The study was conducted in a recently renovated public administration building in Luxembourg, the identity of which is not being made public. The site had undergone a complete renovation incorporating low-emission materials, in line with current environmental standards. The project was financed by the Luxembourg Ministry of Energy and Spatial Planning, via the Climate and Energy Fund, and carried out by a consortium involving Neobuild, the University of Liège, as well as several players specialising in environmental measurement and indoor greening systems.
For 46 weeks, the researchers monitored the air quality in this occupied building, comparing areas equipped with interior green walls to similar spaces without vegetation. The measurements covered several indoor air pollutants - including formaldehyde and other volatile organic compounds - as well as the presence of mould spores in suspension.
The main pollutant analysed was formaldehyde, an irritant and carcinogenic gas widely present in buildings, mainly emitted by wood panels, furniture, glues, paints and certain coatings. Even in buildings renovated with so-called “low-emission” materials, this pollutant is still frequently detected and is a key indicator of indoor air quality.
Up to 40% benefits
The results show that, formaldehyde concentrations are systematically lower in areas with green walls than in reference areas without green walls. Reductions of up to 40% were observed, depending on the areas studied. This effect was statistically significant and observed repeatedly over the duration of the study.
The authors do, however, stress that these results must be interpreted with caution. The building studied already had relatively low pollution levels, due to the use of low-emission materials and ventilation in line with recent standards. In this context, a reduction of 40% represents a significant relative reduction, but a more modest absolute reduction in real terms. The study therefore does not allow us to conclude that there has been a major change in the health risk, but rather a marginal improvement in an environment that is already under control.
The impact of green walls on other volatile organic compounds also appears to be more limited. No overall reduction in VOCs has been observed. On the other hand, certain specific families, notably aromatic compounds and ketones, show lower concentrations in the vicinity of green walls, mainly in confined or well-ventilated spaces. These effects remain closely linked to the configuration of the premises, the volume of the rooms and the ventilation strategy.
More fungi potentially
The study also highlights a biological point of vigilance. In several areas, the presence of green walls is associated with increased concentrations of mould spores in the air, linked to higher humidity and irrigation conditions. No toxic or pathogenic moulds were detected during the measurement campaign. However, the authors believe that inadequate maintenance could, in other contexts, accentuate this risk.
The researchers emphasise that green walls must be considered as living technical systems, requiring appropriate sizing, coherent integration with the building’s ventilation and regular maintenance. Without these conditions, the chemical benefits observed could be neutralised, or even counterbalanced, by undesirable biological effects.
In conclusion, the study shows that indoor green walls can be a complementary tool for improving certain air quality parameters, including in recent and already efficient public buildings. However, their effect remains moderate, highly context-dependent and insufficient on its own to justify an indoor air quality strategy.
For public building managers, the message is clear: green walls can bring measurable benefit, but they are no substitute for effective ventilation or a rigorous policy of materials selection and maintenance.
