Air quality, past, present and future

Many human activities result in emissions of contaminants to the atmosphere, releasing a range of gases and particulate matter. These pollutants were recognised as a problem for human health and ecosystems long before their chemical identity was known and prior to measurements of their ambient concentrations. In fact widespread direct measurements to show the change in atmospheric composition for the reactive pollutants are limited to the last 100 years or so.

The early evidence that air pollutants were harmful was sufficient to lead to regulations to limit emissions, before measurements and thresholds for effects were established. The presentation explores the early evidence of air pollution and some of the steps taken to control it followed by a chronology developed from models and measurements of the main pollutant emissions of sulphur, volatile organic and nitrogen compounds and the products of their chemical interactions in the atmosphere.

Recent global chemistry-transport model intercomparisons provide a description of the rapid increase in sulphur and nitrogen pollutants through the early 20th century and the gradual decline in sulphur, VOC and to a lesser extent oxidized nitrogen emissions between 1980 and 2010. Measurement networks at the surface and more recently satellite remote sensing reveal the spatial and temporal detail of the changes in chemical composition over the last few decades. The presentation describes the time course of anthropogenic emissions to the atmosphere and their effects on global and regional air quality.

A wealth of research has shown that exposure to ambient air pollution is associated with increased risk of adverse human health outcomes such as premature mortality, hospital admissions, birth outcomes, and asthma, among others. The Global Burden of Disease project estimates that air pollution is among the top ten causes of morbidity and mortality worldwide. These associations have been demonstrated through different study designs and in different populations and regions. Still many critical research questions remain. This presentation will review key findings and research needs in the field of air pollution and human health. These include the analysis of complex mixtures and the impact of climate change on air pollution and subsequently on human health. The talk will focus on future directions for such work and paths forward to address air quality and climate change in order to improve public health.

Over the last 100 years the abundance of different air pollutants, their geographic distributions and impacts on managed and natural habitats, have undergone large changes. Some pollutants, including oxidized and reduced nitrogen compounds have steadily increased, while others have declined. The concentrations and effects of sulphur compounds peaked and then declined over this time period allowing some ecosystems to recover. The potential for air quality to have an impact on the natural environment was recognised long before the twentieth century. During the nineteenth and first half of the twentieth centuries, impacts including changes in lichen distribution, moth pigmentation, bird feathers and black snow were noted. During the second half of the twentieth century, studies on the effects of air pollution became broader and much more systematic, notably due to acid deposition, tropospheric ozone and nitrogen deposition and we now know a great deal about the damage that air pollutants can cause to sensitive habitats. This presentation reviews some of the most important impacts that air pollution has had on terrestrial habitats the developed world, including the lichen deserts, forest dieback on both continents, the loss of heathland habitats in the Netherlands, and reductions in vegetation species richness. Although our knowledge base regarding the impacts of air pollution on ecosystems has increased considerably since the early twentieth century, there remain many gaps. This is especially the case in less developed regions of the world, which are experiencing increasing air pollution problems coincident with economic development.

The damage and injury that ground level ozone causes vegetation has become increasingly evident over the past half century with a large body of observational and experimental evidence demonstrating a variety of effects at ambient concentrations that include: visible injury, changes to plant physiology, alterations in the developmental rate of plant growth, reductions in yield, productivity and forage quality, shifts in species composition and reductions in carbon sequestration for a variety of crop, forest and semi-natural vegetation species and ecosystems.  This paper explores the use of experimental data to develop dose-response relationships for use in risk assessment studies; these studies have typically identified the US mid-West, much of Europe, the Indo Gangetic plain in South Asia and the Eastern coastal region of China as global regions where ozone is currently likely to threaten food supply. Experimental data can also be used to identify thresholds for effects to support establishment of air quality guidelines for use in policy development of emission reductions for mitigation efforts. In this respect, understanding emission trends is crucial as ozone concentrations tend to follow patterns of industrialisation with key precursor pollutants being NOx and NMVOCs. However, methane (CH4), which is less tightly linked to industrialisation,  is increasingly being recognised as an important determinant of global background levels of O3 pollution with implications on the appropriate geographical scale at which effective policy responses might be required. Finally, the paper also explores adaptation options to enhance ozone tolerance. Most work has focused on arable crops through identification of  crop cultivars resistant to ozone or management practices that will reduce ozone exposure and uptake. Such adaptation options should ideally also consider other stresses (heat stress, water stress, soil fertility stress) that often co-occur with pollution episodes to ensure responses are appropriate. This requires an improved understanding of the mechanisms by which ozone impacts on plants (i.e. physiological processes that trigger the cascade of plant growth, development, productivity and ecosystem function responses). Such understanding is also critical for the new generation of Earth System Models that hope to incorporate pollution impacts and feedbacks on terrestrial systems.

The acid rain issue was first identified as a problem in the early 1970s and was during the following decades one of the most discussed environmental problems in Europe and North America. Although it was initially postulated that transboundary transport was an important cause, there was initial scientific scepticism on the transboundary question, and there were certainly no coordinated international attempts to solve the problem. Eventually the scientific findings provided the foundation for a determined policy response resulting in a treaty under the United Nations’ Economic Commission for Europe (UNECE), the Convention on Long-range Transboundary Air Pollution (the Air Convention) agreed in 1979. The Convention has successively been extended with eight protocols requiring monitoring and modelling and subsequently reductions in emissions to combat acid rain and in later versions, which were based on effects-based strategies, to reduce impacts of ozone, NOx, VOCs, ammonia, particulate matter as well several heavy metals and persistent organic pollutants. Alongside the activity in the UNECE which initially focussed on damage to ecosystems, the European Union produced air quality Directives which dealt with the effects on human health, primarily in urban areas. The first Directive appeared in 1980 and dealt with ‘smoke’ and sulphur dioxide. Since then there has followed a series of ambient air legislation and Directives dealing with industrial emissions and emissions from road vehicles, as well as national emissions where the obligations mirror those in the Air Convention and extend them to longer timeframes. These actions in the EU and under the Air Convention have led to substantial reductions in emissions of many pollutants, which have resulted in positive effects on ecosystems and human health and in a virtual solution of the acid rain problem. However, despite a significantly improved situation over the last 30 years, problems still remain, especially concerning the health effects from particulate matter and the wider ecosystem issues around excess nutrient nitrogen. There is a continuing need for concerted action as well as increased cooperation in a global context to clean up the air.

Airborne particulate matter (PM) is a pollutant of concern not only because of its adverse effects on human health but because of its ability to reduce visibility and soil buildings and materials. It can be regarded as a suite of pollutants since PM covers a very wide range of particle sizes and also has a diverse chemical composition. Historically, much of the PM arose from coal burning and was measured as black smoke. However, in the second half of the 20th century there was a reduction in black smoke emissions from coal burning and PM steadily became dominated by carbonaceous particles from road traffic exhaust and the secondary pollutants, ammonium salts and secondary organic carbon. This is exemplified by the composition of fine particles (referred to as PM2.5) as measured in London, Delhi and Beijing.  Steadily, as control strategies have addressed the more tractable sources of emissions, so sources previously regarded as unconventional have emerged and been seen to make a significant contribution to airborne PM concentrations. Amongst these are non-exhaust particles from road traffic, cooking aerosol and wood smoke. The particle size distribution of airborne PM is hugely diverse, ranging from newly formed particles of a few nanometres diameter through to particles of tens of micrometres diameter. There has been a great deal of interest in ultrafine (nano) particles because of suspicions of enhanced toxicity, and as traffic emissions decrease as a source, so regional nucleation processes have become much bigger contributors to particle number, but not mass.

Because of its high leaf area index and microphysical properties dry deposition to vegetation is more efficient than dry deposition to non-vegetated surfaces such as bare soil, sealed urban spaces, buildings and water. As governments around the world struggle to adhere to air quality limit levels for particulate matter (PM) and nitrogen dioxide (NO2), there is an acute interest in the potential of using green infrastructure to mediate air pollution levels. This presentation investigates the effectiveness of intervention across a range of scales, from the effect of national vegetation cover to town planning to microscale intervention. Using an atmospheric chemistry and transport model with a range of landcover scenarios, the total UK vegetation is estimated to lower average pollutant concentrations significantly, by 10% for PM2.5, 6% for PM10, 30% for SO2, 24% for NH3 and 13% for O3. This reflects the accumulated effect of vegetation-enhanced deposition during pollutant transport from all UK and non-UK sources to a given receptor site. However, the national vegetation cover only has a negligible net impact on NO2: although vegetation captures NO2, vegetated soils also emit more NO. For city-scale intervention the effect is much reduced for all pollutants, and significant conversion to tree vegetation is required to see appreciable effects even for PM. By contrast, green infrastructure micro-interventions have a negligible effect on local air pollution levels through deposition, but can affect local dispersion leading to increases or reductions in concentrations very close to sources of pollution. An increase in urban vegetation results in multiple benefits, but where pollution control is the sole aim, micro-interventions tend to compare economically poorly with interventions aimed at reducing emissions, especially when population-level exposure is considered.

Much of the early epidemiological evidence on the health effects of air pollution related to changes in routine medical statistics (mortality and hospital admissions) in response to day-to-day variations in concentrations of air pollutants. Later, cohort studies reported larger increases in mortality risk associated with long-term average concentrations of pollutants, particularly fine particulate matter (PM2.5). Experimental evidence also suggests that associations reported in epidemiological studies are likely to be causal. Therefore, methods for health impact assessment were developed to allow policy makers to predict the benefits that could be expected from implementation of interventions to reduce emissions of pollutants. A need to communicate the size of the effect of air pollution on public health was also identified. Estimates of annual mortality burdens attributable to current levels of pollution were developed to meet this need. Burden estimates represent the total health effect on a population, but do not provide information about how the burden is distributed across the population. They are based on a number of underlying assumptions and approximations and require decisions to be made about input parameters and extrapolations. These have implications for the interpretation of the estimates produced. The presentation will use examples to illustrate these points, drawing particularly on work by the UK government’s independent advisory Committee on the Medical Effects of Air Pollutants (COMEAP). It will also touch upon the additional complexities introduced by consideration of pollutant mixtures or by incorporation of morbidity into assessments.

Particulate matter (PM) is a complex, heterogeneous mixture that changes in time and space. It encompasses many different chemical components and physical characteristics, many of which have been cited as potential contributors to toxicity. Each component has multiple sources, and each source generates multiple components. Identifying and quantifying the influences of specific components or source-related mixtures on measures of health-related impacts, especially when particles interact with other co-pollutants, therefore represents one of the most challenging areas of environmental health research. Current knowledge does not allow precise quantification or definitive ranking of the health effects of PM emissions from different sources or of individual PM components and indeed, associations may be the result of multiple components acting on different physiological mechanisms. Some results do suggest a degree of differential toxicity, namely more consistent associations with traffic-related PM emissions, fine and ultrafine particles, specific metals and elemental carbon and a range of serious health effects, including increased morbidity and mortality from cardiovascular and respiratory conditions. Exposure to combustion-related PM, at concentrations experienced by populations throughout the world, contributes to pulmonary and cardiac disease through multiple mechanistic pathways that are complex and interdependent. Current evidence supports an interactive chain of events linking PM-induced pulmonary and systemic oxidative stress, inflammatory events, and translocation of particle constituents with an associated risk of vascular dysfunction, atherosclerosis, altered cardiac autonomic function, and ischemic cardiovascular and obstructive pulmonary diseases. Because oxidative stress is believed to play such an instrumental role in these pathways, the capacity of particulate pollution to cause damaging oxidative reactions (the oxidative potential) has been used as an effective exposure metric, identifying toxic components and sources within diverse ambient PM mixes that vast populations are subjected to from traffic emissions on busy roads in urban areas to biomass smoke that fills homes in rural areas of the developing world.

Air pollution ranks among the leading risk factors contributing to the disease burden in India. In the most recent assessment of state-level disease burden in India, approximately 0.67 million premature deaths and 21.3 million disability-adjusted life years (DALYs) were attributable to ambient air pollution (AAP) and 0.48 million premature deaths and nearly 15.8 million DALYs to household air pollution (HAP) resulting from use of solid cooking fuels, in the form of fine particulate matter ≤ 2.5 μm in aerodynamic diameter (PM 2.5). This places air pollution near or at the top of the list of all known risk factors for ill health in the country, above high blood pressure, tobacco smoking, child and maternal malnutrition. The disease burden from air pollution, borne disproportionately by poor populations who rely on solid fuels for cooking, poses an enormous challenge for air quality management within public health programs in India. The evidence for required inter-sectoral solutions is strong and is reflected in on-going regulatory actions. There is a however a need to create seamless breathing spaces that straddles across all exposure micro-environments to both  diminish the health burden and  the associated disparities among population sub-groups. Strategic epidemiological approaches that can advantage of on-going programmatic initiatives (especially in household energy sector) can advance the cause for air quality actions in India, while filling critical research gaps in the national and global pool of evidence.  The talk will cover on-going and planned initiatives that can greatly benefit from global collaboration.