Air pollution: does it get worse before it gets better?

The World Health Organization (WHO) highlights air pollution as the biggest driver of environment-related deaths. It’s estimated to be the cause of seven million premature deaths every year – 4.3 million from indoor air pollution, and 3 million from ambient outdoor pollution.1

It’s therefore a pressing global challenge. Effectively addressing air pollution risks requires some understanding of patterns of change through time, and the drivers of this evolution.

Here, we look at the long-term perspective of one key air pollutant, sulphur dioxide (SO2), highlighting an interesting development which could prove important in understanding current and future air pollution. Sulphur dioxide is produced primarily through the burning of fossil fuels—most fuels contain a small amount of sulphur which is released as SO2 during combustion—but is also formed in other industrial processes.

Sulphur dioxide creates a number of health and environment-related problems. Exposure to SO2 (even at low concentrations and short periods) is known to affect pulmonary function, and aggravate a number of respiratory illnesses. Furthermore, SO2 can react with other compounds in the atmosphere to form particulate matter (PM)—another air pollutant which can penetrate the lungs and cause severe health problems.

Fully disentangling the causality between pollutants, their co-pollutants and the number of air pollution deaths is complex, making it difficult to directly attribute a given number of deaths to SO2 pollutants. Nonetheless, a number of empirical studies have found a relationship between SO2 concentrations and the number of cardiovascular-related deaths.2,3

How have global emissions evolved?

Industrialization marked a key transition point in the magnitude of SO2 emissions as a result of large-scale burning of sulphur-containing fuels and industrial processing.

The chart shows total global annual emissions of SO2 by world region, extending from 1850 to 2010. Europe was first to see a rapid rise in sulphur pollution, followed closely by North America in the mid-19th century. Driven by the increased energy demand of industrialization, emissions in Europe and North America continued to grow through the 19th and 20th century. But then the increase came to an end.

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In Northern America emissions peaked in 1970, in Europe in 1980, and in South America a decade later. Since then emissions have been on a downward trend in these regions. At the beginning of the 21st century emissions in Northern America are at a lower level than at any time in the 20th century. The reason for which we will discuss later.

With the exception of Japan, industrialization in Latin America, Asia and Africa began much later. The rise in SO2 emissions in the rest of the world was delayed until the 20th century. SO2, as a by-product of energy and industrial production, is closely linked to prosperity. So, despite increasing throughout the 20th century, per capita emissions from Asia and Africa were, and still are, small relative to Europe and North America. The dominance of Europe and North America in total global emissions means that the world’s SO2 emissions peaked in the 1980s, despite a continued increase across the other continents.

Today, the regional trends are split: Annual SO2 emissions in Europe and the Americas continue to fall, while emissions in Asia and Africa are increasing.

Dirty then clean: patterns of change

The evolution of regional and global pollution pattern appears to show an increase-peak-reduction shape. This trend is characteristic of what is called the ‘Environmental Kuznets Curve’ (EKC). The concept of the EKC was first discussed in the 1990s within the 1992 World Development Report, but builds upon the stylized relationship between income inequality and economic development as described by Simon Kuznets in 1955.4

The EKC provides a hypothesis of the link between environmental degradation and economic development: environmental quality initially worsens with the onset of industrial growth, but then peaks at a certain stage of economic development and from then on environmental quality begins to improve with increased development. Evidence of the EKC is very mixed: across various measures it has been widely contested, but for a number of environmental markers there is strong evidence for the existence of EKC relationship. For example, a recent study published in Science suggests the EKC relationship also appears in links between deforestation, afforestation and economic growth.5

Do we see this hypothesized trend in SO2 pollution? It appears we do. Although still controversial6, a number of studies have found an empirical justification of the EKC link to SO2 emissions.7,8

Both Europe and North America went through an initial period of ‘dirty’ industrialization whereby sulphur dioxide production increased. However, in the 1970-80’s, emissions peaked and have since continued to decline.

This transition was achieved through a number of economic, technological, and policy measures. National regulation and regional policy agreements played a crucial role in this clean-up; the first being the 1979 Convention on Long-Range Transboundary Air Pollution (CLRTAP) signed by 35 countries across North America, Western Europe and the Eastern Bloc.

Since then, a number of progressive Sulphur Protocols and European directives have set regulatory limits on SO2 emissions from large power stations and industrial processes. This has forced the adoption of emissions reduction technologies such as ‘scrubbers’ which strip the gases of sulphur before being emitted, as well as a migration away from cheaper coal and oil sources which typically have higher sulphur contents.

However, the EKC links average income, not policy, to environmental quality. Is there therefore a link between higher incomes and better regulatory frameworks? There are three suggested reasons why higher-income nations have stricter regulation: environmental quality achieves a higher priority after basic social demands have been met; richer countries have larger budgets and technical expertise for enforcement and management of regulation; and education-income links provide the necessary levels of local empowerment for enforcement of environmental standards if it’s lacking at a national level.9

SO2 emissions in Asia and Africa have not peaked yet. Are they likely to follow a similar trend? Both regions lag behind Europe and the Americas in terms of economic development, so it is well possible that they are still on the upward rising slope of the EKC and that the peak is yet to come.

The chart here presents the evidence on SO2 emissions in Asia’s two largest economies, India and China, since 2000 and provides some suggestive evidence that this trend will likely recur.

China experienced a ‘peak’ in SO2 emissions in 2006 and has been declining since. Some analysts have suggested that this effort was driven by a target of reducing pollution levels for the 2008 Beijing Olympics. Whilst this may have further accelerated its clean-up efforts, I think it’s likely that such a transition would have happened regardless as a result of heightened concern for public health. As in Europe and North America, this reduction was achieved largely through effective policy implementation and uptake of sulphur-limiting technologies.10 In contrast, India’s SO2 emissions have not yet peaked. However, India is slightly behind China in economic terms.

The evidence therefore suggests that prosperity alone is not responsible for the observed trajectory of SO2 emissions. It is development in a broader sense—encompassing economic growth, technological change, changing attitudes towards environmental concerns, policy and regulation—that is responsible for the increase-peak-reduction shape of the evolution of SO2 emissions.11

Based on the finding that SO2 emissions decline with development, the Intergovernmental Panel on Climate Change (IPCC) projects significant reductions in global SO2 levels in the coming decades. Depending on the scenario SO2, emissions will fall to 35-65 percent of latest estimates (2011) of 101 million tonnes per year, by 2050.

But ultimately it does depend on the technological and political choices we make, and the decrease in emissions of this air pollutant may well be accelerated with the co-beneficial adoption of climate change policies and better technologies.


  1. World Health Organization. Ambient Air Pollution: A global assessment of exposure and burden of disease. World Health Organisation. 1–131 (2016). Available at doi:9789241511353

  2. Bessö, A., Nyberg, F. & Pershagen, G. Air pollution and lung cancer mortality in the vicinity of a nonferrous metal smelter in Sweden. International Journal of Cancer. 107, 448–452 (2003). Available at doi:10.1002/ijc.11412

  3. Amancio, C. T. & Nascimento, L. F. C. Association of sulfur dioxide exposure with circulatory system deaths in a medium-sized city in Brazil. Brazilian Journal of Medical and Biological Research. 45, 1080–1085 (2012). Available at doi:10.1590/S0100-879X2012007500131

  4. Kuznets, S. Economic Growth and Income Inequality. American Economic Review. 45, 1–28 (1955). Available at: JSTOR

  5. Crespo Cuaresma, J. et al. Economic Development and Forest Cover: Evidence from Satellite Data. Scientific Reports. 7, 40678 (2017). Available at: doi:10.1038/srep40678

  6. Stern, D. I. Reversal of the trend in global anthropogenic sulfur emissions. Global Environmental Change. 16, 207–220 (2006). Available at: doi:10.1016/j.gloenvcha.2006.01.001

  7. Kunnas, J. & Myllyntaus, T. The Environmental Kuznets Curve Hypothesis and Air Pollution in Finland. Scandinavian Economic History Review. 55, 101–127 (2007). Available at: doi:10.1080/03585520701435970

  8. Fodha, M. & Zaghdoud, O. Economic growth and pollutant emissions in Tunisia: An empirical analysis of the environmental Kuznets curve. Energy Policy. 38, 1150–1156 (2010). Available at: doi:10.1016/j.enpol.2009.11.002

  9. Dasgupta, S. et al. Confronting the Environmental Kuznets Curve. Journal of Economic Perspectives. 16, 147–168 (2002). Available at: doi:10.1257/0895330027157

  10. Schreifels, J. J., Fu, Y. & Wilson, E. J. Sulfur dioxide control in China: Policy evolution during the 10th and 11th Five-year Plans and lessons for the future. Energy Policy. 48, 779–789 (2012). Available at: doi:10.1016/j.enpol.2012.06.015

  11. Stern, D. I. & Common, M. S. Is There an Environmental Kuznets Curve for Sulfur? Journal of Environmental and Economic Management. 41, 162–178 (2001). Available at: doi:10.1006/jeem.2000.1132

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    author = {Hannah Ritchie},
    title = {Air pollution: does it get worse before it gets better?},
    journal = {Our World in Data},
    year = {2017},
    note = {}
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