# **3.2. Methodology and step-wise computational process**

To estimate the number of people who are exposed to unsafe air pollution levels, this study follows a computational process in four main steps, which are outlined in this section. For further details, refer to Rentschler and Malhab (2020), who offer a detailed description of the stepwise computational process that is replicated in this study.

_Resample the PM2.5 data:_ The air pollution map is resampled to ensure that pixels align with the gridded population density map.

_Define air pollution risk categories:_ While the air pollution map offers average annual PM2.5 concentration levels along a continuous scale, the values are aggregated into six risk categories as summarized in Table 1. These risk categories are defined in line with the air quality guideline (AQG) level by the WHO (2021). The WHO recommends an annual PM2.5 AQG level of up to 5 μg/m. For countries with PM2.5 concentrations above this threshold, it recommends interim targets at 10, 15, 25, and 35 μg/m<sup>3</sup> , which correspond to a linearly increasing mortality rate (Table 1). At higher concentrations, the concentration-response function of mortality may no longer be linear. The epidemiological evidence underpinning these AQG thresholds for PM2.5 is discussed in a systematic review by Chen & Hoek (2020). Each 1 degree cell of each country is assigned one of the six risk categories. This is repeated for the world’s landmass of 149 million square kilometers, which implies the processing of about 300 million data points.

**Table 1. PM2.5 concentration thresholds**

|**Risk**|**classification**|**PM2.5 concentration**|**Mortality**|
|---|---|---|---|
|**Safe**|**No/minor risk**|≤ 5μg/m<sup>3</sup>|_baseline_|
||**Low**|5 – 10μg/m<sup>3</sup>|4% increased mortality rate|
||**Moderate**|10 – 15 μg/m<sup>3</sup>|8% increased mortality rate|
|**Unf**|**High**|15 – 25 μg/m<sup>3</sup>|16% increased mortality rate|
|**sae**|**Very high **|25 - 35μg/m<sup>3</sup>|24% increased mortality rate|
||**Hazardous**|over 35μg/m<sup>3</sup>|> 24% increased mortality rate|

_Assign air pollution risk categories to population headcounts at the pixel level and aggregate to the administrative unit:_ As the air pollution and population density maps are converted into the same spatial resolution, each population map cell can be assigned a unique air pollution risk classification – these cells can then be aggregated to the administrative unit level (e.g. province or district level). This allows the calculation of population headcounts for each risk category and for each (sub-)national administrative unit. This process yields an estimate of the number and share of people exposed to no-, low, moderate, high, very high and hazardous air pollution concentrations throughout the year. We offer these exposure headcount maps as global gridded layers with a resolution of 90 meters. In addition, we aggregate them to administrative units, including for each country and subnational unit. These estimates are further aggregated to yield regional and global estimates.

_Compute the number of poor people exposed to air pollution risk:_ While poverty estimates are not available at the pixel level, the World Bank’s Global Subnational Poverty Atlas (GSPA) of harmonized household surveys provides them at the subnational level for at least 153 countries. These poverty shares are multiplied with the population headcount that is estimated to be exposed to unsafe air pollution, in order to obtain an estimate of the number of poor people in each administrative unit exposed to air pollution risk.

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