This cross-sectional study was conducted among Tehran’s urban green space workers exposed to various air pollutants due to their work environments. The study explored the potential health implications of airborne contaminants, especially heavy metals, and their association with the urinary oxidative stress indicator, 8-OHDG. Despite multiple sources of pollutants, the outcomes of diverse investigations have shown inconsistent findings. The study compared metal concentrations in breathing air, urine, and urinary 8-OHDG levels between urban green space workers and office employees. The demographic factors of the exposure and control groups were the same (due to the study's predetermined inclusion and exclusion criteria). In many studies, for a better comparison, the studied demographic characteristics were the same among the studied groups [29], especially age, work experience, and BMI, which could significantly impact the interpersonal investigations.
Comparing the concentration of heavy metals in the urine and breathing air of green space workers and office staff showed that all heavy metals, except for cadmium, had higher amounts in green space workers’ breathing air and urine. The lack of significant cadmium level differences could be due to non-normal data distribution. Despite this, workers still showed slightly higher levels of cadmium than office staff. In this regard, it is possible to refer to the study of Hung-Hsin Liu et al., which found that the concentration of the studied metals in the breathing air of employees with direct exposure was higher than in office workers, which is remarkably compatible with the present study [33]. In findings on the correlation between metal concentrations in the air and urine of workers, it was expected that as the concentration of metals in the workers' breathing air increased, the concentration of metals in their urine would also increase, and this correlation was found to be significant for all metals studied. A study in 2020 showed that cobalt in air has a rank correlation with cobalt in blood and urine, with coefficients of 0.4 and 0.25, respectively [34]. Also, in another study, in line with the present study, there was a significant correlation between the concentration of zinc in air and urine, with a correlation coefficient of 0.47 [35]. In Hengstler’s study, the results related to cobalt were similar to the present study's findings. However, cadmium in air and urine did not correlate, except in smokers, where a significant correlation was shown [36]. Similar to the current study, Rosati et al., in a cohort study, indicated a significant positive correlation between the cadmium levels in the ambient air and the urine samples [37].
The urinary concentration of heavy metals in green space workers was notably higher than that of in-office employees, indicating the greater exposure of green space workers than office workers. These significant differences are mostly related to the nature of their tasks in the urban green space. The comparison findings may be primarily attributed to factors such as working during rush hour traffic and exposure to car traffic and air pollutants. Urban green space workers mainly experience outdoor work and long-term exposure to urban pollutants. They are exposed to many pollutants, so they are expected to have more exposure than office workers. Numerous studies have demonstrated that air pollutants, such as heavy metals, can lead to a more significant elevation of urine biomarkers than individuals not exposed to such pollutants. For example, Ghaedrahmat et al. showed that the urinary concentration of cadmium in the exposed group was higher than in the control group, which is consistent with the present study.
Contrary to the cadmium, the results related to comparing the concentration of other metals in the urine of the exposed and control groups differed from this study. As an illustration, the concentrations of cobalt and zinc exhibited equivalence in both groups [38]. In another study conducted by Yang et al. in China, surprisingly and contrary to this study, the concentration of cadmium in the urine of the control group was higher than that of the exposure group [29]. The current study indicated that the working settings of green space employees and office workers, in addition to their distinct tasks, significantly influenced the observed differences. Their workplace has a high population density near the city center and various government, private, and scientific institutions. Also, some green space workers act along highways where heavy vehicles travel, and others work near major industrial centers in southern parts of Tehran. Due to the large number of vehicles that travel through these working areas, the increase in pollution can be attributed to the numerous vehicles in these regions. Notably, zinc, cobalt, and cadmium atmospheric emissions are primarily the result of human activities, such as using fossil fuels and direct emissions from automobile exhausts [39]. According to a study by Talebi et al., areas with high traffic congestion and population density have the most significant concentrations of PM10 and heavy metals [40].
As Table 3 shows, the concentration of 8-OHDG in the urine of green space workers was significantly higher than that of office workers, suggesting that green space workers were exposed to a relatively higher risk of DNA damage. In a study conducted by Huang et al. in 2013, similar to the present study, the urinary concentration of the oxidative stress biomarker was higher in the exposed group than in the control group [41]. This result can be attributed to their exposure to air pollution constituents, particularly heavy metals. This inference is supported by the statistically significant increase in the concentration of heavy metals in the urine of workers compared to office workers. Moreover, there was a significant correlation between the urinary concentration of the studied metals and oxidative stress biomarkers in green space workers compared to office workers. A 2009 study investigated oxidative stress biomarker levels in metal industry workers’ urine. The results showed a significant difference between the exposed and control groups, with the exposed group showing significantly higher biomarker levels [33]. Many studies have been conducted on oxidative stress indicators within occupational cohorts associated with metal industries, mainly in indoor occupational settings. However, it is worth noting that some specific studies have also indicated that the urban environment, including car exhaust emissions, plays a significant role in enhancing DNA oxidative stress [42–44]. In a study, Yueh-Ying Han et al. showed that exposure to traffic and car exhaust fumes increases oxidative stress biomarkers in drivers compared to the control group of office workers [43]. Generally, it is found that the levels of oxidative stress biomarkers are higher in groups exposed to respiratory metals compared to control groups. However, it is necessary to note that these findings should not be regarded as definitive due to the influence of various interpersonal and intrapersonal factors on the suitability of these biomarkers as indicators of biological response to metal exposure. Many investigations suggest using the 8-OHDG concentration in urine as a reliable method for investigating DNA damage. Regardless, it is crucial to account for the daily fluctuations of this biomarker. Hence, data from cross-sectional studies should be interpreted cautiously [45].
The multiple linear regression analysis conducted on the exposed group revealed that cobalt and cadmium exhibit the most significant influence on changes in oxidative stress, as indicated by the dependent variable 8-OHDG. These findings were in line with the Pearson correlation of the metals, which determined that cobalt, cadmium, and zinc in the urine of green space workers have the highest to lowest correlation values with urinary 8-OHDG. The results of correlation analysis in a study conducted by Hengstler et al. in 2003 showed that DNA damage has a high correlation with the concentration of cadmium and cobalt metals in the breathing air of the subjects and the concentration of cobalt in their urine, which is in line with the results of the present study. Unlike the present study, in the abovementioned study, there was no correlation between cadmium urinary concentration and DNA damage [36]. Similar to the current study, Huang et al. showed that the concentration of cadmium in the urine of employees and non-smokers is related to and correlated with the urinary 8-OHDG [41]. Measuring urinary cadmium levels has been proposed as a suitable biomarker for assessing respiratory exposure to this particular metal [46]. As previously mentioned, the findings of the present investigation also revealed a strong association between the presence of cadmium in employees' breathing air and its excretion in their urine. Cadmium has been observed to induce the generation of reactive oxygen species (ROS), reducing the antioxidant defense system through the depletion of glutathione levels. This depletion then leads to a decrease in the activity of cellular antioxidant enzymes, ultimately resulting in mitochondrial damage and an increased susceptibility of cells to oxidative stress [16, 46]. Like the current study, the United States Food and Drug Administration (FDA) also declared that cadmium can potentially elevate oxidative stress levels [47].
In contrast to the abovementioned results, Hengsler's investigation of cadmium in urine yielded different effects [36]. The disparity in urine cadmium levels between individuals exposed to cadmium and those working in office settings can support the notion that respiratory exposure to cadmium substantially contributes to the induction of oxidative stress. Cadmium toxicity is essential, and many epidemiological studies have demonstrated that exposure can have long-term impacts [48]. According to a survey conducted in 2020, there was no observed association between urinary biomarkers of oxidative stress and the levels of specific metals discovered in the urine of individuals working at waste incineration centers [29]. The potential cause of this occurrence may be attributed to implementing preventative measures, such as using personal protective equipment and installing ventilation systems, which effectively restrict the exposure period. Nevertheless, the current research has identified significant associations between heavy metals in the air and urine and urinary heavy metals and urinary 8-OHDG levels among urban green space employees. Indeed, the lack of substantial air pollutants in non-industrial environments results in the absence of comparable protective measures and may increase urinary biomarkers by increasing airborne exposure.