Glossary

Factors, their sources, the health outcomes they lead to, the interventions that help, the standards that set limits and the methods that measure — defined once, linked everywhere.

Other terms 2

Chemical classes and vocabulary used across the site.

A

G-0200Acetaldehydeindoor environment factor
A small, highly volatile aldehyde with a sharp fruity smell. Indoors it is the second most abundant carbonyl after formaldehyde, produced by combustion, by fermentation and by the reaction of ozone with the terpenes a household brings in.
G-0010Acroleinindoor environment factor
A colorless to pale-yellow liquid with a strong, pungent odor, used as a biocide to control plants, algae, molluscs, fungi, rodents and microorganisms.
G-0069Activated carbon adsorptionintervention
Porous carbon that traps gas molecules on its enormous internal surface. It is the standard way to remove volatile organic compounds, odours and some ozone from air, complementing particle filters that cannot capture gases.
G-0011Acute (short-term) toxicityhealth outcome
Effects resulting from single or repeated exposures over a day — headaches, nausea, irritation of the eyes, throat and nose, dizziness and asthma.
G-0161Acute respiratory infections (ARI)health outcome
Short-lived infections of the airways, upper or lower, counted as episodes over a defined period. It is the outcome household surveys can collect by asking, which is why it appears throughout the literature on cooking fuel and child health.
G-0140Adhesives and sealantssource
The glues, mastics and joint fillers used to fix and seal building materials. They are applied in small quantities but sit in the joint for the life of the installation, and they often decide what a certified low-emission board actually releases once it is laid.
G-0178Adsorptionintervention
Removing a gas by binding its molecules to the surface of a solid. It is how every gas-phase air cleaner works, and it is a finite process: the surface fills, the removal stops, and what was captured can be released again.
G-0131Air conditioningintervention
Cooling and dehumidifying indoor air mechanically. Most systems recirculate the air of the room rather than replacing it, so air conditioning changes temperature and humidity — and, through them, several exposures — without in itself providing ventilation.
G-0073Air exchange rate (air changes per hour)measurement method
How many times per hour the air in a room is replaced by outdoor air. It is the physical quantity behind ventilation: concentrations of indoor-generated pollutants fall in inverse proportion to it.
G-0068Air filtration and air purifiersintervention
Removing particles from air by passing it through a filter — in the ventilation system or in a portable air purifier. HEPA filters capture over 99.97 % of particles at 0.3 micrometres; activated carbon adds some removal of gases.
G-0096Air freshenerssource
Products that add fragrance to a room — sprays, plug-ins, gels, diffusers and scented candles. They do not remove anything from the air; they release volatile organic compounds, mainly terpenes, which react with ozone indoors to form formaldehyde and secondary particles.
G-0110Air purifiersintervention
A portable device that draws room air through a filter or another removal stage and returns it to the same room. It removes part of what is already airborne, chiefly particles, and does not bring in outdoor air or stop what continues to be emitted.
G-0205Air quality monitoringmeasurement method
Measuring indoor air continuously over time, rather than taking a sample and analysing it later. What monitoring produces is a record of when concentrations changed — which is usually more actionable than knowing the average.
G-0118Airborne bacteriaindoor environment factor
Bacterial cells and fragments carried in indoor air, measured either as colony-forming units that grow on a plate or as genetic material detected without culture. The two methods answer different questions and their numbers are not interchangeable.
G-0120Airborne fungiindoor environment factor
Fungal cells, spores and fragments carried in indoor air. Some are brought in from outdoors with ventilation air and settle with the seasons; the rest are grown indoors on damp material, and it is the second group that indicates a building problem.
G-0169Airborne transmission
Passing an infection through particles small enough to stay suspended in air and travel beyond the person who exhaled them. It makes the ventilation of a room part of how a disease spreads, rather than a matter of comfort.
G-0056Airway and systemic inflammationhealth outcome
The body's defensive response to injury or irritants — swelling, immune-cell recruitment, release of signalling molecules. Low-grade, persistent inflammation is the pathway from indoor air exposure to asthma, cardiovascular disease and cognitive effects.
G-0106Airway obstructionhealth outcome
A narrowing or blockage of the airways that restricts the flow of air in or out of the lungs. It may be sudden or long-standing, partial or complete, and it is the common mechanism behind the obstructive lung diseases rather than a diagnosis in itself.
G-0150Allergic rhinitishealth outcome
Inflammation of the nasal lining driven by an allergic response — sneezing, blockage, runny nose and itching — triggered by an allergen the person is sensitised to. Indoors the relevant allergens are the ones that live in buildings rather than the ones that blow in.
G-0167Allergic sensitisationhealth outcome
The state of having made specific antibodies to an allergen, shown by a skin prick test or a blood test. It is a laboratory finding, not a disease: many sensitised people have no symptoms, and the finding marks readiness to react rather than reaction.
G-0057Allergy and allergic sensitisationhealth outcome
An immune reaction to normally harmless substances — dust mite, pet, mould and pollen allergens indoors. Sensitisation in early life is influenced by dampness, air pollutants and the microbial environment of the home.
G-0012Anthropogenic emissionssource
Atmospheric emissions of pollutants that originate from human activities.
G-0013Anthropogenic VOCssource
Volatile organic compounds emitted by human activities, for example oil-derived solvents.
G-0014Antimicrobialssource
Substances that kill or inhibit microorganisms; triclosan is a common example found in consumer products.
G-0170Anxietyhealth outcome
Persistent worry, tension and apprehension, measured in population research by validated symptom scales. It appears in indoor air research alongside depression, usually in the same studies and with the same difficulty in separating exposure from circumstance.
G-0086ASHRAEstandard or body
The American Society of Heating, Refrigerating and Air-Conditioning Engineers, whose standards 62.1 and 62.2 (ventilation for acceptable indoor air quality) and 55 (thermal comfort) are the most widely adopted engineering benchmarks for buildings worldwide.
G-0206Aspergillusindoor environment factor
A genus of moulds common both outdoors and in buildings, producing small spores that are readily inhaled. It matters more than most indoor fungi because some species can grow at body temperature and cause disease as well as allergy.
G-0051Asthmahealth outcome
A chronic inflammatory disease of the airways causing wheeze, cough and breathlessness in episodes. Indoor factors — particulate matter, nitrogen dioxide, formaldehyde, dampness and mould, dust mites, tobacco smoke — both trigger attacks and, in children, contribute to its development.
G-0181Asthma exacerbationhealth outcome
A worsening of asthma beyond the person's usual state, needing extra treatment, an unscheduled consultation or admission. It is an event in someone who already has the disease, not a new case of it.
G-0052Atopic dermatitis (eczema)health outcome
A chronic, itchy inflammatory skin disease, most common in children, in which a defective skin barrier reacts to irritants and allergens. Indoor air — formaldehyde, volatile organic compounds, particulate matter, low humidity — is an increasingly studied trigger.
G-0107Atopic dermatitis symptomshealth outcome
The severity of atopic dermatitis as measured on a scale rather than the presence of the disease — itching, redness, oozing, thickening and sleep loss, scored by instruments such as SCORAD and the Eczema Area and Severity Index.

B

G-0009Benzeneindoor environment factor
A volatile aromatic hydrocarbon and confirmed human carcinogen. Indoors it comes mainly from tobacco smoke, attached garages, vehicle exhaust entering from outside, solvents and some consumer products. No safe level of exposure has been identified.
G-0083Bioaerosolsindoor environment factor
Airborne particles of biological origin — mould spores and fragments, bacteria, viruses, pollen, dust-mite and pet allergens. They are a subset of particulate matter with their own health effects: infection, allergy and inflammation.
G-0186Bioeffluents (occupant emissions)source
Everything people themselves release into the air of a room — carbon dioxide, water vapour, body odour compounds, skin particles and the products of skin oils reacting with ozone. In a well-built, lightly furnished room, the occupants are the main source.
G-0015Biogenic VOCssource
Volatile organic compounds emitted naturally, for example terpenes released by plants and wood.
G-0108Biomass fuelsource
Plant and animal matter burned in the home for cooking or heating — firewood, crop residues, dung and charcoal. Burned on an open fire or in a simple stove, it is the largest single source of combustion emissions inside dwellings worldwide.
G-0192Birth outcomeshealth outcome
The group of measures describing how a pregnancy ended — gestational age, birth weight, growth for age, stillbirth. The category is used where a study reported them together, and the narrower slugs are used wherever it named one.
G-0154Birth weighthealth outcome
The weight of a baby at birth, recorded in grams as a continuous measurement. Used this way it detects a shift across a whole population that the low-birth-weight threshold would hide, because most of the shift happens above the cut-off.
G-0135Black carbonindoor environment factor
The strongly light-absorbing carbon fraction of particulate matter, formed only by combustion. It is measured optically rather than by composition, and it is used as the marker that separates the combustion part of a particle mass from everything else in it.
G-0152Blood pressurehealth outcome
The pressure in the arteries, recorded as a systolic and a diastolic value. As a continuous measurement repeated on the same people, it is the endpoint through which short indoor interventions are most often tested for a cardiovascular effect.
G-0187Breast cancerhealth outcome
Malignant disease of the breast, the commonest cancer in women worldwide. It reaches this vocabulary through two hypotheses: combustion products acting as carcinogens, and compounds released indoors that interfere with hormonal signalling.
G-0062Building materialssource
Boards, insulation, flooring, adhesives, paints and sealants — the largest and longest-lasting source of formaldehyde and volatile organic compounds indoors. Emissions are highest in the first months after construction or renovation and continue at lower levels for years.

C

G-0066Candles and incensesource
Open flames indoors release fine and ultrafine particles, soot, carbon monoxide and, from incense, benzene and formaldehyde. Incense in particular produces particle concentrations comparable to tobacco smoke.
G-0003Carbon dioxide (CO₂)indoor environment factor
A colourless, odourless gas produced by breathing and combustion. Indoors it is the most reliable indicator of ventilation: rising CO₂ means the air is being breathed more times before it is replaced, and it rises together with every other pollutant that people emit.
G-0004Carbon monoxide (CO)indoor environment factor
A colourless, odourless, toxic gas produced by incomplete combustion — distinct from carbon dioxide. It binds to haemoglobin far more strongly than oxygen does, starving tissues of oxygen at concentrations that cannot be seen, smelled or felt until symptoms appear.
G-0016Cardiovascular disease (CVD)health outcome
Conditions affecting the heart and blood vessels; long-term exposure to particulate matter is among the environmental factors studied in relation to it.
G-0095Carpetssource
Textile floor covering that emits little after the first weeks but acts as a reservoir: dust, allergens, semi-volatile compounds and material carried in on shoes settle into the pile and are released again when the floor is disturbed. Whether that helps or harms the air is disputed.
G-0210Cataracthealth outcome
Clouding of the lens of the eye, the leading cause of blindness worldwide and treatable only by surgery. It reaches this vocabulary through the smoke of household cooking fires, which the eye meets directly and daily.
G-0117Charcoalsource
Wood that has been charred to drive off its volatile fraction, then burned as fuel. It smokes far less than the wood it came from, which is why it is preferred indoors — and it produces more carbon monoxide per unit of heat, which is why that preference is dangerous.
G-0017Chronic (long-term) toxicityhealth outcome
Effects resulting from long-term exposure, including damage to the central nervous system, the kidneys or the liver.
G-0180Chronic bronchitishealth outcome
Cough with sputum on most days for at least three months in each of two consecutive years. The definition is a symptom definition, which is why it can be established by questionnaire in populations where lung function testing is not available.
G-0018Chronic obstructive pulmonary disease (COPD)health outcome
A long-term lung condition characterised by persistently obstructed airflow; indoor air quality is one of the environmental factors relevant to it.
G-0128Clean fuelsintervention
Household fuels and electric appliances that produce little or no particulate emission at the point of use — electricity, liquefied petroleum gas, piped natural gas, biogas, ethanol and solar. The category is defined by what reaches the room, not by how the energy was produced.
G-0101Cleaning productssource
Products used to clean indoor surfaces, floors, bathrooms and glass — sprays, liquids, wipes and powders — considered here as a source of what they release into the air of the room while they are used and while they dry.
G-0116Coalsource
A fossil solid fuel burned indoors for cooking and heating, often in stoves that vent poorly or not at all. Besides the particles and carbon monoxide common to any solid fuel, coal carries sulfur and trace elements that the fuel itself contributes.
G-0158Cognitive dysfunctionhealth outcome
Measurable impairment of memory, attention or executive function, short of dementia and detected by testing rather than by diagnosis. It covers both the transient decline seen within a stuffy room and the persistent decline studied in older people.
G-0059Cognitive performancehealth outcome
Attention, decision-making, memory and reaction time as measured by standardised tasks. Indoor CO₂ and volatile compounds at ordinary office and classroom levels measurably reduce it, and fine particles are linked to long-term cognitive decline.
G-0199Conjunctivitishealth outcome
Inflammation of the membrane covering the white of the eye and the inner eyelid, causing redness, discharge and itching. The allergic form is the one indoor air research meets, triggered by the same allergens that cause rhinitis.
G-0019Continuous and constant sourcessource
Sources that generate uniform emissions over time — linoleum, cork, wood finishes, or furniture emitting formaldehyde.
G-0064Cookingsource
The largest indoor source of particulate matter and nitrogen dioxide in most homes. Frying, grilling and gas burners release particles, aldehydes and nitrogen dioxide in bursts that can exceed outdoor levels tenfold.
G-0132Cookstovessource
The appliance in which household cooking fuel is burned, from a three-stone open fire to a manufactured stove with a chimney. In exposure research the stove, not the fuel, is often the thing that changes — and it is the stove that decides how completely the fuel burns.
G-0204Cosmeticssource
Products applied to the skin, hair and nails for appearance rather than cleaning. They are used close to the face, often as sprays, in the smallest and least ventilated room of the dwelling — which makes a small mass of product a substantial short exposure.
G-0182COVID-19health outcome
The disease caused by the SARS-CoV-2 coronavirus. Its importance in this vocabulary is not clinical: it is the event through which ventilation and air cleaning became building requirements rather than comfort preferences.

D

G-0097Dampness and water damagesource
Persistent moisture in a building — from leaks, rising damp, condensation on cold surfaces or a flood that was never fully dried. It is the precondition for mould and bacterial growth, and in health studies it is the exposure that is recorded, because it can be observed while the growth itself often cannot.
G-0149Depressionhealth outcome
Persistent low mood with loss of interest and function, measured in population research by validated symptom scales rather than by diagnosis. It appears in indoor air research through housing conditions that are unpleasant to live with and through pollutants that reach the brain.
G-0163Dermatitishealth outcome
Inflammation of the skin — redness, itching, scaling and, when severe, weeping or thickening. The term covers several conditions with different causes, and indoor air research meets it mainly as contact dermatitis and as the atopic form.
G-0020Diagnostic spirometermeasurement method
A device measuring the volume and speed of air a person can inhale and exhale, used to assess lung function.
G-0021DIRECTIVE 2004/42/CEstandard or body
EU directive of the European Parliament and of the Council limiting emissions of volatile organic compounds due to the use of organic solvents in paints and varnishes.
G-0179Disinfectionintervention
Inactivating micro-organisms in air or on surfaces rather than removing them. Indoors it is applied as ultraviolet light, as chemical fogging or as devices generating reactive species — and unlike filtration, it can create new pollutants while it works.
G-0077Dose–response relationshipmeasurement method
How the size of a health effect changes with the level of exposure. It is what turns an association into evidence of causation and what guideline values are derived from — the concentration at which effects begin, or the slope of risk per unit of exposure.
G-0156Dry eye diseasehealth outcome
A disorder of the tear film in which the eye is inadequately wetted, causing dryness, burning, grittiness and fluctuating vision. It is the eye complaint most consistently linked to indoor environments, because the tear film is exposed directly to room air.
G-0184Dyspnoea (breathlessness)health outcome
The sensation of laboured or uncomfortable breathing, reported by the person experiencing it and usually graded by how much exertion brings it on. It is a symptom, not a measurement, and it can be present with normal lung function and absent with abnormal.

E

G-0022Electrochemical gas sensorsmeasurement method
Sensors that detect gases through a chemical reaction producing an electrical signal. They feature low power consumption, quick response time, high sensitivity and a longer lifespan.
G-0023Electronic nicotine delivery systems (ENDS)source
Battery-powered devices that deliver nicotine by heating a liquid into an aerosol; a source of indoor airborne constituents.
G-0024Emissionsource
The release of a substance from a point or diffuse source into the atmosphere.
G-0111Emission test chamber methodmeasurement method
A laboratory method in which a specimen of a material is held in a chamber at a set temperature, humidity and air exchange rate, and the air leaving the chamber is analysed. The result is an emission rate or a chamber concentration at a stated age.
G-0092Endotoxinindoor environment factor
A component of the outer membrane of Gram-negative bacteria, carried indoors on dust particles rather than as a gas. It is measured in settled dust or airborne dust and is one of the few biological exposures with an established laboratory assay, which is why it appears in studies where mould and bacteria in general do not.
G-0025Environmental Protection Agency (EPA)standard or body
The United States federal agency responsible for environmental protection, and a widely referenced source of indoor air quality guidance.
G-0026European Chemicals Agency (ECHA)standard or body
The EU agency managing the technical and administrative aspects of chemicals regulation, including REACH.
G-0119Exhaled carbon monoxidehealth outcome
The concentration of carbon monoxide in a breath sample, used as a marker of how much a person has absorbed rather than of what a room contains. It reflects recent exposure from smoking, cooking or heating and falls over hours once the exposure stops.
G-0075Exposure assessmentmeasurement method
Estimating how much of a pollutant a person actually receives — concentration multiplied by time in each place, across a day or a lifetime. Because people spend about 90 % of their time indoors, indoor concentrations dominate personal exposure even for outdoor pollutants.
G-0208Eye irritationhealth outcome
Stinging, burning, grittiness or watering of the eyes reported by the person, without an allergic or infectious cause. It is one of the core symptoms of building-related complaints and among the most sensitive early signals indoor air gives.

F

G-0202Flame retardantsindoor environment factor
Substances added to furniture foam, electronics, insulation and textiles to slow ignition. Most are blended into the material rather than bound to it, so they migrate out over the product's life and are found in indoor air and, far more, in house dust.
G-0094Flooringsource
The largest continuous surface in a room, and therefore the source with the greatest area. Resin-bonded boards, adhesives, backings and finishes release formaldehyde and volatile organic compounds; textile floors also hold dust, allergens and semi-volatile compounds and release them when disturbed.
G-0008Formaldehyde (CH₂O)indoor environment factor
A colourless gas with a sharp smell, classified as a human carcinogen. Indoors it is released for months to years by pressed-wood furniture, laminate flooring, adhesives, textiles and combustion, making it the most important single volatile organic compound in homes.
G-0177Fractional exhaled nitric oxide (FeNO)health outcome
The concentration of nitric oxide in a standardised exhaled breath, measured in parts per billion. It rises with eosinophilic inflammation of the airway, which makes it a non-invasive marker of the response an exposure has provoked.
G-0133Fungal sporesindoor environment factor
The reproductive particles fungi release into the air, typically a few micrometres across and small enough to stay aloft and to be inhaled. They are the countable form of fungal presence indoors, and they remain allergenic whether the spore is alive or dead.

G

G-0124Gas stovessource
A cooking appliance burning natural gas or liquefied petroleum gas at an open flame inside the room. Unlike a boiler, it has no flue: everything the flame produces, and anything the fitting leaks, enters the kitchen directly.
G-0130Gestational diabeteshealth outcome
Glucose intolerance first recognised during pregnancy, screened for routinely in most health systems. Its appearance in indoor air research follows from the same inflammatory and metabolic pathways through which air pollution is linked to diabetes outside pregnancy.

H

G-0027Harmful and potentially harmful constituents (HPHCs)source
Chemicals or chemical compounds in tobacco products and their smoke or aerosol that cause or may cause harm.
G-0028Hazardous air pollutants (HAPs)standard or body
Air pollutants known or suspected to cause cancer or other serious health effects.
G-0166Headachehealth outcome
Pain in the head, reported by the person who has it. In indoor environment research it is one of the core non-specific symptoms — common, easily recalled, and with so many other causes that it carries weight only through comparison.
G-0157Health educationintervention
Changing what occupants do, rather than what the building or the appliance does — teaching when to ventilate, where to cook, how to use a hood or a stove. It is the cheapest indoor intervention and the one whose effect depends entirely on whether the behaviour persists.
G-0076Health risk assessmentmeasurement method
The formal procedure — hazard identification, dose–response, exposure assessment, risk characterisation — by which agencies turn study results into limits and by which the risk of a measured concentration is expressed as a hazard quotient or an excess lifetime cancer risk.
G-0195Heatingsource
Keeping a dwelling warm, considered as an indoor air question. Every heating choice sets three things at once: whether combustion happens in the room, how much the building is ventilated while it is cold, and how warm and dry the surfaces are.
G-0196Hospitalisationhealth outcome
Admission to hospital, counted as an event. It is not a disease but a record of one severe enough to need inpatient care, which is what makes it available in administrative data for whole populations and useful as a hard endpoint.
G-0084House dust mitessource
Microscopic arachnids that live in bedding, carpets and upholstery, feeding on shed skin. Their faecal particles are the most common indoor allergen and a major trigger of asthma and rhinitis; they need relative humidity above about 50 % to thrive.
G-0088HVAC systemsintervention
Heating, ventilation and air-conditioning — the mechanical systems that condition and move air in buildings. Well designed and maintained, they deliver filtered outdoor air at a known rate; neglected, they recirculate stale air, harbour mould and become a pollutant source themselves.
G-0129Hypertensionhealth outcome
Blood pressure persistently above the threshold at which treatment is advised. It is included here because it is the mechanism through which several indoor exposures reach the heart and the brain, and because it is measured often enough to be studied in households.

I

G-0114Improved cookstovesintervention
A stove designed to burn solid fuel more completely or to vent its smoke outside, replacing an open fire or a simple stove. The improvement is judged by what reaches the room, not by what the design promises in a laboratory.
G-0082Indoor environmental quality (IEQ)measurement method
The umbrella concept covering air quality, thermal comfort, lighting and acoustics together — everything about a space that affects the people in it. Used in building certification (WELL, LEED, BREEAM) and in research that asks how the factors interact.
G-0093Indoor fuel combustionsource
Burning fuel inside a dwelling for cooking or heating — wood, coal, charcoal, dung, crop residue, kerosene or gas — with or without a flue. It is the dominant source of indoor particles and combustion gases in households that rely on solid fuels, and a measurable one in kitchens with gas appliances.
G-0071Indoor plants (phytoremediation)intervention
Houseplants can absorb volatile organic compounds in sealed laboratory chambers, but in real rooms the effect on air quality is negligible — hundreds of plants per room would be needed to match ordinary ventilation.
G-0049Indoor temperatureindoor environment factor
The air temperature of a room — the parameter people notice first and the one that shapes every other factor: it drives chemical emissions from materials, sets relative humidity, and decides whether windows get opened.
G-0185Infertilityhealth outcome
Failure to conceive after twelve months of regular unprotected intercourse. It is defined by time rather than by a mechanism, so it covers male and female causes together and is the endpoint that population studies of reproductive exposures can actually count.
G-0074Infiltrationmeasurement method
Uncontrolled leakage of outdoor air into a building through cracks, joints and gaps, driven by wind and temperature difference. It ventilates leaky old buildings by accident and lets outdoor pollutants — and radon from the ground — in.
G-0142Insecticidessource
Products applied indoors to kill insects — sprays, coils, plug-in vaporisers, treated nets and residual surface treatments. They are among the few indoor sources applied deliberately to the air of an occupied room, and the residue outlasts the application by months.
G-0029International Agency for Research on Cancer (IARC)standard or body
The World Health Organization agency that evaluates and classifies the carcinogenicity of substances and exposures.
G-0087ISO 16000 seriesstandard or body
The international standard series for indoor air measurement: sampling strategy, and reference methods for formaldehyde, volatile organic compounds, particulate matter, radon, mould and emission testing of building products in test chambers.

K

G-0153Kerosenesource
A liquid petroleum fuel burned indoors in wick lamps, stoves and portable heaters. It was long counted as a step up from wood, and the measurements have since shown that a kerosene wick burning in a room is a combustion source in its own right.

L

G-0090Laminate flooringsource
A floor covering of a printed decorative layer over a high-density fibreboard core, click-fitted over an underlay. The core is bonded with formaldehyde resins and the surface layers with adhesives, so a newly laid floor is one of the largest single emitting surfaces in a room.
G-0146Limoneneindoor environment factor
The terpene that smells of citrus peel, and the single most abundant fragrance compound in indoor air. It is used in cleaning products, air fresheners and cosmetics, and it reacts with ozone indoors to make compounds that were not in the product.
G-0121Liquefied petroleum gas (LPG)source
Propane and butane stored under pressure as a liquid and burned as a gas, supplied in cylinders where there is no piped network. In the fuel ladder it is the usual first step away from solid fuel, and in most studies it is the comparison group.
G-0145Low birth weighthealth outcome
A birth weight below 2500 g, whatever the cause. It combines two different problems — a baby born too early and a baby born too small for its age — which is why it is easy to measure everywhere and harder to interpret than either of them alone.
G-0079Low-cost sensorsmeasurement method
Compact optical, electrochemical and metal-oxide sensors costing tens rather than thousands of euros, which have made continuous monitoring in homes and schools possible. Their strength is time resolution and coverage; their weakness is accuracy without calibration.
G-0103Low-emission materialsintervention
Building products and furnishings selected because they release less into indoor air than the conventional alternative — boards, adhesives, paints, floor coverings and wallpapers certified against an emission class or a labelling scheme measured in a test chamber.
G-0159Lower respiratory infections (LRI)health outcome
Infections below the larynx — bronchitis, bronchiolitis and pneumonia — as opposed to colds and sore throats above it. The distinction matters because the lower tract is where inhaled particles deposit and where household smoke exposure acts.
G-0053Lung cancerhealth outcome
The cancer most clearly linked to indoor air: radon is its second leading cause after smoking, and secondhand tobacco smoke, coal and biomass smoke from cooking and heating, and fine particulate matter all raise the risk.
G-0058Lung functionhealth outcome
How much air the lungs can move and how fast — measured by spirometry as forced vital capacity and forced expiratory volume in one second. Reduced lung function is the most common objective marker of harm from indoor and outdoor air pollution.

M

G-0160Mass spectrometrymeasurement method
An analytical method that ionises molecules and sorts the ions by mass, giving each compound a pattern that identifies it. Coupled to a separation step, it is what turns a sample of indoor air into a list of named compounds with concentrations.
G-0030Measuring Instrument Directive (MID) 2004/22/ECstandard or body
EU directive harmonising the requirements placed on measuring instruments placed on the market.
G-0126Mechanical ventilationintervention
Supplying or extracting air with a fan rather than relying on wind and temperature difference. Because the flow is driven, it can be sized, filtered, balanced and measured — and it stops when the fan stops or when an occupant switches it off.
G-0078Meta-analysis and cohort studiesmeasurement method
The study designs behind most indoor air evidence: cohort studies follow groups over years to link measured exposure with later disease; meta-analyses pool many studies into one estimate with a confidence interval, and are the highest tier of observational evidence.
G-0031Metal oxide sensorsmeasurement method
Gas sensors whose semiconducting metal-oxide layer changes electrical resistance in the presence of target gases.
G-0032Micronmeasurement method
A micrometre — one millionth of a metre. The unit in which particulate matter sizes are expressed.
G-0098Microplasticsindoor environment factor
Plastic particles below 5 mm, and the fibres shed by synthetic textiles, that occur in indoor air and settled dust. Indoor concentrations are typically higher than outdoors because the sources — clothing, furnishings, floor coverings and packaging — are inside with us.
G-0155Monoterpenesindoor environment factor
Terpenes built from two isoprene units, with ten carbon atoms — limonene, alpha-pinene, delta-3-carene and their relatives. They are the volatile fraction of conifer wood and of citrus and pine fragrances, and the subgroup indoor surveys most often report as a sum.
G-0006Mouldindoor environment factor
Fungi that grow on damp surfaces and release spores, fragments and volatile compounds into the air. Indoors it is a symptom of moisture — condensation, leaks or poor ventilation — and a recognised cause of respiratory symptoms, allergies and asthma.

N

G-0141Naphthaleneindoor environment factor
A two-ring aromatic hydrocarbon that is solid at room temperature yet volatile enough to reach substantial concentrations in air. Indoors it comes from mothballs and deodorant blocks, from combustion, and from building products containing coal tar.
G-0168Natural gassource
Methane delivered by pipe and burned in hobs, ovens, boilers and heaters. Counted as a clean fuel because it produces almost no particles, it remains a combustion source in the room whenever the appliance has no flue.
G-0099Natural ventilationintervention
Air exchange driven by wind and by temperature difference rather than by a fan — open windows, trickle vents and leakage through the building envelope. It is the cheapest way to remove what is already in the air, and the least controllable: the rate varies with weather, with the season and with whoever last closed the window.
G-0151Nicotineindoor environment factor
The alkaloid that makes tobacco addictive, and the most specific marker of tobacco smoke in indoor air. It is semi-volatile: it deposits on every surface in a room where smoking happens and returns to the air for months afterwards.
G-0188Nitric oxide (NO)indoor environment factor
A colourless gas produced by any high-temperature flame and by the body itself. Indoors it is the first product of combustion in air, and it converts to nitrogen dioxide over minutes — so the two are measured together and interpreted together.
G-0048Nitrogen dioxide (NO₂)indoor environment factor
A reddish-brown, irritant gas formed when fuel burns at high temperature. Indoors it comes chiefly from gas cooking and unvented gas or kerosene heaters; outdoors from traffic. It inflames the airways and is linked to childhood asthma.
G-0189Nitrogen oxides (NOₓ)indoor environment factor
Nitric oxide and nitrogen dioxide taken together, as the mixture that combustion actually produces. Instruments and regulations frequently report the sum, because the two interconvert in air and the split depends on how long ago the burning happened.
G-0033Non-dispersive infrared (NDIR) methodmeasurement method
A measurement technique that determines gas concentration — carbon dioxide above all — from the absorption of infrared light.
G-0034Non-methane volatile organic compounds (NMVOCs)indoor environment factor
Organic compounds other than methane that produce photochemical oxidants by reacting with nitrogen oxides under solar radiation.
G-0035Normal temperature and pressuremeasurement method
Reference conditions of 25 °C and 101.3 kPa, used so that measurements from different places remain comparable.

O

G-0183Obesityhealth outcome
Excess body fat sufficient to impair health, in adults usually defined by a body mass index at or above 30 kg/m². It appears in indoor air research both as a metabolic outcome and as the factor that modifies how strongly other exposures act.
G-0036Obstructive lung diseasehealth outcome
A group of conditions in which airflow out of the lungs is obstructed, asthma and COPD among them.
G-0174Optical particle countermeasurement method
An instrument that draws air past a light beam and registers each particle by the light it scatters, converting the signal to a size and a count. It is the principle behind nearly every real-time particle measurement made indoors, from research instruments to consumer sensors.
G-0037Organic compounds associated with particulate matter (POM)indoor environment factor
Organic compounds with a boiling point above 380 °C, which are therefore carried on particles rather than as free gas.
G-0038Organohalogen and organophosphorus compoundsindoor environment factor
Two families of compounds commonly used as flame retardants in furnishings and electronics.
G-0055Oxidative stresshealth outcome
An imbalance between reactive oxygen species and the body's antioxidant defences, damaging cells and tissues. It is the common biological mechanism through which particulate matter, ozone and many volatile compounds cause inflammation and disease.
G-0007Ozone (O₃)indoor environment factor
A reactive form of oxygen with three atoms instead of two. Beneficial in the stratosphere, at ground level it irritates the airways and eyes and reacts with indoor compounds to form further pollutants, formaldehyde and ultrafine particles among them.
G-0039Ozone precursors
Substances that lead to ozone formation: nitrogen oxides, non-methane volatile organic compounds, methane and carbon monoxide.

P

G-0139Paints and coatingssource
Liquid finishes applied to surfaces indoors — wall paints, varnishes, lacquers and stains. They emit heavily while wet and for days afterwards as the film cures, and a low-emission label describes the product in the can rather than the surface after application.
G-0001Particulate matter (PM₂.₅ and PM₁₀)indoor environment factor
Airborne particles small enough to stay suspended and be inhaled: PM₁₀ is 10 micrometres or less in diameter, PM₂.₅ is 2.5 micrometres or less. The smaller the particle, the deeper it travels into the lungs — and, for the finest fractions, into the bloodstream.
G-0040Passive infrared (PIR)measurement method
A sensing method that detects movement from changes in infrared radiation, commonly used for occupancy detection.
G-0080Passive samplingmeasurement method
Small badges or tubes that absorb a pollutant by diffusion over days to weeks, then go to a laboratory. No power, no noise — the standard way to measure average formaldehyde, nitrogen dioxide, benzene and radon in homes at scale.
G-0102Personal care productssource
Products applied to the body indoors — deodorants and antiperspirants, hairsprays, perfumes, creams and cosmetics — considered here as a source of the compounds they release into the air of the room in which they are used.
G-0070Photocatalytic oxidationintervention
Breaking down volatile organic compounds on a titanium-dioxide surface activated by ultraviolet or visible light. Promising in the laboratory; in real rooms it can produce formaldehyde and other by-products if oxidation is incomplete.
G-0050Phthalatesindoor environment factor
A family of plasticisers that make PVC flooring, cables, shower curtains and many consumer products flexible. They are semi-volatile: they migrate slowly into air and house dust, where they are inhaled and ingested, and several are endocrine disruptors.
G-0125Pneumoniahealth outcome
Infection of the lung tissue itself rather than of the airways above it, diagnosed clinically or radiologically. In children under five it is among the leading causes of death worldwide, and household air pollution is one of the exposures consistently associated with it.
G-0201Polychlorinated biphenyls (PCBs)indoor environment factor
A family of chlorinated industrial compounds banned decades ago and still present in buildings that were constructed while they were in use. They were put into sealants, paints and light fittings, and they leave those materials slowly into air and dust.
G-0164Polycyclic aromatic hydrocarbonsindoor environment factor
A family of compounds made of fused benzene rings, formed whenever organic material burns incompletely. Indoors they arrive with smoke from cooking, heating and tobacco, split between the gas phase and the particles according to how many rings they have.
G-0162Preterm birthhealth outcome
Birth before 37 completed weeks of gestation. It is the birth outcome with the clearest definition and the gravest consequences, and the one in which an environmental exposure would have to act on the timing of labour rather than on growth.
G-0123Printers and photocopierssource
Office equipment that heats toner or filament and releases particles and volatile compounds while it runs. Emission is intermittent and close to the person operating the machine, which makes exposure depend on where the device stands rather than on room averages.
G-0060Psoriasishealth outcome
A chronic immune-mediated skin disease with scaly, inflamed plaques. Emerging studies associate flares with air pollution — particulate matter and nitrogen dioxide — and with low indoor humidity, alongside the established triggers of stress, infection and smoking.
G-0144Psychological stresshealth outcome
Self-reported strain, tension or perceived lack of control, measured with validated scales. It enters indoor air research both as an outcome that poor environments produce and as a confounder that travels with the housing conditions being studied.

R

G-0047Radonindoor environment factor
A naturally occurring radioactive gas released from uranium in soil and rock that seeps into buildings through the ground. It is the second leading cause of lung cancer after smoking and the largest source of ionising radiation most people receive. Measured in becquerels per cubic metre.
G-0207Radon mitigationintervention
Building work that lowers indoor radon — most often by extracting soil gas from beneath the floor before it can enter, sometimes by sealing the entry paths or by changing how the building is ventilated. It is the one indoor measure with a legal duty behind it in many countries.
G-0194Radon progenyindoor environment factor
The short-lived radioactive elements formed when radon decays — polonium, lead and bismuth isotopes. Unlike radon itself they are solid, so they attach to airborne particles and to surfaces, and it is they, not the gas, that deliver the dose to the lung.
G-0137Range hoods and kitchen exhaustintervention
An extract fan over the cooking surface that captures the plume before it mixes into the room and discharges it outdoors. Capture, not airflow, is what decides its effect — and a hood that recirculates through a filter captures nothing.
G-0041Recurring intermittent sourcesource
A source that emits repeatedly but not continuously — cooking, for example. Room fragrances are an occasional intermittent source.
G-0005Relative humidityindoor environment factor
The amount of water vapour in the air expressed as a percentage of the maximum the air can hold at its current temperature. It is set by ventilation, moisture sources and temperature, and it is the variable that decides whether mould has a chance to grow.
G-0100Renovation and redecorationsource
Building work carried out in a home, school or office that is still in use — painting, laying new flooring, plasterboard, replacing furniture — together with the weeks afterwards during which the new surfaces release what they were made with.
G-0112Respiratory infectionshealth outcome
Infections of the airways and lungs, from the common cold and sinusitis to bronchiolitis and pneumonia. Indoors they are both transmitted through the air people share and made more likely by what else that air carries.
G-0061Respiratory symptomshealth outcome
Cough, wheeze, phlegm, shortness of breath and irritation of the nose and throat — the everyday complaints that questionnaires record, and the most sensitive indicator of an indoor air problem before any disease is diagnosed.
G-0172Respiratory tract diseaseshealth outcome
The whole group of conditions affecting the airways and lungs — asthma, chronic obstructive disease, infections and the rest. The category is used where a study reported respiratory disease as one outcome rather than naming which.

S

G-0191Semen qualityhealth outcome
The set of measurements made on a semen sample — concentration, total count, motility, morphology and volume — assessed against reference values. It is the standard way male fertility is described, and the level at which most environmental studies report.
G-0042Semi-volatile organic compounds (SVOC)indoor environment factor
Organic compounds with boiling points from 240–260 °C up to 380–400 °C.
G-0054Sick building syndromehealth outcome
A cluster of symptoms — headache, fatigue, irritated eyes, nose and throat, dry skin, difficulty concentrating — that occupants experience in a particular building and that ease after leaving it, without a single identifiable disease.
G-0104Siloxanesindoor environment factor
A family of silicon–oxygen compounds used in personal care products, sealants and polishes. The cyclic members are volatile enough to be measured in indoor air, where they are among the compounds most consistently found wherever cosmetics are used.
G-0171Skin diseaseshealth outcome
The group of conditions affecting the skin that indoor environment research treats together — eczema and dermatitis, urticaria, dryness and irritation. The group exists because studies often record a category rather than a diagnosis.
G-0198Sleep apnoeahealth outcome
Repeated interruption of breathing during sleep, most often because the upper airway collapses. It is diagnosed by counting events per hour of sleep, and it belongs here because the airway that collapses is also the airway that indoor air inflames.
G-0197Sleep durationhealth outcome
How long a person sleeps, measured by diary, questionnaire or a wrist-worn device. It is included here because the bedroom is where people spend the longest continuous period in one indoor environment, with the least control over it.
G-0148Small for gestational age (SGA)health outcome
A birth weight below the tenth percentile for the length of the pregnancy, in a reference population. Unlike low birth weight, it separates growth from prematurity: the baby is small for the time it had, not merely small.
G-0113Solid fuelsource
Any fuel burned in the home in solid form — wood, crop residue, dung, charcoal and coal. The category groups them because what decides the exposure is the way a solid burns in a simple appliance, not the difference between one solid and another.
G-0072Source controlintervention
Removing or reducing the source of a pollutant — no smoking indoors, an extractor hood, low-emission materials, fixing damp — rather than diluting or filtering the air afterwards. It is the first and most cost-effective step in every indoor air guideline.
G-0176Sperm counthealth outcome
The number of sperm in a semen sample, reported as a concentration per millilitre or as a total per ejaculate. It is the oldest and most reported measure of male fertility, and the one that reflects what happened during the roughly three months the sperm took to form.
G-0165Sperm motilityhealth outcome
The proportion of sperm in a sample that are moving, and the vigour with which they move, assessed under a microscope or by computer-assisted analysis. It is one of the standard semen parameters and the one most sensitive to recent exposure.
G-0209Stillbirthhealth outcome
The death of a baby before birth, after a threshold of gestational age that differs between countries — commonly 22 or 28 weeks. It is the gravest birth outcome and the one whose definition most needs recording alongside any figure.
G-0173Strokehealth outcome
Sudden loss of brain function caused by interruption of its blood supply or by bleeding into it. It enters indoor air research as one of the two hard cardiovascular endpoints, reached through the same vascular pathways as heart disease.
G-0203Styreneindoor environment factor
An aromatic volatile compound and the monomer of polystyrene. Indoors it comes from plastics and resins that retain unreacted monomer, from combustion and tobacco smoke, and from heated polymers such as the filament of a desktop printer.
G-0134Sulfur dioxide (SO₂)indoor environment factor
A sharp-smelling, water-soluble gas formed when fuel containing sulfur is burned. Indoors it comes from coal and from some heating oils and kerosene; where those are not used, indoor concentrations simply follow whatever comes in from outside.
G-0043Suspended particulate matter (SPM)indoor environment factor
Solid and liquid particles small enough to remain suspended in air rather than settling out.

T

G-0138Terpenesindoor environment factor
A large family of volatile compounds built from isoprene units, made by plants and by the wood, oils and fragrances taken from them. They are the smell of citrus, pine and lavender, and indoors they are among the most abundant volatile organic compounds present.
G-0081Thermal comfortmeasurement method
The state of mind that expresses satisfaction with the thermal environment — a function of air temperature, radiant temperature, humidity, air movement, clothing and activity. Standardised by ASHRAE 55 and ISO 7730, and the reason people open or close windows.
G-0089Thoron (radon-220)indoor environment factor
A short-lived isotope of radon released from thorium in soil and, notably, from building materials such as clay, earth and some gypsum. Its 56-second half-life means it comes from surfaces close by — walls rather than the ground — and it is often overlooked in radon surveys.
G-0065Tobacco smoke (secondhand smoke)source
The mixture of smoke exhaled by smokers and released from burning tobacco. It contains thousands of compounds — fine particles, carbon monoxide, benzene, formaldehyde, nicotine — and is an established cause of lung cancer, heart disease and childhood asthma in non-smokers.
G-0091Toluene (C₇H₈)indoor environment factor
A clear aromatic solvent used in paints, adhesives, printing inks and cleaning products, and released by tobacco smoke and traffic. Indoors it is one of the most consistently detected volatile organic compounds, usually measured together with benzene, ethylbenzene and the xylenes.
G-0044Total suspended particles (TSP)indoor environment factor
The total mass of all airborne particles in a sample, regardless of size.
G-0109Total volatile organic compounds (TVOC)indoor environment factor
A single summed figure for the volatile organic compounds present in air, obtained by adding the responses measured across a defined range. It indicates the overall load rather than any one compound, and its value depends on the method that produced it.
G-0190Traffic-related air pollutionsource
The mixture emitted by road vehicles — combustion particles, nitrogen oxides, and the dust raised from brakes, tyres and road surface. It is an outdoor source that becomes an indoor exposure through the building envelope, and for many urban dwellings it is the dominant one.
G-0143Tuberculosishealth outcome
A bacterial infection of the lung transmitted between people through the air, and one of the few indoor questions where the pathogen, the ventilation of the room and the susceptibility of the host are all part of the same evidence.
G-0175Type 2 diabeteshealth outcome
Raised blood glucose arising from resistance to insulin and a failing insulin response, diagnosed by blood testing. It is the metabolic endpoint through which air pollution research has extended beyond the lung and the heart.

U

G-0045Ultrafine particles (UFP)indoor environment factor
Particles smaller than 0.1 micrometre. Their small size lets them travel deepest into the respiratory system.
G-0211Under-five mortalityhealth outcome
Death before the fifth birthday, counted per thousand live births. It is the summary measure by which child health is judged worldwide, and the endpoint through which household air pollution enters the global burden estimates.
G-0136Unvented heaterssource
Heating appliances burning gas, kerosene or solid fuel that discharge their combustion products into the room instead of through a flue. Everything the flame makes stays indoors, and the appliance runs for hours at a time in cold weather with the windows shut.

V

G-0067Ventilationintervention
Replacing indoor air with outdoor air, by opening windows (natural ventilation) or by fans and ducts (mechanical ventilation). It is the single most effective measure against the pollutants people and buildings emit, and carbon dioxide is its indicator.
G-0046Very volatile organic compounds (VVOCs)indoor environment factor
Organic compounds with boiling points from below 0 °C up to 50–100 °C.
G-0193Vinyl flooringsource
Floor covering made of polyvinyl chloride, supplied as sheet, tile or plank. Because the polymer is rigid on its own, it is softened with plasticisers that are not chemically bound to it — which is what makes the floor a long-term source rather than a surface.
G-0002Volatile organic compounds (VOCs)indoor environment factor
Carbon-based chemicals that evaporate easily at room temperature — formally, compounds with a vapour pressure of at least 0.01 kPa at 20 °C. Indoors they come from paints, furniture, cleaning products, cosmetics and people, and are measured collectively as total VOCs (TVOC).

W

G-0127Water pipe smokingsource
Smoking tobacco through a vessel of water, using charcoal as the heat source. The session lasts far longer than a cigarette and adds a second combustion source, so the indoor exposure differs from cigarette smoking in both size and composition.
G-0105Wheezehealth outcome
A high-pitched whistling sound made during breathing, usually on breathing out, caused by air moving through narrowed airways. It is a symptom, not a diagnosis: it occurs in asthma, in infections and in other conditions, and in young children it often occurs without either.
G-0122Wildfire smokesource
The mixture of particles and gases from burning vegetation and, where fires reach settlements, from burning buildings and their contents. It is an outdoor event that becomes an indoor exposure, because smoke enters every building that is not sealed against it.
G-0115Woodsource
Wood indoors is two different sources at once: a fuel that is burned, and a material that stands in the room as structure, panelling, flooring and furniture. Burning releases combustion products; standing releases what the wood and its treatment contain.
G-0063Wood-based panels (particleboard, MDF, plywood)source
Boards made of wood particles or fibres glued with resin — the material of most furniture, kitchens and laminate floors. Urea-formaldehyde resins make them the dominant formaldehyde source in homes.
G-0085World Health Organization (WHO)standard or body
The United Nations health agency whose air quality guidelines are the global health-based reference: the 2021 global air quality guidelines (particulate matter, ozone, nitrogen dioxide, sulfur dioxide, carbon monoxide), the 2010 indoor air quality guidelines for selected pollutants, and the 2009 guidelines on dampness and mould.

X

G-0147Xyleneindoor environment factor
An aromatic solvent supplied as a mixture of three isomers, used in paints, adhesives, varnishes and printing inks. Indoors it is one of the group measured together with benzene, toluene and ethylbenzene as a marker of solvent-borne products and of traffic.