# **2.  Prior Research**

Our analysis focuses on two health-critical air pollutants, fine particulates (PM2.5) and NO2, that are measured by ground monitoring in Dhaka. Among widely-measured air pollutants, the international health community currently believes particulate air pollution to be the most damaging. It is a complex mixture of small and large particles of varying origin and chemical composition. Over time, health research has narrowed its focus from all particles, to particles less than 10 microns in diameter (PM10), to fine particles whose diameters are less than 2.5 microns (PM2.5).<sup>2</sup> Coal-fired power plants and motor vehicles are the largest sources of PM2.5 (WHO 1997, World Resources 1998-99). Fine particles are more dangerous because they can penetrate deeply into the lungs, settling in areas where natural clearance mechanisms cannot remove them. The constituent elements of small particles also tend to be more chemically active and therefore more damaging (WHO 1997, World Resources 1998-99). Many studies have found that exposure to particulates is strongly associated with mortality and morbidity (Wells 1994; Xu 1994; World Bank 1996; Cropper et al. 1997; Ostro 1999; Kappos et al. 2004; Pui et al. 2014; Boldo et al. 2006; Lua et al. 2015; Franklin et al. 2007). Identified health effects range in severity from coughing and bronchitis to heart disease and lung cancer. In addition, the research has found no "critical threshold" that governs safe exposure to particulate pollution. Damage increases with exposure, starting at very low concentration levels.

Extensive research has also considered the health impacts of NO2, with particular attention to respiratory problems for children and cardio/pulmonary problems for the elderly. These impacts have been studied both singly and in combination with other air pollutants (Brauer et al. 2002, 2007; Cesaroni et al. 2008; Chen et al. 2012; Gauderman et al. 2005; Gehring et al. 2002, 2006; Hoeck et al. 2002; Morgenstern et al. 2007; Naess et al. 2007; Nafstad et al. 2004; Sunyer et al. 2006). The findings highlight significant associations with rhinitis, asthma, bronchitis and other respiratory illnesses, as well as cardiovascular morbidity and mortality.

Recent research has also suggested that PM2.5 and NO2 pollution aggravate Covid-19 infections and increase fatalities because fine particulates and nitrogen dioxide cause lung inflammation and compromise immune systems (Andrée 2020; Liang et al. 2020; Xiao et al. 2020).

Accumulating research on health damage from PM2.5, NO2 and other air pollutants has led developing-country policy makers to consider more stringent pollution control measures. However, limited air pollution monitoring has left considerable uncertainty about location-specific pollution impacts and the design of locationally-targeted, cost-effective measures for addressing the problem.

> 2  Large particles usually contain dust and smoke from industrial processes, construction, agriculture and road traffic, as well as plant pollen and other natural sources.  Smaller particles generally originate from combustion of fossil fuels.  These particles include soot from vehicle exhaust, which is often coated with chemical contaminants or metals, and fine sulfate and nitrate aerosols that form when sulfur dioxide and nitrogen oxide emissions condense in the atmosphere.

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