## 4 Discussion
Recent studies have found that personal exposure is higher than those measured in fixed sites [23; 60] and that it is determined by people's activities, especially in indoor environments [27; 61]. This study showed that the spatial distribution of PM\({}_{2.5}\) identified two areas in which PM\({}_{2.5}\) levels were very high, and two more with lower ones; these areas were distant from each other, with very different social, economic, and topographic conditions, where local stratified sampling methods might be used in further studies. When the variables were chosen, we expected to find a spatial relationship among them, as well as between their locations and interactions [62]. The basin of Mexico is not a flat surface with standard meteorological and topographical conditions; it is neither homogenous in its social, economic, commercial, and transport networks. On the contrary, the city is a mixture of micro-environments entangled within an intricate collection of geomorphological features in which conditions can change from one place to another, including the induced biological effects caused by a different composition of the particulate matter [63; 64]. The results showed how the abovementioned conditions might influence the spatial distribution of contaminants such as PM\({}_{2.5}\). For instance, the spatial distribution of PM\({}_{2.5}\) was the lowest in the northern area of Xochimilco, located in the south part of the basin (Figure 1, borough 13), which is well known for its _chinampas_ (part of the UNESCO's list of human heritage). This area is one of the least contaminated regions in the city. This could be related to the high levels of humidity due to the amount of lakes, canals, and trees. This region presented some of the highest continuous NDVI indexes in the study area (approximately 174 ha with an index value of more than 0.3, just in the wetlands with a maximum of 0.489). Furthermore, this borough is in some ways protected by the Iztapalapa peninsula and the Huizachtecatl volcano, which serve as a wall that impedes the free flow of wind from north to south. Moreover, most of the population is located on a flat surface that is interrupted by the piedmont of the volcanic field of the Sierra Chichinautzin; therefore, the dispersion of contaminants could be more efficient and the region cleaner. It is worth mentioning that the spatial distribution of PM\({}_{2.5}\) was higher in other areas. For instance, the spatial distribution of PM\({}_{2.5}\) was among the highest in the borough of Iztapalapa, located in the southeast part of the basin (Figure 1, borough 7) (Figure 3). This does not come as a surprise since the borough has intrinsic characteristics such as its high population density (Instituto de Vivienda del Distrito Federal, Comision Nacional de Fomento a la Vivienda, & Universidad AutonomaMetropolitana, 2007) and its particular topography consisting of a small mountain formation called Iztapalapa Peninsula. This formation extends all the way to the Santa Catalina range. The Huizachtecatl volcano, or the Star mountain, is located almost in the center of the borough. As previously said, these three formations range from 2400 to 2900 masl and constitute a natural wall that prevents the wind from flowing from north to south. Therefore, it is possible for the wind to deposit the pollution at the north facing slope of these topographical formations. The Central de Abasto (Supply Center) is also located in Iztapalapa, north of these mountains, and is one of the busiest places in the whole country. According to the Supply Center webpage [65], this market distributes up to 35% of fruits and vegetables in the country, handles over 30,000 tons of merchandise per day, is attended by more than 500,000 people per day, and has 62,000 vehicles in transit within its borders. The spatial distribution of PM\({}_{2.5}\) in the western and northwestern parts of the basin was the highest (Figure 3).