Indoor activities and materials are primary drivers of poor indoor air quality, releasing fine particles and reactive gases that accumulate when ventilation is limited. Source control, adequate air exchange, and careful selection of building materials and products are essential to reducing pollutant buildup and protecting health.

Combustion-related sources — including cooking, tobacco or cannabis use, home heating, and candle burning — emit PM_2.5, UFPs, CO, NO_x, VOCs, PAHs, and trace metals that contribute to respiratory, cardiovascular, and carcinogenic risks. Emissions depend on fuel type, appliance design, and airflow, with poorly ventilated or unvented systems producing the highest concentrations. Improving kitchen exhaust, eliminating indoor smoking, upgrading to vented or electric heating systems, and limiting candle use are key strategies to reduce exposure.

Chemical and product-based sources — such as cleaning agents, building materials, scented and personal care products, and ultrasonic humidifiers — emit VOCs, SVOCs, secondary oxidation products, and, in some cases, airborne metals or microorganisms. Emissions and by-products contribute to adverse respiratory, neurological, and endocrine effects and can be mitigated through safer formulations, informed use, and adequate ventilation.

The following subsections examine each of these indoor-origin sources of pollution in detail.

3.2.1 Cooking

Cooking is a major indoor pollution source whose emissions and health impacts depend on fuel, temperature, and ventilation. Managing airflow and source control through effective range hoods, natural ventilation, and regular maintenance is critical to reducing exposure and protecting health.

Cooking is one of the most significant and variable sources of degraded IAQ. It releases fine particles and gases, including VOCs, whose composition varies with fuel type and temperature. Gas burners emit NO₂, CO, and formaldehyde, while frying and grilling at high temperatures generate UFPs, aldehydes, and PAHs. Oils and seasonings add further chemical complexity, and emissions rise sharply in homes and restaurants when ventilation is limited.

Exposure to these pollutants can irritate the eyes and airways and aggravate asthma. Repeated or high-level exposure contributes to chronic respiratory disease and has been linked to elevated lung cancer risk, especially among cooks and in poorly ventilated dwellings. Pollutants from gas appliances have also been associated with subtle neurodevelopmental effects in early childhood.

Quantifying and characterizing cooking emissions remains complex. Real-time mass spectrometric and modeling studies show that alcohols, aldehydes, and terpenes dominate primary emissions, while their oxidation by ozone and NO_x forms secondary pollutants such as formaldehyde, organic nitrates, and peroxyacetyl nitrates. Reactions on indoor surfaces — including wood, plastics, and fabrics — further promote secondary formation, making surface area and material composition key factors in pollutant persistence. Reviews indicate that cooking-derived particles are rich in elemental carbon with large surface area, enhancing the adsorption of organic compounds and trace metals.

Indoor concentrations from cooking vary widely with building design, ventilation, and occupant behavior. Tightly sealed or energy-efficient homes retain pollutants longer, and many range hoods fail to deliver their rated airflow or are seldom used. Field studies show that most apartments with gas stoves do not meet code ventilation requirements, while controlled tests in passive houses found that temporary mechanical boosts had little effect compared with direct exhaust. Natural ventilation — such as opening doors or windows — removes particles and gases more effectively than recirculating fans, and well-designed range hoods can reduce particle peaks by more than 75%. Commercial kitchens with high air exchange rates experience lower exposures during cooking but can still see sharp increases in particulate and chlorinated gases during cleaning.

Research demonstrates that cooking contributes substantially to indoor pollutant exposure, especially where ventilation is inadequate or equipment underperforms. Improving range hood performance and ensuring proper airflow are key steps toward healthier indoor environments.

3.2.2 Tobacco and Cannabis Smoke

Tobacco and cannabis smoke are leading contributors to indoor air pollution, releasing fine particles, gases, and reactive compounds harmful to health. Eliminating indoor smoking and vaping is essential to preventing exposure and maintaining clean indoor air.

Tobacco and cannabis smoke are among the most pervasive sources of indoor air pollution. Combustion and vaporization of these products release complex mixtures of fine and UFPs, CO, VOCs, and trace metals that linger in indoor air long after use. Cigarettes, cigars, and waterpipes emit dense aerosols containing nicotine and numerous toxic or carcinogenic species, while cannabis adds terpenes and cannabinoids such as THC. Even without combustion, newer devices — including electronic cigarettes, heated tobacco systems, and cannabis vaporizers — generate measurable emissions of particles, aldehydes, and other VOCs. Indoor studies consistently show that pollutant concentrations rise sharply during active use and remain elevated in enclosed or poorly ventilated spaces.

Exposure to tobacco and cannabis smoke causes a wide spectrum of health effects, from immediate irritation to long-term disease. Acute exposure can inflame the eyes and airways, worsen asthma, and impair cardiovascular function through elevated heart rate and reduced vascular capacity. Chronic exposure to secondhand smoke is a proven cause of lung cancer, coronary heart disease, and COPD. Studies in children show that household smoking increases asthma symptoms and cotinine levels, while prenatal or early-life exposure can impair neurodevelopment. Cannabis smoke and vapor cause similar respiratory irritation and oxidative stress, and emerging evidence suggests potential cardiovascular and cognitive effects with repeated exposure. Although emissions from electronic cigarettes and heated tobacco systems are lower, they still contain nicotine, aldehydes, and UFPs capable of provoking inflammation and oxidative damage.

Measured concentrations from smoking and vaping often reach extreme levels indoors, shaped mainly by product type, duration, and ventilation. Controlled experiments demonstrate that cigarette smoke can raise PM_2.5 concentrations above 1,500 µg/m³, accompanied by sharp increases in CO, NO_x, and VOCs that linger for hours before returning to baseline. Waterpipe sessions have been observed to elevate indoor PM_2.5 to several hundred µg/m³ and CO above 40 mg/m³ in cafés and homes, with pollutant levels rising in proportion to the number of active smokers and ventilation conditions. Field measurements show that these pollutants readily disperse beyond smoking areas and remain elevated long after sessions end. In contrast, emissions from electronic cigarettes and heated tobacco systems are lower but still substantial, generating UFPs, nicotine, and aldehydes that accumulate in enclosed or crowded spaces such as vape shops and conventions. Cannabis smoking yields PM_2.5 concentrations similar to or greater than tobacco, while vaping cannabis produces shorter, more localized peaks that drop quickly with ventilation.

Because pollutants from smoking and vaping persist even after active use, the only fully effective strategy is to keep indoor environments smoke- and vape-free. Research demonstrating the hazards of secondhand tobacco smoke provided the foundation for smoke-free air laws, which have since expanded to cover waterpipes, e-cigarettes, and other emerging products. These studies show that ventilation or air cleaning cannot fully prevent exposure once combustion or aerosol generation occurs. Modern devices such as heated tobacco systems and vaporizers reduce emissions but still release measurable nicotine, aldehydes, and metals, reinforcing the need for clear testing standards and product oversight. Recent work has also broadened policy attention to cannabis use and occupational sources such as vape shops and cultivation facilities, where aerosols and biogenic VOCs can affect both workers and nearby residents.

Research identifies smoking and vaping as dominant yet fully preventable sources of indoor air pollution. Maintaining smoke-free environments and strengthening compliance with existing restrictions are the most effective measures for safeguarding IAQ and respiratory health.

3.2.3 Home Heating

Combustion-based heating is a dominant winter source of indoor pollution, emitting fine particles and gases that impair respiratory health. Transitioning to cleaner and properly vented systems is key to reducing exposure.

Heating is essential for comfort in cold climates but remains one of the largest seasonal sources of indoor air pollution. Combustion-based systems — including wood stoves, fireplaces, coal burners, and unvented kerosene or gas heaters — emit fine particles and gaseous pollutants such as CO, NO₂, VOCs, and PAHs. These emissions result from incomplete combustion, while limited ventilation allows them to accumulate indoors. Modern appliances emit fewer pollutants than open fires but still release PM_2.5 and trace metals during operation and maintenance. In homes that rely on solid or liquid fuels, particularly older or unvented systems, heating often dominates wintertime pollutant levels and persistently degrades IAQ.

Exposure to emissions from residential heating is associated with a variety of adverse health outcomes. Inhalation of particles from wood combustion causes airway inflammation, oxidative stress, reduced lung function, and DNA damage, particularly when incomplete combustion generates fine organic and soot particles rich in PAHs and metals. Epidemiologic studies link coal heating and unvented gas or kerosene heaters to higher asthma prevalence and greater symptom severity among children, with improvements observed after replacement by vented or electric systems. Residents in homes using open fireplaces or poorly maintained wood-burning heating appliances report higher rates of irritation, cough, and other respiratory symptoms, highlighting the health impacts of combustion-based heating.

Pollution from household heating varies widely with fuel type, appliance design, and building conditions. Monitoring shows that wood stoves and fireplaces produce sharp peaks in fine particles and PAHs during ignition and maintenance, influenced by ventilation and combustion practices. Coal-heated rural homes often exceed indoor standards for PM_2.5, CO₂, and formaldehyde, while traditional stove–kang systems and burning caves yield even higher CO and particle concentrations. Field studies also show that unvented kerosene and gas heaters emit submicron particles and NO₂ that build up in poorly ventilated rooms. Across regions, older or unvented systems and restricted airflow amplify exposures, whereas modern high-efficiency or vented appliances emit fewer pollutants. Socioeconomic and cultural factors also shape heating choices, as shown in the Navajo Nation and in Beijing’s coal-to-electricity transition, where cleaner technologies improved IAQ for wealthier households but imposed cost burdens on lower-income residents.

Effective control strategies focus on eliminating indoor combustion and replacing high-emission heaters. Replacing coal, kerosene, or unvented gas heaters with vented or electric systems lowers indoor PM_2.5 and NO₂ and is linked to better respiratory outcomes. Large-scale fuel switching has been shown to reduce indoor PM_2.5 and improve indoor temperatures, though benefits were smaller where costs limited adoption. In rural settings, air-source heat pumps maintained cleaner indoor air than coal boilers or stove–kang systems. Where solid fuels persist, appliance design and practice matter: advanced wood appliances and adequate ventilation reduce ignition-related pollutant peaks, and careful maintenance limits ash-related particle and metal releases. Evidence supports regulating unvented kerosene heaters given their high emissions. Program design should address affordability and cultural fit to sustain uptake.

Research shows that emissions from residential heating are a leading source of seasonal indoor pollution but can be reduced through cleaner fuels and improved appliance design. Transitioning to vented or electric systems, maintaining equipment, and ensuring adequate ventilation are effective strategies to lower exposure and protect respiratory health.

3.2.4 Cleaning and Disinfectants

Cleaning and disinfecting products are vital for hygiene but also major sources of indoor chemical exposure. Selecting safer formulations, improving ventilation, and avoiding excessive or improper application are essential to maintaining healthy indoor air.

Cleaning and disinfecting products are widely used to maintain hygiene and control disease, but their ingredients and by-products can unintentionally degrade IAQ. Sprays, detergents, solvents, and air fresheners emit VOCs, chlorinated gases, and other reactive chemicals during use. Many formulations contain irritant or toxic ingredients, and routine cleaning in homes, schools, and workplaces contributes substantially to total indoor chemical load. Their use rose sharply during the COVID-19 pandemic, often driven by public misunderstanding of disinfection needs, which led to excessive and unsafe application and increased indoor exposure risks.

Exposure to these products is consistently linked to respiratory illness and other adverse health outcomes. Occupational and household studies show elevated risks of asthma, chronic bronchitis, and COPD among frequent users and professional cleaners. Epidemiological evidence indicates that even weekly use of sprays and disinfectants increases adult asthma risk, while prolonged exposure contributes to lung function decline and COPD. Children exposed during pregnancy or early life have higher rates of wheezing and reduced lung function. Population studies during the COVID-19 pandemic identified additional symptoms including respiratory irritation, throat and eye inflammation, and, in some cases, neurological and metabolic effects from overuse of chlorinated and alcohol-based disinfectants.

Exposure to cleaning and disinfecting chemicals occurs mainly through inhalation and dermal contact, with residues on indoor surfaces and dust extending exposure after use. Reactive ingredients — including chlorine, ammonia, aldehydes, and QACs — act as strong airway irritants that can damage epithelial tissue, induce oxidative stress, and trigger allergic sensitization. Mixing bleach with other cleaning agents can release toxic gases capable of causing acute airway injury. VOCs from scented and solvent-based products further react indoors with ozone and radicals to form harmful secondary pollutants such as formaldehyde and UFPs through ozone–terpene chemistry.

Reducing exposure from cleaning and disinfecting products requires a combination of safer product selection, proper use, and adequate ventilation. Substituting low-emission, fragrance-free, or hydrogen peroxide-based products for formulations containing bleach or QACs limits toxic emissions and secondary reactions. Cleaning should precede disinfection, and disinfectants should be used only when needed and at recommended dilutions. Opening windows, running exhaust fans, or operating HVAC systems during and after cleaning helps remove airborne VOCs and byproducts. Staff and occupants should avoid aerosol sprays, never mix cleaning agents, and allow surfaces to dry before reoccupancy. In schools and workplaces, training, clear labeling, and ventilation planning are essential to maintain both infection control and healthy indoor air.

Studies highlight cleaning and disinfection as a major but controllable contributor to indoor chemical exposure. Prioritizing safer formulations, limiting unnecessary use, and ensuring ventilation during and after cleaning are effective strategies for protecting both hygiene and respiratory health.

3.2.5 Building Materials and Furnishings

Building materials and furnishings are persistent indoor pollution sources that continuously emit volatile and semi-volatile compounds long after installation. Choosing low-emission materials, ensuring adequate ventilation, and managing moisture and temperature are critical to reducing chemical buildup and protecting indoor air quality.

Building materials and furnishings are major, persistent sources of indoor air pollution. Products such as particleboard, paints, adhesives, and flooring emit VOCs — including reactive species like formaldehyde and other aldehydes — along with plasticizers and semi-volatile additives that contribute to the characteristic “new-building” odor and sustained indoor chemical load. Emission strength depends on material composition, installation phase, and environmental conditions such as temperature, humidity, and ventilation. Because many materials continue releasing pollutants for months or years after installation, they represent one of the most consistent and long-term contributors to indoor exposure.

Exposure to pollutants and microbes released from building materials affects multiple aspects of human health. Emitted compounds act through genotoxic, endocrine-disrupting, and immunological mechanisms, with children and pregnant women most vulnerable. VOCs — particularly BTEX compounds, terpenes, and aldehydes — pose carcinogenic and mutagenic risks, while preservatives and other additives are linked to respiratory and hormonal effects. Fungal contamination of damp materials adds spores and mycotoxins associated with respiratory and allergic illness. Modeling and life cycle studies show that conventional materials cause far greater long-term health burdens than low-emission or bio-based alternatives, with gypsum board producing roughly fifteen-fold higher damage than bamboo flooring and biocomposites reducing total human health impacts by more than 50%.

Indoor exposure depends on how materials interact with their environment. Emission rates rise with temperature, humidity, and limited ventilation, making pollutant loads highest in airtight or newly finished buildings. Over time, concentrations follow decay curves — rapid initially, then gradually declining but often persisting for years. Field and chamber studies show that flooring, particleboard, and furniture dominate emissions during and after construction, while gypsum walls can temporarily adsorb and later re-emit VOCs, altering indoor dynamics. Chemical reactions within and between materials further transform pollutants, as terpenes and other VOCs react with ozone and NO_x to generate secondary aldehydes, organic acids, and UFPs. These secondary products often have stronger odors and irritation potential than the original compounds, contributing to discomfort and occupant complaints.

Reducing pollution from building materials relies on source control, ventilation management, and material innovation. Studies show that replacing high-emitting products with certified low-emission alternatives improves perceived air quality and reduces sick building symptoms more effectively than increasing airflow alone. Optimizing ventilation allows lower energy use while keeping pollutant levels below guideline limits, and life-cycle analyses show that bio-based materials further reduce indoor and environmental health burdens.

Studies demonstrate that emissions from building materials and furnishings can be minimized through careful material selection and environmental control. Using certified low-emission products, maintaining ventilation, and managing moisture and temperature help sustain healthy IAQ and occupant well-being.

3.2.6 Scented Products

Scented products emit particles and gases that degrade indoor air quality and stress the respiratory system. Limiting use and ensuring adequate ventilation is essential to reducing exposure.

Scented products are widely used indoors to improve odor and create a calming atmosphere but are also significant sources of airborne pollutants. They include candles, incense, air fresheners, and essential-oil diffusers, all of which release chemicals during burning, heating, or evaporation. Combustion-based products such as candles and incense emit smoke and gases as waxes, wicks, and resins decompose, while non-combustion devices continuously evaporate fragrance mixtures into the air. In both cases, VOCs and SVOCs disperse throughout occupied spaces and can accumulate when ventilation is limited. Their widespread and repeated use in homes, offices, and places of worship makes them an important contributor to overall indoor pollutant exposure.

Exposure to emissions from scented products can irritate the eyes and airways and provoke inflammatory responses. Short-term chamber studies show that burning candles elevates markers of oxidative stress and airway irritation, while animal experiments confirm systemic inflammation, fibrosis, and lung injury under repeated exposure. Surveys of frequent users report headaches, coughing, and shortness of breath, particularly in enclosed or poorly ventilated rooms. Occupational studies of temple workers chronically exposed to incense smoke reveal elevated biomarkers of benzene, 1,3-butadiene, and PAHs together with increased DNA damage and impaired repair capacity, indicating carcinogenic potential from long-term exposure. Reviews further identify phthalates, PAHs, and reactive VOCs as contributors to respiratory, reproductive, and endocrine effects. Although large population data show no measurable rise in hospitalizations from routine candle use, mechanistic and toxicological evidence indicates that frequent or prolonged exposure can trigger oxidative and genotoxic pathways relevant to chronic disease.

Emissions from scented products depend on material composition and combustion stability. Chamber studies demonstrate that wax purity, wick metal content, and fragrance additives determine the release of VOCs, PAHs, and PM_2.5 or UFPs. Flickering or oxygen-limited flames generate short, intense bursts of BC and PAHs, and real-time measurements show that burning candles and incense sharply increases indoor particle concentrations, with levels and persistence determined by ventilation and surface deposition. Modeling work indicates that under normal air exchange rates, most pollutant concentrations remain below health-based limits, but exceedances of NO₂, acrolein, or benzo[a]pyrene can occur when air movement is poor or combustion is unstable.

Adequate ventilation and source control remain the most effective ways to limit pollutant buildup from scented products. Increasing airflow during and after burning, shortening burn duration, and avoiding combustion in enclosed spaces all substantially reduce exposure. Cleaner-burning alternatives such as soy or beeswax candles, cotton or paper wicks without metal cores, and fragrance-free or low-additive formulations emit fewer VOCs, PAHs, and soot than paraffin or synthetic products. Reviews recommend minimizing routine use, improving product labeling, and adopting eco-design standards to lower emissions at the source. For incense, substituting low-emission resins and ensuring sufficient ventilation in places of worship can markedly reduce PAH and benzene exposure among temple workers.

Studies show that scented products are frequent sources of indoor chemical exposure but can be managed through informed use and ventilation. Reducing combustion-based burning, choosing low-emission or fragrance-free options, and ensuring adequate air exchange during and after use can improve IAQ and respiratory irritation.

3.2.7 Personal Care Products

Personal care products are a widespread but underrecognized source of indoor pollution. Limiting emissions and improving product transparency are key to protecting indoor air quality and health.

PCPs are everyday hygiene and cosmetic items — such as shampoos, deodorants, lotions, hair sprays, nail products, and perfumes — used routinely across all age groups. Their ingredients serve as solvents, preservatives, propellants, or fragrances, many of which are chemically reactive or persistent indoors. Because they are applied directly to the body and used in confined spaces, PCPs represent a continuous and intimate source of indoor chemical exposure.

Exposure to emissions from PCPs is consistently associated with respiratory, neurological, and endocrine effects. Population studies show that frequent users experience measurable declines in lung function, with regular use of scented body and hair products linked to reduced breathing capacity. Surveys in the United Kingdom and United States report that over a quarter of adults — and nearly two-thirds of people with asthma — develop symptoms such as respiratory irritation, migraines, and asthma attacks when exposed to fragranced products. Occupational studies of salon workers document widespread coughing, throat irritation, and other respiratory complaints that correlate with elevated indoor pollutant levels. Toxicological reviews further implicate ingredients such as preservatives, aluminum compounds, phthalates, and synthetic fragrances in allergic, neurological, and endocrine-disrupting responses.

PCPs release pollutants through both inhalation and skin contact, with residues accumulating on indoor surfaces and in poorly ventilated spaces. Perfumes, deodorants, and body sprays generate PM_2.5 and UFPs — most smaller than 0.3 microns — that remain airborne and can penetrate deep into the lungs. Routine use of shampoos, lotions, and other products emits a broad range of VOCs, including alcohols, siloxanes, and terpenes such as limonene. In hair and nail salons, pollutant concentrations often exceed outdoor levels by more than tenfold, reflecting the combined effects of heavy product use and limited ventilation. Once released, these compounds can react with O₃ and radicals to form secondary pollutants such as formaldehyde, organic aerosols, and new UFPs.

Reducing exposure from PCPs requires a combination of better ventilation, safer formulations, and clearer ingredient disclosure. Studies of hair and nail salons show that pollutant buildup results primarily from inadequate outdoor air exchange, highlighting the importance of improved ventilation and product handling practices to protect workers and clients. Researchers emphasize substituting high-emission sprays and solvent-rich products with lower-emission or non-aerosol alternatives, since formulation type strongly influences particle and VOC release. Broader mitigation also depends on transparency and regulation: analyses of fragranced consumer goods found that many emit hazardous or carcinogenic VOCs without labeling them, underscoring the need for clearer disclosure and oversight. Reviews further call for stronger regulation, realistic testing of “natural” products, and waste-treatment improvements to limit the persistence of PCP ingredients in indoor and environmental media.

PCPs are common yet often overlooked contributors to poor IAQ. Reducing ingredient emissions, increasing labeling transparency, and maintaining adequate ventilation help limit exposure and support healthier indoor environments.

3.2.8 Ultrasonic Humidifiers

Ultrasonic humidifiers often improve comfort but can introduce airborne metals, minerals, and microbes that degrade indoor air quality. Their safety depends on water quality, ventilation, and cleaning, making regular maintenance and use of distilled water essential to preventing exposure and protecting health.

Ultrasonic humidifiers are widely used to enhance comfort and relieve dryness but can also generate unintended airborne pollutants. They operate by vibrating a piezoelectric transducer at ultrasonic frequency, breaking water into fine droplets that are dispersed as a cool mist. While this process efficiently adds moisture to indoor air, it also aerosolizes dissolved minerals, metals, and microorganisms present in the fill water. As these submicron droplets evaporate, the remaining solids form inhalable particles that can spread throughout enclosed spaces. Their frequent use in homes, offices, and childcare environments makes them an important yet often overlooked contributor to indoor particle and microbial exposure.

Exposure to emissions from ultrasonic humidifiers is linked to respiratory and systemic effects. Reported outcomes include hypersensitivity pneumonitis, infant lung impairment, and elevated non-cancer risks from inhaled arsenic, cadmium, chromium, manganese, and lead, even at drinking-water-standard levels. Experimental and modeling studies show deep lung deposition of mineral particles, macrophage uptake, and altered gene expression in mice, while microbial tests reveal efficient aerosolization of bacteria, endotoxin, and pathogens such as Legionella. Community surveys associate infrequent cleaning with irritation, cough, and other respiratory symptoms.

Mitigation depends primarily on limiting what the humidifier can aerosolize. Using distilled or demineralized water eliminates mineral and metal emissions, while cleaning reservoirs weekly prevents microbial growth and endotoxin release. Increased ventilation and operation in larger rooms lower airborne concentrations and lung deposition, especially for children. Maintaining moderate humidity and avoiding disinfectant additives further reduce risks from particle and microbial exposure.

Studies show that ultrasonic humidifiers can contribute to indoor particle and microbial pollution when poorly maintained or filled with tap water. Using distilled water, cleaning regularly, and maintaining adequate ventilation are effective measures to ensure safe operation and healthy IAQ.