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The hidden chemicals in the indoor air we breathe: understanding VOCs

Published Date 11/27/25 8:00 AM

Most people think of air pollution as something that happens outside, near factories, highways, or in big cities. But here’s the surprising truth: we spend around 90% of our time indoors, and the air inside our homes, offices, or classrooms can often be 2 to 5 times more polluted than outdoor air. From sleeping and studying to cooking and working, nearly every part of modern life happens inside buildings. That means indoor air quality directly influences our health, comfort, and performance every single day. When indoor air is clean and fresh, people feel more alert and comfortable, students learn better, workers are more productive and focused, and sick leave and allergies decrease.

The invisible threat: VOCs

Indoor air contains a mix of pollutants that come from everyday sources. The main indoor air pollutants include volatile organic compounds, or VOCs. Volatile organic compounds (VOCs) aren’t a single specific substance, but rather a large group of carbon-based chemicals that are easily emitted at room temperature as gases into the air from products and processes. Exposure to VOCs can cause a range of short-term and chronic health effects, including headaches, eye and respiratory tract irritation, and in some cases, organ toxicity or carcinogenicity. Because there are so many types of VOCs, they are often grouped together and monitored as TVOCs (total volatile organic compounds).

Major indoor VOCs and their key sources & health effects

Formaldehyde (aldehyde):

Released from pressed wood (MDF, particleboard, plywood), urea-formaldehyde resins, insulation, tobacco smoke, and wrinkle-resistant textiles.
Health effects: eye, nose, and throat irritation; classified as a known human carcinogen.

Benzene (aromatic hydrocarbon):

Found in tobacco smoke, paints, varnishes, solvents, stored fuels, and adhesives.
Health effects: neurological symptoms, bone marrow suppression, and carcinogenic (linked to leukaemia).

Toluene (aromatic hydrocarbon):

Present in paint thinners, lacquers, varnishes, nail polish removers, adhesives, and air fresheners.
Health effects: fatigue, confusion, and long-term neurological damage.

Xylenes (aromatic hydrocarbons):

Emitted from paints, varnishes, solvents, cleaning agents, and aerosol sprays.
Health effects: dizziness, eye and skin irritation, and potential liver and kidney damage.

Ethylbenzene (aromatic hydrocarbon):

Found in paints, coatings, tobacco smoke, and styrene-based building materials.
Health effects: throat irritation; chronic exposure affects the liver and kidneys.

Styrene (vinyl aromatic compound):

Emitted from plastics, foam insulation, carpets, and office equipment (printers, copiers).
Health effects: mucous membrane irritation; potential carcinogen.

Acetone (ketone):

Present in nail polish removers, paint thinners, and furniture polish.
Health effects: eye and throat irritation; headaches; nervous system effects.

Limonene (terpene):

Found in citrus-scented cleaners, air fresheners, and wax polishes.
Health effects: low toxicity but can form formaldehyde and organic aerosols when reacting with ozone.

Ethanol & isopropanol (alcohols):

Common in disinfectants, sanitisers, and cleaning sprays.
Health effects: respiratory irritation and drowsiness at high concentrations.

Terpenes (α-pinene, β-pinene, limonene):

Released from pine-scented cleaners, air fresheners, and essential oils.
Health effects: can form secondary pollutants such as formaldehyde and fine particles.

Dichlorobenzene (chlorinated aromatic hydrocarbon):

Present in mothballs, air fresheners, and toilet deodorisers.
Health effects: liver and kidney damage; potential carcinogen.

Collectively, these VOCs highlight the complexity of indoor chemical emissions and underscore the importance of source control, product selection, and adequate ventilation to reduce exposure and protect human health.

International guidelines and thresholds for indoor TVOC exposure

Organisation / Country	Guideline / Classification	TVOC Concentration Limit	Equivalent (approx.)	Interpretation / Category	Reference
World Health Organization (WHO) – Regional Office for Europe	Based on comfort and sensory irritation levels	<100–200 ppb	—	Harmless	WHO, 2000
	Based on comfort and sensory irritation levels	200–610 ppb	—	Tolerable only for short-term exposure	WHO, 2000
German Federal Environment Agency (UBA)	Hygienic evaluation bands	≤0.3 mg/m³	≈80 ppb	Hygienically safe	Umweltbundesamt, 2007
		0.3–1 mg/m³	≈80–260 ppb	Still acceptable
		1–3 mg/m³	≈260–780 ppb	Noticeable
		3–10 mg/m³	≈780–2600 ppb	Alarming
		>10 mg/m³	>2600 ppb	Unacceptable
Japan (Ministry of Health, Labor and Welfare – MHLW)	Provisional indoor target value	400 µg/m³	≈100 ppb	Recommended limit for residential settings	MHLW, 2002
China (GB 50325-2010)	National indoor air quality standard	≤0.5 mg/m³	≈130 ppb	Limit for residences, schools, and hospitals	MOHURD, 2010
China (GB 50325-2010)	National indoor air quality standard	≤0.6 mg/m³	≈160 ppb	Limit for offices and public buildings	MOHURD, 2010
United States (EPA)	Operational threshold (not formal standard	500 ppb	≈1.9 mg/m³	Indicator for elevated exposure events	U.S. EPA, 2017

Overall, these guidelines indicate that, while no universal standard exists, indoor TVOC concentrations below approximately 200–500 ppb are generally regarded as acceptable for health and comfort in non-industrial indoor environments.

A closer look: CAREL’s case study

To assess how volatile organic compounds (VOCs) behave under real-world conditions, TVOC concentration data from indoor air quality (IAQ) monitoring at CAREL Industries offices were analysed. The analysis compares the concentration and emission rates of TVOCs between two offices, referred to as Office 1 and Office 2. Office 1 is larger, with a volume of 553.5 m³, while Office 2 is considerably smaller, with a volume of 51.66 m³. Notably, an air freshener emitting fragrance was present in Office 2.

TVOC concentration over a week

The graph illustrates the time variation in TVOC concentrations in two offices (Office 1 and Office 2) from 1 September to 8 September 2025. During working hours, when the ventilation system was operating (grey-shaded areas), both offices had lower and more stable TVOC concentrations, highlighting the effectiveness of ventilation in reducing indoor pollutant levels at the workplace. In contrast, when the ventilation was turned off, TVOC concentrations gradually increased in both offices, with a markedly sharper rise observed in Office 2 due to the use of air freshener fragrances.

On weekdays, during nighttime periods without ventilation, TVOC concentrations exceeded 400 ppb in Office 1 and 1000 ppb in Office 2. Over the weekend, when the offices were unoccupied and ventilation remained off, TVOC levels spiked dramatically, surpassing 1200 ppb in Office 1 and approaching 2400 ppb in Office 2. Overall, Office 2 consistently recorded higher TVOC levels than Office 1, primarily due to emissions from air fresheners, in addition to contributions from furniture, cleaning products, and other sources. The estimated emission rates during non-ventilated periods (at night) ranged from 0.27 to 2.46 mg/min in Office 1 and from 0.62 to 19 mg/min in Office 2.

Ventilation made a significant difference. Once the system resumed operation, TVOC levels dropped below 400 ppb within 25–45 minutes, clearly demonstrating that fresh air is one of the most effective measures against indoor pollution. Maintaining adequate ventilation helps ensure a more comfortable, healthier, and productive working environment. The CAREL case study demonstrates how continuous monitoring and intelligent building systems can help identify hidden pollution sources and optimise ventilation schedules.

Lord Kelvin’s saying, “to measure is to know. If you cannot measure it, you cannot improve it”, perfectly captures the essence of indoor environmental quality (IEQ) monitoring. Measuring IEQ parameters—such as air temperature, humidity, CO₂, particulate matter, and VOCs—provides the necessary data to understand the indoor environment and identify sources of discomfort or health risks. Without continuous and accurate monitoring, it is impossible to detect poor air quality, evaluate the effectiveness of ventilation, or implement targeted improvements. Therefore, IEQ monitoring is fundamental for maintaining healthy, comfortable, and productive indoor spaces, enabling evidence-based decisions that enhance both occupant well-being and building performance.

Healthy air is not a luxury - it’s a necessity for a sustainable indoor environment. By combining data-driven insights with smart control systems and effective ventilation, we can enhance our indoor environmental quality.

References:

China Ministry of Housing and Urban-Rural Development. (2010). Code for indoor environmental pollution control of civil building engineering (GB 50325-2010). Beijing: China Architecture & Building Press.
Ministry of Health, Labour and Welfare [Japan]. (2002). Interim target value for total volatile organic compounds (TVOC) in indoor air. Tokyo: MHLW.
Umweltbundesamt (German Federal Environment Agency). (2007). Hygienic guide values for total volatile organic compounds (TVOC) in indoor air. Dessau-Roßlau, Germany: UBA
U.S. Environmental Protection Agency. (2017). Quality assurance handbook for air pollution measurement systems: Volume I – General procedures. Washington, DC: EPA Office of Air Quality Planning and Standards.
World Health Organization Regional Office for Europe. (2000). Air quality guidelines for Europe (2nd ed.). Copenhagen: WHO Regional Office for Europe. Available at: https://iris.who.int/server/api/core/bitstreams/7107999d-7e53-47aa-90e8-bb1d162ff46e/content