3.1. Spatial and temporal analysis of PM2.5
The monthly average maps of PM2.5 concentration for the period of March to December 2021 were generated using spatial interpolation and are shown in Fig. 2. PM2.5 mass concentrations in Curitiba during the period of this study exceeded the annual average WHO guideline of 5 µg.m-3 (WHO, 2021b) in all locations. These concentrations were similar to those reported in a previous study conducted in the city between 2008 and 2015, with an average value of 9.7 µg.m3 (Polezer et al. 2019), and within the observed average of 11.7 µg.m-3 for urban cities in Brazil (Health Effects Institute -HEI, 2020). The maps illustrate the regional distribution of the pollutant, indicating a general gradual increase in pollutant concentration along an imaginary line from the north and northeast towards the south and industrial regions. The differences in PM2.5 levels reflect the characteristics of each city region, as represented in Fig. 1. Therefore, the study identified three main clusters of regions based on the daily and annual levels of pollution, as well as the Pearson correlation generated using the daily mean concentrations of PM2.5 for all monitored regions (detailed in the supplementary material, Fig. S5).
The first cluster consists of the industrial areas of SDTQ, SDCIC, and the city of Araucaria, which exhibit the highest concentrations of PM2.5, with about 28% of the days exceeding the value recommended by WHO air quality guideline (15 µg.m-3) (WHO, 2021b) and there is a high average Pearson correlation coefficient of 0.91 among these regions, Fig S5(b).
The second cluster includes the residential districts of SDSF and SDCJ, which exhibit lower pollution concentrations and demonstrate a high correlation with each other (r = 0.90).
The third cluster comprises the intermediated regions of SDBV, SDPR, SDBQ, along with Curitiba Downtown (SDMZ) which are highly correlated with each other (ranging between 0.81 and 0.94) and have lower correlations with the residential sites. These urban regions are categorized as intermediate regions in terms of their PM2.5 concentrations as showed in Fig. 1, positioned between the highly polluted industrial sites and the less polluted residential regions, Consequently, these regions are influenced by traffic flow from nearby highways, high-flow lanes with commercial establishments, and proximity to industries. Moreover, the cities situated north of Curitiba have the highest concentrations of limestone mines in the state of Parana, which could result in a significant environmental impact due to the emission of particulate matter from these activities, s as observed in the SDBV region. Despite being predominantly residential (Fig. 1), this district demonstrated higher concentrations of particulate matter (Fig. S5 (a)) compared to the regions in the second cluster and exhibited intermediate correlations when compared to the other regions (Fig. S5 (b)).
The maps also indicate that the months with the highest PM2.5 concentration are from June to September, whereas October, November, and December exhibited the lowest concentrations. Increased pollutant levels coincide with periods of reduced rainfall (INMET, 2021), due to the “washout“ effect" (Seinfeld et al., 2016). Furthermore, lower temperatures during winter days and the absence of rain (Fig. S6) exacerbate the occurrence of thermal inversion, leading to the accumulation of atmospheric pollutants in the lower atmospheric layers.
The seasonal variation of PM2.5, with higher concentrations in winter months compared to summer, has been observed in other locations worldwide, including Beijing in China (Liu et al., 2017) and urban cities in Europe, such as Vienna, Amsterdam, and Stockholm (Adães and Pires, 2019). Furthermore, other studies conducted in Curitiba have also observed seasonal variation, with lower values reported by de Miranda et al. (2012) in summer (Dec-Feb) ranging from 8 µg.m-3 to higher values of 16,7 µg.m-3 in winter months (Jun-Aug). Polezer et al. (2018) also identified in Curitiba that the summer months presented a lower number of days with values above the 10 µg.m-3.
3.2 Spatial Assessment of COVID-19 Health Impacts, socioeconomics impacts, and PM2.5 Mass Concentration
The spatial analysis of health impacts reveals regional variations in mortality rates and incidence of COVID-19 cases (Fig. S7) during the two-year period. Furthermore, the majority of the regions examined exhibited annual rates (per 100,000 inhabitants) exceeding the Brazilian average for both years. In the case of Curitiba as a whole, the mortality rate in 2020 exceeded this average by approximately 35%, while the incidence rate was 59% higher. The city of Araucaria (9) is an exception, with a 6% lower mortality rate in 2020 compared to the national average in Brazil. However, it recorded the highest number of cases per 100,000 inhabitants during the two-year period, with 21,934 cases, surpassing the Brazilian rate by more than 96% in 2020 and accumulating a 33% higher rate in 2021.
Among the study regions, the city of Araucaria showcased the lowest MHDI (Fig. 3), ranking it 54th in the state of Paraná (IBGE, 2021). Furthermore, owing to its industrial nature, the city exhibited the highest average concentration of PM2.5 in the monitoring conducted in 2021 as part of this study.
In general, regions that attained higher averages of particulate matter corresponded to areas with lower MHDI values (Fig. 3), aligning with previous studies (Clougherty et al., 2022; da Motta Singer et al., 2023).
The region of SDPN, situated in the intermediate regions of Curitiba, recorded the highest mortality rate per 100,000 inhabitants during the analyzed two-year period, with 499 deaths. Moreover, this region this region is also distinguished by being the second lowest index in Curitiba. These socioeconomic conditions may contribute to increased vulnerability among the population regarding COVID-19 transmission and may also impede access to medical services for diagnosis and treatment, as suggested in previous studies (Bermudi et al., 2021).
Furthermore, the central regions of the map, comprising SDPN, downtown SDMZ, and SDPR, exhibited significantly higher morbidity and mortality rates compared to the other regions. These areas are renowned for their significant influx of people, which serves as a contributing factor to the transmission of COVID-19 (WHO, 2023). For instance, SDPN host the two largest bus terminals in Curitiba, capable of accommodating over 252,000 people daily (IPPUC, 2021). Likewise, SDPR and SDMZ boast a significant concentration of commercial and service establishments, leading to an intensified influx of people in the respective regions.
In contrast, the northern/north-eastern regions (SDBV and SDSF) of Curitiba municipality displayed comparatively lower mortality and incidence rates of COVID-19. Notably, these areas corresponded to regions with a reduced spatial concentration of PM2.5 and are predominantly residential in nature. Moreover, these regions possess higher MDHIs in comparison to the Southern/Southwestern regions depicted on the map.
Similar findings linking socioeconomic and geographic indicators, as well as air pollution, to COVID-19 case fatality rates and incidence have been reported in studies conducted in various countries, including the United States (Liang et al., 2020), Brazil (Lorenz et al., 2021), England (Travaglio et al., 2021), and China (Zheng et al., 2021).
In the statistical analysis conducted using the Pearson correlation through a distributed lag model over a time lag ranging from 0 to 28 days, most of the studied regions showed positive and significant Pearson correlations (Fig. S8) between PM2.5 concentration and COVID-19 cases. The influence was particularly pronounced a lag period of 1 to 6 days (p-value > 0.01). Moreover, a significant positive correlation (p-value < 0.05) was identified between COVID-19 deaths and the spatialized concentration of PM2.5, but this association was found only in the most polluted regions of the study.
These findings are in line with previous research that demonstrates the correlation between exposure to air pollution and the exacerbation of infectious diseases (Ciencewicki and Jaspers, 2007). Moreover, they are consistent with recent systematic reviews that have reported a positive correlation between PM2.5 and both COVID-19 cases and deaths (Bhaskar et al., 2020; Copat et al., 2020).
3.3 Time Evolution of COVID-19 in Curitiba
The data presented in Fig. 4 provides a comprehensive evaluation of atmospheric pollution due to PM2.5 and other significant factors related to the phases of COVID-19 and urban mobility in Curitiba. Notably, the SDMZ location, situated in the city center, exhibited a strong correlation of PM2.5 mass concentration and all other measurement points (0.76 < r < 0.93), suggesting its characterization as a typical pollution hotspot in the city (Fig. S5b). Moreover, the data collection period in this area was the longest (2020–2021), further enhancing the significance of the findings.
The number of passengers using public transportation has frequently been used as an indicator of urban mobility during the pandemic period (Marra et al., 2022). However, it has been noticed that there is no direct relationship between the number of passengers and the levels of pollutants in the city. This is because PM2.5 emission sources in the city are not exclusive to public transport. A study conducted in the city of Curitiba also found that, in addition to public transport, emission sources from private vehicles (cars) and subsequently from the local industries all contribute to the levels of particulate matter in the city (Gidhagen et al., 2021).
A comparative analysis was conducted on the periods before (May 2020 – April 2021) and after vaccination (May 2021) phases was performed to investigate their potential effects on the environment. The evaluation revealed a gradual increase in pollution compared to the corresponding months in previous years, coinciding with an increase in passenger traffic. These findings suggest that the gradual easing of mobility restrictions for the population and service operations following increased inoculation has contributed to higher pollution levels in the city. Thus, the observed deterioration in air quality can be attributed to these relaxations.
To examine this trend, we computed the Pearson correlation coefficient using a lag method, considering the concentration of PM2.5 at the central location (SDMZ) as a representative measure of Curitiba. We analyzed four distinct periods - scenario: (1) Reference data from 2020 - "Curitiba 2020"; (2) Reference data from 2021 - "Curitiba 2021"; (3) Curitiba data from June to October 2020 - "Curitiba Jun/Oct 2020"; and (4) Curitiba data from July to October 2021 - "Curitiba Jul/Oct 2021." The comprehensive results, including p-values, can be found in Supplementary Materials, Fig. S9.
The highest significant coefficients were found for scenario 4, with mean values of 0.29 for the number of deaths and 0.28 for the number of cases. Furthermore, the findings demonstrated that shortening the evaluation period to the most polluted months of PM2.5 exposure - "Curitiba Jun/Oct 2020" and "Curitiba Jul/Oct 2021" - exerted a more substantial impact on the health of the COVID-19-affected population compared to the broader annual correlations - "Curitiba 2020" and "Curitiba 2021" - which yielded negative mean r results (p-value < 0.05).