Air pollution, particularly exposure to particulate matter (PM2.5) and nitrogen dioxide (NO2), poses a significant threat to public health and imposes substantial economic costs. In 2016, exposure to particulate matter caused 4.1 million premature deaths globally. In the UK alone, the financial burden on the NHS and social care due to PM2.5 and NO2 was estimated at £42.88 million in 2017, rising to £157 million when accounting for emerging evidence of other diseases. These numbers highlight the pervasive impact of air pollution, particularly on vulnerable communities. AN UNEVEN BURDEN Environmental injustice occurs when certain populations - often low-income or minority communities - are disproportionately exposed to environmental hazards. The concept emerged in the 1980s in the United States, closely tied to grassroots movements against environmental racism. In 2020, the tragic case of Ella AdooKissi-Debrah, a nine-year-old who died from asthma aggravated by air pollution in London, brought the health impacts of environmental injustice into sharp focus in the United Kingdom. The case highlights the urgent need for stricter environmental laws and more comprehensive policies to protect vulnerable populations. The issue of disparities in exposure to pollution has spawned interest across multiple fields, including law, sociology, public policy, geosciences, and economics. Early work mostly focused on understanding the extent of environmental injustice and providing quantitative evidence of the uneven burden of pollution that falls on communities of colour or lowincome households. Quantifying pollution gaps across different pollutants and population groups is important for policymakers to allocate limited resources efficiently when working toward equity-related goals. While this is a critical first step, identifying the underlying mechanisms leading to these pollution disparities is essential for developing effective solutions. One such mechanism is residential mobility, i.e. the movement of people from one home to another. Our study reveals how household income influences where people choose to live and the quality of their environment, painting a concerning picture of environmental inequality in the United States. BEYOND SIMPLE CHOICES We often hear about the voting with your feet phenomenon – the idea that people choose to live in areas that best match their preferences and needs, including environmental quality. But this is not just about personal preferences. If clean air is an attribute everybody wants, this will drive up the price of clean neighbourhoods. What happens then, when clean air becomes a luxury only some can afford? Our study reveals a self-reinforcing cycle that helps explain why environmental inequalities persist in America. When wealthier residents consistently choose cleaner areas, it leaves lower-income households concentrated in more polluted regions. This pattern does not just happen by chance – it is driven by the economic reality that cleaner environments often come with higher housing costs. This dynamic creates what we as economists call a ‘sorting effect’, where households naturally separate themselves based on their ability to pay for environmental quality. It is similar to how people might sort themselves based on school quality or crime rates, but with one crucial difference: while you can choose to invest in private education or security systems, you cannot easily escape poor air quality in your neighbourhood. TRACKING MOVEMENT AND INCOME Using detailed IRS migration data from 2010 to 2014, we analysed how Americans moved between counties – local administrative divisions in US states. This unique dataset captured information from nearly 98% of all tax filers, making it the most comprehensive view of population movements within the United States ever studied in this context. What makes our study particularly interesting is that we did not just look at where people moved – but also at how much money they made compared to those who stayed behind. We looked at two key measures of environmental quality: 36 |
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