spectrum of urban settings (i.e. from small urban centers to megacities) sufficient enough to be linked to adverse health effects.

In fact, one of the first major efforts to contextualize urban air pollution from a global perspective commissioned by the WHO and UNEP found pollution levels that could be associated with serious health effects in 20 megacities (i.e. cities with a population of more than 10 million) across the world. Generally, exceedances of WHO limits were much higher in megacities in developing countries (largely situated in Asia) with Suspended Particulate Matter (SPM) being the most severe, whereas gaseous pollution levels were somewhat spread throughout the sampled megacities. While air pollution seemed particularly more severe in megacities in developing countries, each megacity had at least one pollutant level exceeding WHO health protection guidelines with vehicle traffic being the single most important ambient pollution source accounting for CO, NOx, HC and Pb. This was largely based on data from some of the 270 established monitoring sites under the Global Environment Monitoring System/Air Programme covering 45 countries by 1992 (WHO & UNEP, 1993; Mage et al., 1996).

Likewise, subsequent studies investigating urban air quality on a global scale which also focused primarily on megacities and major urban centers found considerable levels of poor air quality. A snapshot of further insights from the global perspective studies (non-encyclopedic list) have been presented as follows: An assessment of air quality from the years 1990-2000 in the ‘principal cities’ of the world selected from Europe, Africa, North America, Latin America, Asia and Oceania by Baldasano et al (2003) which identified O3, PM10 and NO2 as pollutants of concern. Particularly, nearly half of the cities had maximum daily hourly O3 levels above 200μgm<sup>-3</sup> with North America (Mexico City), and Latin America (São Paulo, Santiago) recording the highest levels i.e. 491μgm<sup>-</sup> 3 -3 -3 , 403μgm , 351μgm r <mark>espectively, whereas most cities in Asia had high PM levels with a number of cities exceeding 300μ</mark> gm<sup>-3</sup> <mark>. These pollutant concentrations were several times higher than the WHO threshold values at the time (i.e. 120</mark> μgm<sup>-3</sup> and <mark>60-90μ</mark> gm<sup>-3</sup> for daily 8-hour O3 and suspended particulate matter respectively).<sup>6</sup> A similar study by Gurjar et al. (2008) classified air quality in 18 megacities based on a Multi-Pollutant Index (MPI) and found poor air quality in 13 of the megacities studied. Work on air pollution trends for Kolkata, Mumbai and Delhi (3 megacities in India) by Gurjar et al (2016) based on historical monitoring data (1991-2012) highlighted the severity of NOx and Particulate Matter ambient concentrations largely coming from transportation and industry. The geographic span of these sampled megacities during various timescales i.e. North America, Europe, Africa, Central and South America and Asia, highlights the historical significance of urban air pollution as a major health and environmental issue for both developed and developing countries, although there has been significant decline in SO2 and Pb levels attributable to regulatory enforcement.

Rapid population increase coupled with air pollution precursors and economic drivers e.g. motorization, combustion energy, industrialization, and other infrastructural developments may lead to inferring that air quality may be deteriorating in fast-growing urban centers in Africa. However, actual evidence is limited given the lack of adequate real-time monitoring data sets. Countries in Sub-Saharan Africa particularly have had minimal or oftentimes unsuccessful attempts to implement effective air quality control programs (Mage et al., 1996; Mayer, 1999; Baldasano et al., 2003; Petkova et al., 2013), and for a number of cases, data are nonexistent e.g. the WHO (2016) compilation of urban ambient air pollution in global cities had only 9 SubSaharan African countries with active monitoring in place, a possible reduction from what was reported in Schwela (2012).

> 6 This study referred to WHO guidelines of 1979, but the standards have since been updated and revised. the current standards are the WHO Air Quality Guidelines 2005.

3