The role of ambient outdoor air quality in human health is well known, with rising public awareness and regulatory moves to limit concentrations of key air pollutants. However, less attention has been given to the ecological impact of these pollutants and the opportunity for action on these to benefit people and planet. Christopher Crompton looks deeper into the impacts and co-benefits.
With almost every jurisdiction on earth falling short of implementing the World Health Organization’s recommended limit values into legislation, there is clearly still much work to be done in this area. In the UK, ongoing readings in Parliament of the proposed Clean Air (Human Rights) Bill, against a setting of Britain’s divergence from more stringent EU legislation, makes the health implications of air quality a hot topic.
However, the negative effects of air pollution are not contained to human health: they also profoundly impact the environment. Key concerns include the following:
• Ground-level ozone (O3) exposure harms forests and crops, impacting growth and yields. It can also make plants more susceptible to drought. Ozone forms when other pollutants such as nitrogen oxides and volatile organic compounds including methane interact with sunlight. Global economic losses resulting from the effects on ozone on crops have been estimated at up to $26 billion annually. Nitrogen oxides in the UK primarily arise from transport, energy and manufacturing, while major sources of methane include agriculture and landfill sites.
• Nitrogen (N) deposition, mostly resulting from air pollution, has caused eutrophication (excess nutrients) in 73% of EU ecosystems, changing the structure of ecological systems and harming biodiversity. While nitrogen is an essential plant nutrient, high concentrations in fog and cloudwater can directly damage plant foliage, and high ammonium uptake can cause loss of other nutrients such as magnesium from leaves, as well as contributing to soil acidification. Some plants are more susceptible to excess nitrogen than others, leading to shifts in species composition. While pure nitrogen is a major component of the atmosphere, the main nitrogen sources leading to deposition that harms ecosystems are nitrogen oxides and ammonia, which are emitted principally by agriculture, fossil fuel combustion, and industrial processes (see also IEEP UK’s policy gap analysis on nitrogen pollution)
• Sulphur dioxide (SO2) can alter the chemical composition of soil and water systems, causing acidification. which leads to biodiversity loss and disrupts ecosystems. The main sources of SO2 are the industrial and energy sectors.
• Heavy metals are toxic pollutants resulting mostly from manufacturing, energy, and road transport that can travel long distances in the atmosphere before being deposited into ecosystems. They concentrate in soils and subsequently bioaccumulate (build up within living organisms). The strongest effects are often felt further up the food chain, as concentrations bio-magnify (build up throughout the food web). Concentrations of heavy metals in agricultural soils and in animals eaten by humans in turn have implications for human health. Heavy metals also pollute water systems; for example, the European Environment Agency recently reported that almost half of Europe’s surface waters are contaminated with mercury.
UK leading the world on scientific research
The UK Centre for Ecology and Hydrology has led international efforts to study the impacts of air pollution on vegetation through an International Cooperative Programme. The programme involves over 250 scientists from around 50 countries and studies impacts of nitrogen pollution on vegetation, considers impacts of pollutant mixtures (e.g., ozone and nitrogen), as well as consequences for biodiversity and the modifying influence of climate change on the impacts of air pollutants on vegetation.
Recent work includes the use of mosses as ‘biomonitors’ of pollution to identify areas most at risk.
Does legislation protect ecosystems from air pollution?
The EU’s Ambient Air Quality Directives set standards to protect vegetation from air pollution, including a target value and a long-term objective for ozone, and critical levels for NOX and SO2. The UK adopted the same critical levels for vegetation through the Air Quality Standards 2010, although they have not been updated since.
Nonetheless, the National Emission Ceilings Regulations 2018, arising from the international Convention on Long-Range Transboundary Air Pollution set legally binding emission reduction commitments for key pollutants including ammonia and nitrogen oxides. The Environment Act 2021 also addresses the impact of air pollution on biodiversity and requires local authorities to consider these impacts in their planning and nature recovery strategies. Yet, the Office for Environmental Protection (OEP) reported last year that the UK government is not on track to meet four out of five of its 2030 air pollution emissions reduction targets by 2030.
It is also clear that for sensitive ecosystems, existing national ambitions do not go far enough. Approximately 84% of the UK’s sensitive habitats are exposed to nitrogen deposition above their critical loads for eutrophication, the threshold at which ecological harm occurs. The UK’s Clean Air Strategy as it currently stands projects only small reductions in nitrogen deposition (around 17% by 2030 in England), which is insufficient to bring levels below critical loads.
At the same time, some have raised concerns that the UK’s departure from the EU and the removal of some retained EU laws could weaken environmental protections without adequate replacement, and that strong EU enforcement mechanisms (such as infringement proceedings for not complying with regulations) are no longer in place.
Air quality, ecology, and health – intertwined issues
Awareness and improved monitoring of air pollution effects on ecosystems lend still greater weight to health-based arguments for reducing air pollution. The interactions and feedback between these areas are also important in a more holistic understanding of air quality policy.
Where pollution such as heavy metals enters our food or water systems, the crossover of effects on human and ecosystem health becomes especially apparent and concerning.
While improved air quality has positive ecological effects, the relationship also works both ways, with vegetation playing a role in purifying the air. For example, scientists have called for strategic introduction of green spaces targeting areas of high pollution, noting how green spaces are particularly effective in reducing particulate matter (PM) pollution concentrations in urban areas. In many cases, there is clear potential for multiple wins, as reducing pollution brings benefits for human health, ecosystems, and can in some cases also support climate goals, all of which also contribute economic benefits. Action to limit the biggest sources of air pollution can therefore bring wider benefits beyond air quality.
There are also clear benefits of the health and environmental communities working together for this common cause, which may help create the tipping point for action. And in doing so, there may be more that both can take from the policies and protections that have evolved in the rest of Europe since Brexit, and may provide an achievable milestone in the UK’s ambitions.