A new global air quality health index based on the 2021 WHO air quality guideline values

As Africa continues to grow with rapid urbanization and industrialization, the downside is that air pollution is the second leading cause of death on the continent; yet its health effects remain largely unquantified. This is evidenced by global systematic reviews and multi-city studies in which South Africa was the only country included. This thesis addressed two major research gaps in South Africa. First, it examined the short-term effects of multiple ambient air pollutants on cardiorespiratory hospitalizations and mortality. Second, it proposed a revised methodology for the Air Quality Health Index (AQHI) that references the long-term WHO 2021 Air Quality Guideline values. This would allow the scientific evidence to be properly reflected in the interpretation of short-term concentrations. The constructed index was applied using air pollution data from Cape Town.
The study used daily air pollution, temperature, relative humidity data from the City of Cape Town, with hospital admissions (2011-2016) and mortality (2006 – 2015) data collected from six private hospitals and the South African Department of Statistics, respectively. For both hospital admissions and mortality due to cardiovascular and respiratory diseases, time-series analyses of daily health outcomes and air pollution were conducted using generalized additive quasi-Poisson models within a distributed lag non-linear modelling framework to estimate the cumulative effects. In addition, independent effects of multi-pollutant were examined and analyses were stratified by age group, sex, and season. The AQHI was constructed using effect estimates from two global systematic reviews and meta-analyses. Excess risks (ER) were derived for PM2.5, PM10, NO2, SO2 and O3. Single pollutant AQHIs were developed and scaled using the ERs at the WHO 2021 long-term Air Quality Guideline (AQG) values to define the “low risk”. The daily total AQHI is the weighted average of the single AQHIs.
In the first part, we found a robust association between air pollution and cardiorespiratory health outcomes, with stronger effects among the female group and the elderly, with evidence of mortality displacement among the elderly. The second part of the thesis showed that the daily air quality posed “low risk” to the Cape Town population on 11% of the days within our study period. In addition, there was inconsistent improvement in the daily air quality over the decade, with the last year (2015) having the highest number of “low risk” days (28%).
This thesis adds to the growing body of evidence on the cardiorespiratory health effects of short-term exposure to air pollution and proposes a new tool for communicating air quality.