Pollutants of major public health concern include particulate matter, carbon monoxide, ozone, nitrogen dioxide and sulfur dioxide, known as air criteria pollutants. The combined effects of ambient and household air pollution are associated with 6.7 million premature deaths annually (WHO, 2022). The relevance given by the WHO to the worldwide monitoring of air quality in large urban centers and metropolises has generated great interest from the scientific community and the media (de Vasconcellos et al., 2022). Air pollution is also linked to climate change (Ramanathan and Feng, 2009) as well as negative impacts on global economy (Aguilar-Gómez et al., 2022).
Urban and economic development is causing the functional areas of many cities around the world to transcend their political borders with labor, service and financial markets, as well as the physical extension of these cities across the jurisdictional territories of several neighboring municipalities (Gómez-Álvarez et al., 2017). The informal urban growth represents a defect in urban development in cities, the absence of design and planning standards together with the uncontrolled application of organizing laws lead to severe problems as the insufficiency of services, facilities and infrastructure (Bek and Elkafrawy, 2018). Environmental problems, such as low efficiency in solid waste management, fresh water supply and wastewater treatment, as well as poor air quality, are also related to unplanned urban growth which also increases public health risks.
Mexico City has transcended its State limit to the surrounding municipalities of the State of Mexico forming the Mexico City Metropolitan Area (MCMA). This is also the case of MATV, which is currently integrated by 15 municipalities (SEDATU, 2018). Both cities have grown without a proper process of planned urbanization, but instead there has been rapid urban expansion, diffuse and fragmented growth, and a conurbation with small and medium-sized peripheral settlements (Cruz-Bello, Galeana-Pizaña and González-Arellano, 2023). Despite the fact that MATV is located 60 km west from MCMA, it has been impacted by the immigration of labor and various industries, as well as by the increase in traffic density with the consequent related environmental problems (Iracheta, 2017). MATV has currently become the city with the highest PM2.5 concentrations in Mexico since 2019 (IQ Air 2023).
MATV is located between north latitude 18°59′07′′ and 19°34′47′′ and parallels, 99°38′22′′ and 99°56′13′′ west longitude (Fig. 1a) and has a territorial extension of 2,410.5 km2 (SEDATU, 2018). According with the last 2020 census, MATV has a population of 4,188,804 inhabitants (INEGI, 2021). Some of the MATV air quality problems are related to geophysical factors such as its high average altitude of 2,610 masl, being one of the highest cities of Mexico, were the atmospheric oxygen concentration is lower with respect to sea level, causing a less efficient combustion (Wang et al., 2013); in addition, MATV is partially surrounded by mountains affecting the air pollutants dispersion and increasing thermal inversions occurrence and high pressure systems (SMAGEM, 2012). Toluca Valley has three weather seasons: Dry-cold (DC) season, from November to February, Dry-hot (DH) season from March to May and Rainy (Ra) season, from June to October (SMAGEM, 2012).
Since 1960, Toluca Valley had an industrialization process promoted by the government and based on the offer of land, low-cost energy and water supplies with virtually no environmental regulations, resulting in the establishment of several industrial enterprises, the consequent increase in traffic density, as well as labor immigration (Orozco Hernández et al., 2003). Currently, there are more than 11,000 manufacturing economic units registered (SMAGEM, 2012) and 1,047,661 registered motor vehicles in circulation (INEGI, 2021) with a rate of 250 vehicles per 1000 habitants. This large amount of vehicles is related to the absence of a massive transport system due to the unplanned historical expansion of the metropolitan area provoking a high amount of scattered suburban areas (CMM, 2014).
There are relatively few research publications analyzing the MATV air pollution phenomena, even so, serious health risks have been evidenced, as the presence of bioaerosols (pollen, spores, microorganisms, fragments and diatoms) with aerodynamic sizes of 0.3 µm to 45 µm (Romero-Guzmán et al., 2021). Ávila-Pérez et al., (2019) found a significant enrichment of Pb followed by Zn, Cu and Cr accumulated in mosses biomonitors. Romero-Guzman et al., (2018) reported high concentrations of heavy metals in rainwater TSP in the Northern zone of Toluca Valley. Unlike MATV, MCMA has been the subject of several studies to analyze the trends of air pollutants, their impact on public health and their emission sources (Mugica et al., 2009; Molina et al., 2010; Cromar et al., 2021).
The present study aims to contribute to the understanding of the serious air quality problems in cities with unplanned urban growth through the case study of MATV, by analyzing ten years (2011–2021) of ACP monitoring data, describing temporal and spatial distributions to identify the areas with higher risk to the population, in compliance with the WHO AQG. A comparison between MATV and MCMA ACP evolution levels was made along these 10 years to analyze the effects of the application of environmental policies taken by the Mexico City government and not in MATV. The impact on public health was assessed through the estimation of the number of natural deaths associated with PM2.5 exposure using the AirQ + software developed by WHO for MATV and Mexico City. The changes in the ACP emissions in MATV related to the COVID-19 lockdown were analyzed to identify which pollutants were significantly reduced in relation to the decline in economic activities.