Volatile organic compounds (VOCs) are carbon-based chemicals that evaporate readily at room temperature, releasing into the air ('off-gassing'). Hundreds are present in typical indoor air, from the smell of fresh paint to the 'new car' or 'new furniture' smell. Indoor VOC levels are often several times higher than outdoors because the sources are inside and ventilation is limited.
Common indoor sources
| Source | Typical VOCs | Pattern |
|---|---|---|
| Paints, varnishes, solvents | Toluene, xylene, white spirit | High when fresh, decays over days/weeks |
| Engineered wood (MDF, chipboard, plywood) | Formaldehyde | Ongoing off-gassing, worse when new/warm |
| New furniture, carpets, foams | Mixed VOCs, formaldehyde | Highest when new; 'new furniture' smell |
| Cleaning products, air fresheners | Limonene, terpenes, alcohols | Spikes during/after use |
| Adhesives, sealants, flooring | Various solvents | High during/after installation |
Formaldehyde — the one to watch
Formaldehyde is a specific VOC of concern: emitted by the resins in MDF, chipboard, plywood and some insulation and furnishings, it's an irritant and a recognised carcinogen at sufficient exposure. The WHO sets a short-term guideline of around 0.1 mg/m³ (0.08 ppm). Off-gassing is highest when materials are new and is accelerated by heat and humidity, easing over time.
Health effects
At typical indoor levels, VOCs are associated with the 'sick building' symptoms: eye, nose and throat irritation, headaches, and aggravation of asthma and allergies, with sensitive individuals affected more. Some specific VOCs (formaldehyde, benzene) carry more serious long-term risks. Because the mix is complex and individual sensitivities vary, the sensible strategy is precaution: reduce the sources and ventilate well.
What TVOC readings can and can't tell you
Controlling VOCs — source first, then ventilation
- Source control — choose low-VOC paints, finishes and adhesives, and low-formaldehyde (E1/E0 or formaldehyde-free) engineered wood and furnishings. This is by far the most effective step, because it removes the emission at source.
- Air out new materials — let new furniture, flooring and decoration off-gas with extra ventilation when first installed, when emissions are highest.
- Ventilate continuously — background and mechanical ventilation dilute the steady, low-level off-gassing of materials already in place; MVHR provides continuous fresh air to keep levels down.
- Manage activities — ventilate during and after painting, cleaning and any solvent use; reconsider routine air fresheners and scented candles, which add VOCs rather than removing them.
Choosing low-emitting materials — the labels that matter
Because source control is the most effective lever, it pays to know the labelling that identifies low-emitting products. For engineered wood, the formaldehyde-emission classes E1 and the stricter E0 (and 'formaldehyde-free' or no-added-formaldehyde boards) indicate progressively lower off-gassing — worth specifying for the MDF, chipboard and plywood that make up so much modern furniture and joinery. For paints, finishes and adhesives, look for low- or zero-VOC formulations; the EU Decopaint Directive limits VOC content and most reputable paints now state their VOC level on the tin. Independent emissions labels — such as the French A+ rating, the German Blue Angel and Eurofins/GUI Indoor Air Comfort — test the finished product's emissions over time and are a more reliable guide than vague 'eco' marketing. The Passive House approach goes further, treating low-emitting material selection as part of designing healthy indoor air from the outset, rather than ventilating away an avoidable problem after the fact.
Beyond VOCs — the other gaseous pollutants
VOCs are not the only invisible gases worth understanding. Nitrogen dioxide (NO₂) is produced indoors by gas cooking and any unflued combustion, and infiltrates from traffic outdoors; it irritates the airways and is a recognised concern in homes with gas hobs and poor kitchen extract — a strong argument for an externally-ducted cooker hood and, increasingly, for induction cooking. Radon, a naturally-occurring radioactive gas, seeps from the ground in certain geologies and is the second-leading cause of lung cancer after smoking; in affected areas it is managed by sub-floor ventilation or membranes rather than by indoor air filtration. Ozone is generated by some 'ionising' or ozone-producing air purifiers — a reason to avoid that type of device, since ozone is itself a respiratory irritant. The common thread is that gaseous pollutants are best dealt with at source (the right appliance, the right ground detailing, avoiding ozone-generating gadgets) and then diluted by good ventilation — the same hierarchy that governs VOCs.
