Pollution solution: The role of scrubbers in cutting emissions and improving health

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Air pollution hurts both people and planet, with emissions contributing to higher hospitalisations and deaths in countries around the world. Coal is particularly harmful but remains an important energy source worldwide. This presents policy-makers with a tricky trade-off between energy supply, emissions goals and human health. But technology can help. This column presents evidence from the United States, exploring how the installation of systems called ‘scrubbers’ on coal power station exhausts can reduce SO2 emissions, leading to lower instances of heart disease in nearby and downwind areas. This research could help clarify cost benefit analyses of different climate measures, while also feeding into policies promoting health.

Air pollution is harmful to human health (Dominici et al., 2014). A recent study in Science (Hanneman et al., 2023) suggests that air pollutants released from coal power plants were associated with nearly half a million premature deaths of elderly Americans from 1999 to 2020. This is a staggering statistic. 

But technology can help. There are devices available to reduce the harmful pollution emitted from industrial processes (such as burning coal). For example, installing de-sulfurisation ‘scrubbers’ – which work a bit like giant filters – can remove over 90% of the flue gasses emitted by a coal exhaust.  Investment in scrubbers at coal plants across the United States is one key feature that has driven a stark decline in certain types of air pollution, showing that a better, cleaner, and healthier production process is possible.   


Scrubbed clean 

In a recent paper (Zigler et al., 2023), we try to answer a more specific question: what are the causal effects of specific interventions in terms of reducing pollution emissions from power plants? Using data from the United States, we evaluate the effectiveness of installing scrubbers on 472 coal-burning power plants in reducing hospitalisations for Ischemic Heart Disease (IHD) among people aged 65+ years. Our dataset consists of 21.5 million Medicare beneficiaries, living across 25,553 ZIP codes. The idea is to assess whether the installation of scrubbers on power plants has a measurable effect on the number of people turning up to hospital with heart problems. A healthier climate means healthier people, we propose. 

Our paper confronted one key challenge: air pollution moves. Somebody’s risk of heart disease may depend on one particularly influential power plant but may also depend on the pollution from the constellation of plants located hundreds of kilometres upwind. Our results indicate that, on average, having a scrubber installed on the most influential power plant causes a reduction of approximately 23 hospitalisations per 10,000 person-years (a unit of measurement calculated by adjusting for the fact that some people at risk for heart disease died or otherwise left during the study year). Additionally, installing more scrubbers on the constellation of other upwind power plants led to a reduction of around 16 IHD hospitalisations per 10,000 person-years. This second finding makes clear that efforts to reduce pollution from power plants can have health impacts in other geographies.  So, installing scrubbers – even at distant locations – could reduce heart disease levels. 

To validate our results, we also study the effect of scrubbers on ambient PM2.5 (pollutant particles that are less than 2.5 microns in diameter). In fact, the link between power plant pollution and ambient PM2.5 is reasonably well understood. Research suggests that having a scrubber on a most influential power plant may reduce ambient PM2.5 substantially. These results, combined with our results on IHD hospitalisations, are consistent with the recent findings in the paper in Science (Henneman et al., 2023). Air pollution particles from coal-fired power plants are more than twice as likely to contribute to premature deaths as air pollution particles from other sources. So, reducing the flow of such particles is a matter of life and death. 


Breathing easy 

Many studies (Currie and Walker, 2011; Chay et al., 2003) have contributed to linking PM2.5 to increased risk of early death, lung and heart disease, cancer, dementia and other diseases. Following this research, the Environmental Protection Agency (EPA) began regulating PM2.5 concentrations in 1997, and has lowered the acceptable limit of particulate flows over time. Further, starting with Title IV of the 1990 Amendments to the Clean Air Act, there have been increasing efforts to reduce SO2 emissions – a known precursor to the atmospheric formation of PM2.5, which itself has been linked to myriad adverse health outcomes (Pope III et al., 2009; Dominici et al., 2014). A major focus of efforts to reduce population pollution exposure is the reduction of SO2 (and other) emissions from coal-fired electricity generating power plants – the dominant source of SO2 emissions in the United States. Clearly, cleaning up power generation should remain high up on the policy agenda.  

There are many strategies for reducing emissions for coal-fired power plants. But, at present, there is no specific study quantifying the effect on health of a specific intervention. As mentioned, the strategy that is studied in our paper is the installation (or not) of flue-gas de-sulfurisation scrubbers on coal-fired power plants in the United States during 2005 – a year of significant regulatory action on power plants. The idea here is to gain a clearer picture of what such measures can do in terms of reducing the number of people going to hospital with heart problems. This study could then provide the basis for investigations into other interventions, in other jurisdictions. 

But why, until now, has there been so little available evidence? The lack of evaluation of specific air quality policies stems from the fact that such studies are confronted with the challenge of interference: interventions at a particular pollution source affect air quality and health at distant locations, and air quality and health at any given location are likely affected by interventions at many sources. It is a chaotic system. An exception is the study by Papadogeorgou et al. (2019). The structure of interference in this context is dictated by complex atmospheric processes governing how pollution emitted from a particular source is transformed and transported across space. These phenomena are known as pollution transport and chemistry; chemical compounds such as SO2 emitted from a power plant smokestack are transported through the atmosphere and react chemically with atmospheric co-constituents. The primary end product of atmospheric SO2 is sulfate, which condenses quickly and contributes to increased fine particulate air pollution. In this setting, an intervention employed at one power plant will likely affect health outcomes in the locations where chemical compounds are transported, and the health outcomes at a given location are dictated in part by actions taken at many power plants.  

Turning back to our latest study, we use a reduced-complexity atmospheric model called HYSPLIT Average Dispersion (HyADS), to evaluate the movement of pollution through space and time (Henneman et al., 2019) and to characterise the structure of the interference network. This is then combined in a novel estimation approach to account for possible sources of confounding at both the ZIP codes and the power plant levels. This constitutes the first attempt to evaluate the direct impact of air quality interventions on health outcomes. The hope is that this method can then be applied in new settings, deepening our understanding of the interplay between pollution and health outcomes. 


What next? 

Our study makes it clear that the use of pollution-control devices can have direct impact on human health, even in areas far away from the upwind pollution sources. Climate policies involve a balance of mitigation and adaption, with co-benefits through reduction in air pollution routinely included when projecting economic benefits of proposed rules (New York Times, 2023).  The use of scrubbers could be one way in which the negative outcomes of coal-burning are reduced (at least in terms of the human health impact).   

The world is changing. At present, coal still contributes about 27% of electricity in the United States. While substantial, this is less than half the level it was in 1999 (56%). Coal burning has a place, but its role is shrinking. Policy-makers should now focus on cleaning up the remaining plants.  

Taking a step back, making full use of emissions controls, as well as dialing up the use renewable energy sources, are both ways to reduce the overall negative impact of coal. Burning coal harms both people and planet, and reducing this damage involves interventions such as installing scrubbers. Understanding the true cost of coal power, as well as what it might take to ease the flow of harmful pollutants, is vital information for policy-makers – in the United States and further afield. A cleaner atmosphere is possible, but only with the right tools, based on the information, deployed at the right time. 


Author: Fabrizia Mealli 

Author’s note: Author’s note: to read the original work, please see: Corwin Zigler, Vera Liu, Fabrizia Mealli, and Laura Forastiere (2023), ‘Bipartite Interference and Air Pollution Transport: Estimating Health Effects of Power Plant Interventions’, arXiv: 2012.04831