## 1 Introduction  
In the last four decades, air pollution (AP) has been considered one of the most relevant environmental problems in the world. The increase of mobility, the continuous expansion of industrial production in new locations, and the increase of chemicals in agriculture contribute to the growth of air pollution and to enlarging the impacts of it in several domains, namely the recent emergence in the public health agenda [1]. This type of pollution expands to its maximum levels in urban areas, especially in big cities and metropolitan areas, due not only to the intense traffic and general concentration of pollutants, but also to the contribution of climate change in enhancing the environmental variables for the propagation of air pollution [2; 3; 4]. Europe is not an exception, as 77% of its urban population has been exposed to unsafe levels of air pollution [5; 6]. Although being a relatively recent public health concern, the relation between air pollution and health has long been discussed in academia, where many epidemiological studies have supported that air pollution has an effect on the individual's health, while those impacts can be more or less harmful [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19]. In fact, the share of population who are more prone to the dangerous consequencesare the most vulnerable groups, the young and the elderly, but also those with preexistent morbidity conditions, in which are included habits such as smoking, that compromise populations' health and enhance risks [20; 21; 22; 23]. Furthermore, while the young population seems to have a generally faster and fuller recovery from the contact and impact of air pollution, the elderly group is more severely affected in the decline of their expected years of life [24; 25], a trend that is particularly evident in Europe and North America, where the populations have been ageing, and that shows no signs of reversing [26]. The increasing importance due to its noxious levels, climate change contribution (including inherent effects on other determinants), and its concrete influence on populations health status highlights air pollution's role in public health [17], an aspect that becomes particularly evident in cities that take up the leading role in terms of air pollution--in a great extent due to traffic [27; 28; 29], but also in terms of share of affected population and due to the higher population density [30; 31]. So, air pollution is a determinant of health--exogenous--since it plays a role in the health of populations, being affected and affecting other determinants (like the aforementioned climate change, traffic, or population density). Every pollutant has its own characteristics and, therefore, different impacts on the human being. This means that the health effects of air pollution are dependent (among other factors, like exposure) on the type of pollutant. However, the main diseases associated with air pollution are respiratory and circulatory diseases, such as chronic lower respiratory diseases, upper respiratory infections, cardiac diseases, or cerebrovascular disease. Although the pollutants' emissions have plenty of sources, for instance households, industry, or traffic; in an urban context, the traffic concentration is the main source of air pollution [32]. In the last few years, among the most studied air pollutants are inhalable particles PM\({}_{10}\) and PM\({}_{2.5}\), pollutants that have been related to the decline of pulmonary function and increased risk of developing cardiovascular and circulatory diseases [9; 10; 11; 21; 33; 34; 35; 36; 37]. Other important pollutants are nitrogen oxides (big contributors to the climate change and appearance of smog), particularly the nitrogen dioxide (NO\({}_{2}\)) and nitric oxide (NO), these also demonstrate--primarily the former, which has the most impacts on health (Table 1)--to prompt the individual increase of risk in developing diseases of the respiratory tract [38; 39; 40; 41; 42; 43; 44].