The air inside a building is shaped largely by what the building is made of. I find this genuinely interesting, and a little unsettling, because most people assume indoor air is cleaner than outdoor air. The chemistry says otherwise.

During my training in indoor environment science I looked at how the indoor environment is affected by the compounds that buildings and their contents release over time. What surprised me most was not the obvious sources like paint and cleaning products. It was the materials themselves, the walls, the floors, the structural panels sitting quietly in the background, slowly releasing compounds into the air you breathe at home.

Let me walk you through what the chemistry actually shows.

 

What Your Building Is Releasing Into the Air

Volatile organic compounds are carbon-based molecules that evaporate readily at room temperature. Every building releases them, not just when freshly built, but for months and sometimes years after construction.

The sources are wider than most people realise. Pressed wood products like particleboard and medium-density fibreboard, MDF, release formaldehyde from the urea-formaldehyde resins used as binders. Paints and varnishes release glycol ethers, aromatic hydrocarbons, and aldehydes. Adhesives used in flooring release toluene and xylene. Vinyl flooring off-gases plasticisers. Carpets release 4-phenylcyclohexene, which is the compound behind that distinctive new carpet smell most of us recognise immediately.

The reason concentrations build up indoors is simple. Buildings restrict the airflow that would otherwise dilute these compounds outdoors. Studies consistently find indoor VOC concentrations higher than outdoor concentrations, sometimes significantly so, particularly in newly built or recently renovated spaces.

Formaldehyde deserves its own mention here because it shows up so often and its chemistry is specific. It is a small, highly reactive aldehyde that interacts with proteins in biological tissue. The International Agency for Research on Cancer classifies it as a group 1 carcinogen. The main source in buildings is not paint or cleaning products but pressed wood, the particleboard in your kitchen cabinets, the MDF in your shelving, the flooring underlay. That resin off-gases slowly over time, with emission rates highest when the material is new and declining over months to years as the reservoir depletes.

 

 

 

Why the Material Chemistry Makes All the Difference

The honest distinction between a conventional building material and a genuinely eco-friendly one often comes down to binder chemistry and finish chemistry. Not the branding on the packet.

Conventional particleboard uses urea-formaldehyde resin because it is cheap, bonds well, and is water-resistant. The trade-off is a slow, steady formaldehyde release that peaks early and tails off but does not stop quickly. Low-emission alternatives use phenol-formaldehyde resins, which form more stable cross-links and release far less formaldehyde, or non-formaldehyde binders altogether.

Natural materials like solid timber, bamboo, Bambusoideae subfamily, rammed earth, and cork do not use these binders. Their off-gassing profile is different. I find solid timber interesting on this front because it does release terpenes, particularly softwoods like Scots pine, Pinus sylvestris, from resin canals in the wood. Those terpenes are not inert. They can react with indoor ozone to form secondary compounds including formaldehyde and ultrafine particles. So the picture is always more nuanced than “natural equals safe.” The concentrations matter, the ventilation matters, and the wood species matters.

 

The Difference Between Real Low-VOC and Greenwash

This is the part I want people to pay attention to because the marketing language here is genuinely slippery.

Low-VOC paint is a real category. VOC content in paints is regulated in most markets and the claim is measurable and verifiable. A paint meeting low-VOC limits does genuinely contain fewer volatile organic compounds by mass than a conventional paint. That is real.

The complication is that some biocides and preservatives added to water-based paints to extend shelf life are not classified as VOCs for regulatory purposes, but they do evaporate and they do have biological activity. Isothiazolinones used as preservatives are one example. Not classified as VOCs by standard definitions, but reactive, and they have been associated with sensitisation. This does not mean low-VOC paints are unsafe. It means the label answers one specific question and does not answer all the others.

Certifications like FSC from the Forest Stewardship Council and independently assessed schemes like Cradle to Cradle are more reliable than self-declared eco labels on product packaging. They are not perfect but they involve third-party verification rather than the manufacturer rating their own product.

 

What the Natural Materials Actually Do

I find hempcrete particularly interesting from a chemistry point of view. It is made from the woody core of Cannabis sativa stems mixed with lime. The lime binder, calcium hydroxide, carbonates slowly over time as it absorbs CO2 from indoor air, converting to calcium carbonate. That process sequesters carbon within the wall material itself. The hemp component also fixed carbon during plant growth. The net carbon balance over a building’s lifetime is more complicated than the marketing suggests, production and transport have real costs, but the carbonation chemistry happening inside the wall is genuine and measurable.

Rammed earth uses no binder chemistry at all. Compacted layers of subsoil are stable through mechanical interlocking and the cohesive properties of clay minerals. No synthetic resins, no formaldehyde reservoir, essentially zero VOC off-gassing. I appreciate the elegance of that from a materials science perspective.

Straw bale construction, using dried cereal stalks like wheat, Triticum aestivum, provides excellent thermal insulation with very low embodied VOC content. The main concern in practice is moisture, since damp straw supports microbial growth. But a well-built, dry straw bale wall has a genuinely benign indoor air chemistry profile.

 

 

 

Ventilation Is the Other Half of the Equation

Here is something I think gets overlooked. Material choice reduces how much is being released into your indoor air. Ventilation determines how much of that builds up.

The two work together, and you need both. In a well-ventilated building, even materials with moderate off-gassing rates produce lower indoor concentrations than a poorly ventilated building using low-emission materials. The ventilation rate, measured as air changes per hour, determines how quickly the indoor air pool turns over.

Modern energy-efficient buildings are often tightly sealed to reduce heating and cooling loads. That is the right instinct from an energy perspective, but it creates a tension. Lower air exchange rates allow VOC concentrations to build higher from the same emission source. Heat recovery ventilation systems address this directly by exchanging indoor and outdoor air while recovering most of the thermal energy, keeping ventilation rate and energy efficiency both acceptable at the same time.

The chemistry is the same regardless of the building’s energy performance. Compounds released from materials accumulate in proportion to their emission rate and in inverse proportion to the ventilation rate. Reduce the emission rate, increase the ventilation, and you get better indoor air. Both levers work.

 

Questions You May Have

What are VOCs and why do they build up indoors?

Volatile organic compounds are carbon-based molecules that evaporate at room temperature. Indoors they accumulate because buildings restrict the airflow that would dilute them outdoors. Concentrations can be significantly higher inside than outside, particularly in new or recently renovated buildings.

What is the main source of formaldehyde in buildings?

The largest source is usually pressed wood products, particleboard and MDF, which use urea-formaldehyde resin as a binder. The resin releases formaldehyde slowly over time, highest when the material is new and declining over months to years.

Are natural building materials always better for indoor air?

Not automatically. Solid timber releases terpenes that can react with indoor ozone to form secondary compounds. The specific material, its finish, and the ventilation rate all shape the outcome. Natural materials generally have a lower VOC profile than synthetic alternatives but they are not zero-emission.

What does low-VOC paint actually mean?

It means the paint contains fewer volatile organic compounds by mass than conventional paint, measured against regulatory limits. The claim is verifiable. The complication is that some preservatives in water-based paints are not classified as VOCs but do evaporate and have biological activity.

How does hempcrete absorb carbon?

The lime binder carbonates over time, absorbing CO2 and converting calcium hydroxide to calcium carbonate. The hemp component also fixed carbon during plant growth. The net balance depends on the full production and transport picture, but the carbonation chemistry within the wall is real.

What is rammed earth and why does it off-gas so little?

Rammed earth uses compacted subsoil with no synthetic binders. Stability comes from mechanical interlocking of soil particles and clay mineral cohesion. Without synthetic resins there is no formaldehyde reservoir depleting over time.

Does bamboo off-gas VOCs?

Solid bamboo has a low VOC profile. Bamboo composite products that use adhesive binders can off-gas similarly to other bonded wood products depending on the binder chemistry. The binder type matters more than the bamboo label.

How does ventilation affect indoor VOC concentrations?

Higher ventilation dilutes compounds faster. In tightly sealed energy-efficient buildings, lower air exchange rates allow VOC concentrations to build higher from the same source strength. This makes material choice more important in those buildings, not less.

What certifications are worth looking for?

FSC covers responsible forest management for timber. LEED and Cradle to Cradle assess broader environmental performance including emissions. Independently verified schemes are more reliable than self-declared eco labels on product packaging.