Asthma, pollen and pollution: selecting the best plants for healthy living
22 September 2021
By Shenagh Hume (asthma and allergy nurse specialist, retired) and Jackie Herald (freelance writer and Society of Garden Designers award-winning designer of children’s gardens)
There is much talk of tree planting and green infrastructure to mitigate London’s air pollution problems. But not all plant species are of equal benefit. If London Asthma Standards for children and young people are to achieve safe air, both in and out of the home, and access to green space rich with opportunities to exercise, which species to plant is critical to the prevention of further asthma and allergy.
The important goal is not to cause an overload of airborne pollen. And we need to take extra care, as global warming and climate change are causing trees to grow faster and produce more pollen.
Though the primary trigger for asthma is not airborne pollen, there is a significant link between asthma and hay fever among individuals who are sensitised to allergenic pollen. In the UK, three species in particular – birch, alder and hazel – are the cause of sending children to hospital for desensitisation. These links are described in The Walk to School poster published by Allergy UK.
The ever-popular silver birch is cited in London’s Urban Forest Plan (December 2020) as one of the most common species across the capital. Unfortunately, the combination of noxious urban pollution – which birch foliage is intended to clean up – and the production of the birch’s own airborne allergenic pollen may sensitise more people to become allergic to the pollen. Symptoms are not just respiratory, but may include eye irritation and even allergies to pollen related foods, such as fresh apples. The pollen food syndrome (aka Oral Allergy Syndrome) causes irritation and swelling in the mouth, throat and gut and – in rare cases – anaphylaxis.
Such symptoms may impact negatively on children and young people’s quality of life, affecting sleep, concentration, performance at school and causing breathing difficulties when playing outdoors. Airborne pollen gets caught up in hair and clothing and hangs around indoors long after exposure, extending the pollen season for two months or more. As a sufferer’s response to pollen is often delayed and persistent, they may not link their breathing difficulties to the plant source. Boundaries between school, home and the great outdoors merge, making avoidance of the trigger almost impossible.
Recent scientific evidence offers detailed data on which plant species are most effective in helping to improve air quality and, perhaps more significantly, to deliver the psychological and social benefits of contact with nature. Biodiversity is key. Birch is no longer recommended to mitigate pollution, though the legacy of mass planting remains evident in school playgrounds, parks and streetscapes – including around City Hall. Now that all schools should work towards achieving ASF (Asthma Friendly School) status, we do not propose that all silver birches should be chopped down; tip pruning or pollarding will remove the pollen-bearing catkins for 2-3 years and will then grow back. Most importantly, serious attention should be paid to the selection of species for London’s tree canopy, hedging and green walls – especially where the building is dense and hard landscaping predominates.
Lewisham has published clear, comprehensive species guidance for its Street Trees for Living programme that takes allergies and asthma into account. Hopefully tree officers in other boroughs will follow this excellent example.
See more from #AskAboutAsthma 2021