1. Introduction

#### 1.1. Pollution

The WHO (World Health Organization) estimates that 12.6 million deaths each year are attributable to unhealthy environments [1], confirming the urgent need for investment in strategies to reduce environmental risks in cities, homes, and workplaces. Of these deaths, 4.3 million (34%) and 3.7 million (29%) have been linked to exposure to indoor and outdoor pollution, respectively [2]. More broadly, it is considered that 92% of the population lives in areas that are exposed to respirable particulate matter of <2.5 μm in diameter (PM2.5) at an annual mean concentration of >10 μg/m3 (or a 24 h mean of >25 μg/m<sup>3</sup> ), which exceeds the WHO Air Quality Guidelines (AQG). The worst affected areas are China, the Indian subcontinent, the Middle East, and northern Africa.

Particulate and gaseous air pollution results largely from industrial and motor vehicular sources, although these emissions are exacerbated by those generated by heating [3]. The combustion of solid biomass and impure liquid fuels is the main source of pollution leading to health problems, as evidenced by numerous recent studies. For example, in Ethiopia, the average concentration of respirable particulate matter of diameter of <PM2.5 reaches as high as a 24-h mean concentration of 280 μg/m<sup>3</sup> , indoor CO levels are greater than the regulatory limits for the US, and the average NO2 level is 97 ppb [4]. In Southeast Asia, the large-scale burning of biomass causes heavy emissions of combustion by-products, which photochemically transform to other more dangerous molecules [5], such as polycyclic aromatic hydrocarbons (PAHs) [6]. Europe is affected mainly by emissions of nitrogen oxides (NOx) [7]. Although outdoor pollution tends to be a focal point, indoor air in the US can be two to five times more harmful than outdoor air [8]. Almost 96% of houses in North America are reported to have at least one indoor air quality problem [9]. Cigarette smoke contains more than 7000 chemicals, of which 69 are known to be carcinogenic and many more to be poisonous [10]. Household items, such as cleaning products, air fresheners, and sanitation products, also contribute to indoor pollution by releasing chemicals. Consequently, there are environmental health imperatives to develop technologies that address these while satisfying other demands in the areas of energy efficiency, economic efficiency, utility, and esthetics.
