Outdoor green walls in heatwave-prone, polluted environments could be increasing pollution in office buildings and affecting worker health, new research from the University of York has found.

Concentrations of ultrafine particles – which are a health concern as they can carry potentially toxic species into the lungs – were simulated for offices in Athens, Helsinki and Milan during heatwave conditions and for typical summer conditions.

The researchers found indoor concentrations of UFPs were highest in the Milan and Athens offices, reflecting high outdoor air pollution levels in these cities (the pollutants can make their way indoors through doors, windows and ventilation systems, as well as through gaps in the building fabric).

Indoor UFP levels were predicted to be higher in heatwave conditions, however they were well above those expected through penetration of outdoor particles alone. Further investigation found that reactive volatile organic compounds emitted by plants and trees were to blame.

Once in the atmosphere, VOCs from plants rapidly oxidise to form a range of secondary gas and particle-phase products, the researchers said, which exist in a dynamic equilibrium. During heatwaves, emissions of VOCs increase.

When outdoor air is drawn into an office air inlet, it is often filtered to partially remove outdoor particles. However, removing these particles disturbs the equilibrium of the secondary products and in order to re-establish a balance, new particles quickly form once the air reaches the office environment.

Therefore, the researchers said, indoor UFP concentrations are seen to be much higher if reactive VOCs from greenery exist outdoors near an office air inlet, as the impact of air filtration is lessened.

The researchers said the findings were significant, as it is the first time indoor UFP formation has been linked to the oxidation of outdoor plant and tree species in heatwave conditions.

The researchers warned that the increased popularity of green walls could exacerbate indoor air pollution in hot, heatwave-prone and polluted environments.

“Although significant attention is paid to the role of outdoor air pollution and its adverse impact on health, little thought is given to indoor air quality even though in developed countries we spend 90 per cent of our time indoors,” the University of York’s Dr Nicola Carslaw said.

“If we want to fully understand our exposure to air pollution, it is crucial to understand the processes that lead to exposure to pollutants indoors, whether in the office or in our homes.

“In this study we investigated indoor air quality in offices in varying climates and found that emissions of reactive species from vegetation in hot temperatures can have a direct adverse effect on air quality in air-filtered office environments in polluted locations.”

Dr Carslaw said there were important implications of the findings given the current trend of green walls, “which could provide a potential source of biogenic emissions near to air inlet systems”.

“Whilst no doubt visually arresting and aesthetically pleasing to many, such features may inadvertently introduce a problem for indoor air quality in city centre offices in hot, polluted locations,” she said.

“This research shows that we may need to find other ways to maintain safe pollutant levels indoors, particularly under heatwave conditions.”

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  1. The title of the article is incredibly misleading as the authors never actually studied any greenwall; in fact they have no empirical evidence for any of their claims.

    There is no doubt they pick on green walls as it is a hot topic, as opposed to the incredibly low impact estimate modelling chemistry based research they have conducted.

    Further, there estimates are solely based on a worst case scenario whereby a heatwave (so extremely high temperatures for only a few days of the year) can cause biogenic volatile organic compounds, which if you had a green wall conveniently placed near the air conditioning intake it would lead to higher VOCs in a building.

    It is true that, plants in extreme temperatures can cause VOCs -> it is that smell of plants dying on extremely hot days. However, The VOCs they emit are nowhere near the volume produced by non-biological sources and nowhere near as toxic, which during a heatwave event would surely subsume the contribution of plants!

    Further, all plants in the area would be doing the same if exposed to the same heat, so it would be an urban forestry phenomenon as opposed to blaming a green wall on a single building.

    In fact the authors even fail to have ambient outdoor VOC data for the heatwave days in which they have based their model on…

    Urban vegetation can cool city environments preventing the urban heat island effect, a phenomenon that accelerates the formation of harmful smog, as ozone precursors such as nitrous oxides and VOCs combine photochemically to produce ground level ozone.

    Funnily enough, both the VOCs and PMs generated by the plants outside the building could be remediated in an indoor environment with green walls placed indoors (assuming the indoor environment isn’t also the same temp as the heatwave temps (<40 deg C)

    Unbelievable and incredibly sensationalist article.