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| In hot and polluted environments, indoor air pollution may be worse with 'green' walls. |
In fact, they could contribute to poorer air quality indoors, experts say.
Researchers of the University of York recently looked at the levels of ultrafine particles (UFPs) in hot and polluted environments. Such particles are a health concern as they can carry potentially toxic substances into the lungs.
The scientists simulated typical UFP levels in Athens, Helsinki and Milan offices during a heatwave and typical summer temperatures. The three cities were selected to compare contrasting climates and locations across Europe.
The researchers found that indoor concentrations of UFPs were highest in the Milan and Athens offices, reflecting high outdoor air pollution levels in these cities.
The pollutants make their way indoors through doors, windows and ventilation systems as well as through gaps in building materials.
However, indoor UFP concentrations were well above those expected through penetration of outdoor particles alone.
The researchers wanted to know why and realized they were a result of high concentrations of reactive volatile organic compounds (VOCs) outdoors, emitted by plants and trees.
These reactive VOCs include limonene, a naturally occurring compound emitted by plants and trees responsible for the citrusy smell in lemons and oranges, and pinene, emitted by pine trees.
Once in the atmosphere, such compounds rapidly oxidise to form a range of gas-phase and particle-phase products, which exist in a dynamic equilibrium depending on the conditions.
During heatwaves such as that experienced during 2003, emissions of VOCs increase in high temperatures and the formation of the secondary gas and particle-phase products becomes very efficient.
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, indoor UFP concentrations are seen to be much higher if reactive VOCs exist outdoors near an office air inlet, as the impact of air filtration is lessened.
This finding is significant as, for the first time, indoor UFP formation is shown to be linked to the oxidation of outdoor plant and tree species in heatwave conditions.
Given the increasing popularity of green walls covered in plants and vegetation, their prevalence in hot, polluted locations could exacerbate indoor air pollution.
The filtration of air in modern office blocks is also seen to be less effective than expected, and this may explain why expected health benefits are often not realised when particle filters are added to a building.
Source: University of York
