people are in the space. Since people are not the only source of pollution, more outdoor air may be needed if there are strong indoor pollution sources.
When outdoor air is polluted, bringing more of it indoors can make indoor air worse and cause more health problems. Generally, outdoor air in rural areas is cleaner than indoor air in cities.
Air filters can trap some pollutants. Portable air cleaners with HEPA filters can be used if ventilation is poor or if outdoor air has high levels of PM 2.5. Air filters also help reduce dust that can grow mold on wet parts of HVAC systems and lower the efficiency of these systems.
Trickle vents on windows can help keep air moving constantly. They can prevent mold and allergens from building up and reduce the spread of some respiratory infections.
Managing moisture and controlling humidity requires HVAC systems to work as they were designed. Managing moisture and humidity may conflict with saving energy. For example, moisture control may require systems to supply air at lower temperatures instead of higher ones used to save energy in hot climates. However, in most parts of the US and many places in Europe and Japan, outdoor air is cool enough during most of the year to provide comfort indoors without extra cooling. High outdoor humidity, though, requires careful attention to indoor humidity levels. High humidity can cause mold to grow and increase the risk of respiratory problems.
The "dew point temperature" is a measure of how much moisture is in the air. Some buildings are designed to have dew points in the lower 50s °F, and some in the upper and lower 40s °F. Some buildings use desiccant wheels with gas-fired heaters to remove moisture from air and reach the needed dew points. After moisture is removed, a cooling coil is used to lower the air temperature to the desired level.
Commercial buildings, and sometimes homes, are often kept at slightly higher air pressure than the outside to reduce the amount of air that leaks in. This helps with managing moisture and humidity.
Bringing outdoor air into a building can reduce indoor pollution if the outdoor air is clean. However, ozone in outdoor air can react with chemicals indoors, creating harmful substances like formaldehyde, aldehydes, and fine particles. These reactions can happen even at low ozone levels. This means ozone should be removed from ventilation air, especially in areas with high outdoor ozone levels.
Houseplants and the soil they grow in can help reduce certain indoor pollutants, like volatile organic compounds (VOCs) such as benzene, toluene, and xylene. Plants remove carbon dioxide and release oxygen and water, but the effect of houseplants on air quality is small. Interest in using plants for removing VOCs began after a 1989 NASA study in sealed spaces similar to those on space stations. However, these results are not useful for regular buildings, where outdoor air already removes VOCs at a rate that would require 10–1000 plants per square meter of floor space to match.
Plants may also reduce airborne microbes and molds and increase humidity. However, higher humidity can lead to more mold and VOCs.
Since very high humidity increases mold growth and health problems, adding moisture from houseplants might not be good in all indoor spaces if plants are watered improperly.
Institutional programs
The topic of indoor air quality (IAQ) has become important because more people understand the health risks caused by mold and substances that can worsen asthma and allergies.
In the United States, the Environmental Protection Agency (EPA) created a program called "IAQ Tools for Schools" to help improve air quality in schools and other educational buildings. The National Institute for Occupational Safety and Health (NIOSH) performs Health Hazard Evaluations (HHEs) in workplaces when requested by employees, their representatives, or employers. These evaluations check if any substances found in the workplace could be harmful, including those affecting indoor air quality.
Many scientists study indoor air quality. These experts include chemists, physicists, mechanical engineers, biologists, bacteriologists, epidemiologists, and computer scientists. Some of these professionals are trained by groups such as the American Industrial Hygiene Association, the American Indoor Air Quality Council, and the Indoor Environmental Air Quality Council.
In the United Kingdom, the Department for Environment, Food, and Rural Affairs (DEFRA) oversees the Air Quality Expert Group. This group reviews current knowledge about indoor air quality and gives advice to government officials.
Globally, the International Society of Indoor Air Quality and Climate (ISIAQ), which was formed in 1991, holds two major conferences called "Indoor Air" and "Healthy Buildings."