The World Health Organisation’s landmark review established health-based guidelines for nine key indoor pollutants, including formaldehyde, benzene, and nitrogen dioxide. The review draws on epidemiological, toxicological, and clinical evidence. It identifies indoor sources of these pollutants (primarily building materials, finishes, and furnishings) and sets concentration limits for both short-term and year-long exposure.

Multiple monitoring studies across European homes consistently find that indoor VOC concentrations exceed outdoor levels by a factor of 2–10, even in low-traffic rural areas. Synthetic paints, laminate flooring, MDF furniture, and adhesive-bonded products are the dominant emission sources. Concentrations are highest in newly built or recently renovated homes, but remain elevated for years as materials continue to off-gas.

Researchers at the Technical University of Denmark tested clay wall plaster alongside carpet as a VOC source, in the presence of ozone. Clay plaster acted as a passive removal material — absorbing ozone and reducing concentrations of aldehydes produced by ozone-carpet reactions. Panels of 24 human subjects assessed perceived air quality as measurably better in chambers containing clay plaster.

A cross-sectional study of 252 asthmatic adults found that VOC and formaldehyde emissions from newly painted wall surfaces were associated with worsening asthma symptoms, compared to 310 non-asthmatic controls. The study underlines why ventilation after painting is critical, and why zero-VOC or natural paint alternatives offer a real health benefit for occupants with pre-existing respiratory conditions.

The landmark DTU field study examined sleep in single-occupancy dormitory rooms under two ventilation conditions for one week each. When bedroom CO₂ dropped from an average of 2,395 ppm (closed room) to 835 ppm (ventilated), objectively measured sleep efficiency improved. Participants fell asleep faster, reported fresher air on waking, felt less sleepy during the day, and performed better on a logical thinking test. The researchers used actigraphy (wrist-worn movement trackers) rather than relying solely on self-report.

A single-blind field intervention in 50 actual Danish bedrooms during the heating season found that participants had less deep sleep, more light sleep, and more awakenings under low ventilation. Fan speeds were covertly altered so participants were unaware of the intervention, making this one of the more rigorous bedroom ventilation studies to date. CO₂ concentration served as the marker of ventilation quality.

Researchers compared the dynamic sorption properties of clay materials (rammed earth, unburned brick, clay plaster) against conventional materials (concrete, lime plaster, gypsum board) under controlled humidity conditions. After 8 hours, clay plaster reduced relative indoor humidity by 27%, compared to 17–20% for non-clay materials. Rammed earth performed best at 34%, followed by unburned brick at 30%. Clay materials showed a superior positive effect on rapid adsorption and desorption of indoor air moisture.

Arundel et al. established that the 40–60% relative humidity range is the optimal zone for indoor health. Below this range, respiratory mucous membranes dry out and viral survival increases. Above it, mould growth, dust mite proliferation, and chemical off-gassing all accelerate. This range is widely cited in building biology standards and underpins the IBN Baubiologie SBM humidity threshold values. Hygroscopic materials like clay and lime contribute to passive maintenance of this range without mechanical intervention.

A review from EPFL’s Laboratory of Construction and Architecture found that clay and lime plasters — due to their hygroscopic porous structure — absorb moisture when indoor relative humidity is high and release it as the air dries. This passive buffering effect reduces mould risk and improves indoor air quality, while also lowering the ventilation rates required for moisture removal, with potential energy savings. VOC emissions from eco-friendly plasters were comparable to or lower than conventional materials.

The WHO’s dedicated guidelines on indoor dampness and mould conclude that the evidence for health effects is “sufficient” across multiple outcomes — including respiratory symptoms, asthma development, allergic rhinitis, and respiratory infections. Poor ventilation and moisture-trapping building materials (including impermeable wall finishes and vinyl wallpaper) are identified as key risk factors. The populations most affected include children, the elderly, and those with existing respiratory conditions.

Children breathe at a higher rate relative to body weight than adults, spend more time on floors (where settled dust and chemical concentrations are highest), and have developing organ systems with reduced capacity to metabolise toxicants. Multiple studies confirm that children in homes with high VOC concentrations, dampness, or mould have elevated risk of developing asthma, allergic rhinitis, and eczema. The first seven years represent the highest-risk window for chemical exposure effects on respiratory health.

Epidemiological follow-up studies consistently find that early-life exposure to indoor mould — in bedrooms and nurseries especially — is one of the strongest independent predictors of asthma development by school age. The mechanism is believed to involve sensitisation of the developing immune system. Dampness-related exposures in the first year of life appear to carry the highest risk. The IBN SBM-2015 reflects this evidence by assigning children’s rooms a separate, more stringent set of evaluation thresholds across all three assessment pillars.