# **2. Existing evidence**

## **Air pollution is pervasive, caused by a wide range of economic activities.**

<mark>Air pollution refers to the heightened concentration of a range of different pollutants, most notably particulate matter (PM), ozone (O3), nitrogen dioxide (NO2), carbon monoxide (CO), and sulfur dioxide (SO2). While also occurring naturally, hazardous concentration levels of these pollutants are predominantly the result of human activities, including transport, fossil fuel combustion, and agriculture. This also means that their occurrence has high spatial correlation. For instance, major urban centers typically have increased concentrations of all of the above-mentioned pollutants. Using remotely sensed carbon dioxide (CO2) measurements from over 1,200 cities in 138 countries, Dasgupta et al (2021) show that emission levels correlate strongly with population size and economic development levels, though the presence of public transport systems (subways) can reduce local emissions.</mark> The vast majority of the world’s population faces air pollution levels that are considered unsafe by the <mark>WHO. For PM2.5, one of the most pervasive pollutants, the WHO’s 2005 guidelines recommend for average annual concentrations not to exceed 10 μg/m</mark><sup>3</sup> <mark>(WHO, 2005). Southerland et al (2022) study PM2.5 concentration trends over two decades for over 10,000 urban centers, and show that 86% (2.5 billion inhabitants) of urban inhabitants lived in urban areas that exceeded the WHO’s 2005 guideline threshold of 10 μg/m</mark><sup>3</sup> <mark>for PM2.5. The WHO’s (2021) Global Air Quality Guidelines further revise the 2005 guideline threshold down to 5 μg/m</mark><sup>3</sup> <mark>, meaning that an even higher percentage of the world’s population is exposed to unsafe concentrations.</mark>

## **Exposure to unsafe levels of particle concentration has a wide range of health impacts.**

<mark>The WHO’s revised threshold reflects a growing body of evidence on the wide-ranging global burden of disease associated with air pollution. There is strong evidence on the causal relationship between air pollution – especially PM, O3, NO2, SO2 – and all-cause mortality, as summarized by the WHO (2021). Heightened air pollution exposure has been shown to increase the risk of chronic obstructive pulmonary disease, ischemic heart disease, lung cancer, stroke, as well as chronic and acute respiratory diseases such as asthma (Cohen et al., 2017; WHO, 2018). Besides cardiovascular and respiratory diseases, growing evidence exists on the role of PM2.5 exposure in increasing the risk of type II diabetes and neurological diseases such as Alzheimer’s (Peters et al., 2019; GBD 2019 Risk Factors Collaborators, 2020). Evidence also suggests that air pollution exposure increases neonatal mortality due to low birth weight and short gestation (GBD 2019 Risk Factors Collaborators, 2020; Patel, et al. 2021). Cumulative exposure to air</mark> pollution can also result in a range of health issues that lower long-term quality of life (O’Neill, et al. 2003).

<mark>Estimates of the number of people affected by air pollution related diseases are staggering. The WHO (2018) estimated that ambient (i.e. outdoor) air pollution caused 4.2 million premature deaths in 2016. Of these deaths, 58 percent were due to ischemic heart disease and stroke, 18 percent were due to chronic obstructive pulmonary disease and acute lower respiratory infections, and 6 percent due to lung cancer. Such ambient air pollution is driven mainly by fossil fuel combustion, transport and industry. In addition, the WHO highlights the serious health risks due to indoor air pollution faced by about 2.6 billion people who rely on coal and biomass for cooking and heating their homes. In 2016, the WHO attributed about 3.8 million premature deaths to such indoor air pollution. The 2019 Global Burden of Disease study (GBD) estimates that 4.5 million premature deaths were due to ambient air pollution and another 2.3 million premature deaths due to indoor air pollution. PM2.5 is identified as the pollutant responsible for</mark> the vast majority of air pollution deaths.

<mark>Focusing specifically on PM2.5 exposure in urban areas, Southerland et al (2022) show that average pollution levels have decreased in most regions between 2000 and 2019. Nevertheless, PM2.5 attributable deaths have increased in the same period in all regions except Europe, and North and Latin</mark> 3