Guardians of Our Garden City? Importance of Trees in Preserving Air Quality

Brimming with lush greenery and having roads lined with neatly pruned trees; Singapore is renowned for its reputation as a garden city. Besides serving as an aesthetic function and being widely associated as the lungs of our Earth by supplying us with fresh oxygen, do trees serve any other purpose in our urban city? 

An interesting study conducted in the United States by the USDA Forest Service and The Davey Institute on the value of trees in removing air pollutants. The study estimated that trees save more than 850 human lives and prevent 670,000 acute respiratory cases. 

According to the study, trees can help to absorb the gaseous pollutants such as ozone, sulfur dioxide and nitrate dioxide (the 6 commonly monitored pollutants discussed in the previous post) through its leaf stomata which facilitate gaseous exchange. In addition, trees are also particularly effective in trapping hazardous  particulate matter such as PM2.5 on their leaves. These air pollutants are then washed off from the leaves when it rains and eventually absorbed by the soil. 

Based on environmental modelling and analyses, the study estimates that the trees grown in the urban areas can remove up to 711,000 tonnes of air pollutants per year, translating to US$7 billion direct human health effects from the reduced air pollution annually. The study also emphasised that trees grown in the urban areas have significantly greater impact on our health than those grown in the rural areas due to their proximity to people and probably pollution sources. 

Bringing this context back to Singapore, it is undeniable that our air quality is significantly better than many other countries laden with air pollution. This can be largely accredited to our comprehensive Urban Redevelopment Authority (URA) Master Plan in balancing our land use needs such as ensuring that potentially polluting industries are situated away from residence areas or even offshore. Moreover, there are also strict air pollution regulations set by national agency such as the National Environmental Agency (NEA) in monitoring and restricting vehicle emissions.

However, it is reported that the particulate matter level in Singapore is still exceeding the limits set by the World Health Organisation (WHO) and probably due to Singapore being a tiny island that houses one of the world's largest oil refineries. Perhaps it is time for us to give more recognition to our trees that are the unsung "guardians of our garden city" in trapping some these particulate matter and maintaining the air quality.