Air pollution is defined as the presence of substances in the atmosphere resulting from human activity or natural processes. When these materials surpass pre-established concentrations, they can be harmful to health, as well as to normal community activities, such as activities of daily living and social participation.^{1 , 2}

Impacts of air pollution on cardiovascular morbidity and mortality have been described,³ and can be explained by: 1) an increase in fibrinogen and circulating inflammatory factors that lead to an increase in blood viscosity and coagulation disorders; and 2) changes in autonomic nervous system (ANS) modulation to the heart,^{4 – 9} which can be evaluated through heart rate variability (HRV).

Studies have shown that some factors like sex and functional capacity can influence ANS modulation.^{10 – 12} Women have a higher resting HRV than men of the same age group, indicating a parasympathetic predominance in females and a sympathetic predominance in males.^{10 , 12 – 14}

Assessment of HRV is a non-invasive, simple and inexpensive method, widely used in the quantification of cardiac sympathetic and parasympathetic modulation using linear and non-linear measures.^{11 , 15 – 19} An individual who exhibits efficient cardiac autonomic control mechanisms has a high resting HRV, characterized by a parasympathetic predominance.^{8 , 19 , 20} However, some cardiac conditions (e.g., conduction disturbances and atrial fibrillation) can also induce a high HRV.¹⁹

Thus, the research hypothesis is that individuals exposed to low concentrations of air pollution have better cardiac autonomic modulation compared to those exposed to high concentrations, and that women have better cardiac autonomic control than men. Thus, this study aimed to analyze the long-term impact of exposure to pollutants, specifically to smoke, and the influence of sex on cardiac autonomic modulation in healthy young people exposed to two different levels of air pollution.

The Shapiro-Wilk test was used to verify data normality. The impact of exposure to pollutants and of sex on HRV were analyzed by the two-way ANOVA statistical test, taking sex (male and female) and pollution (most polluted location – São Paulo and least polluted location – São Carlos) as factors, and analyzing the interaction between them using the post hoc Tukey's Test. Data were expressed as mean ± standard deviation. A significance level of 5% (p<0.05) was considered. For this purpose, the Sigma Plot 11.0 for Windows software was used.

Anthropometric characteristics and age of participants are described in Table 2 . There were statistical differences in body mass, height and estimated VO₂max between the sexes, where women showed lower values compared to men (as expected), regardless of the city in which they lived ( Table 2 ). Group 1 volunteers lived in São Paulo for 21.94 ± 1.43 years and group 2 volunteers in São Carlos for 22.55 ± 4.40 years.

Table 3 shows the results of linear (time and frequency domain) and non-linear (conditional entropy) HRV analysis in supine rest of the evaluated groups. Mean R-Ri showed the impact of sex and its interaction with air pollution exposure, which was higher in men compared to women in the city of São Paulo. Likely, HFnu (cardiac parasympathetic modulation) showed an interaction between sex and air pollution exposure, which was higher in women compared to men living in the city of São Carlos.

Moreover, HFnu was higher in women living in São Carlos, compared to those living in São Paulo. On the other hand, LFnu (cardiac sympathetic modulation) and variables related to the HRV complexity (conditional entropy) did not show an impact from pollution or sex.

The main findings of the present study were: 1) there was an impact of sex on mean R-Ri in subjects living the city with the highest concentration of air pollution (São Paulo/SP), as men had higher values than women; 2) there was an impact of sex on cardiac parasympathetic modulation in the city with the lowest concentration of air pollution (São Carlos, SP), as women had greater parasympathetic modulation compared to men; and 3) there was an impact of air pollution on cardiac parasympathetic modulation among women, as women from São Carlos, SP had greater modulation than those from São Paulo, SP.^{31 , 32}

The main finding of the present study was that long-term exposure to pollutants, specifically smoke, interferes with cardiac autonomic modulation, and female residents of the city with lower air pollution levels showed greater parasympathetic modulation compared to male residents.

Mean R-Ri at rest was higher in men than in women only in group 1, i.e ., among individuals exposed to a higher concentration of pollution, indicating an impact of sex on R-Ri regardless of air pollution levels. This may be explained by the fact that male individuals have a larger heart size³³ and, consequently, greater cardiac output and blood volume ejected by systole, presenting a lower basal heart rate compared to women.³⁴ A possible adaptive process related to long-term exposure to pollutants in both sexes should also be considered. Furthermore, men had a better functional capacity than women according to the AHA functional classification.²⁷

Parasympathetic modulation was greater in women than in men only in group 2 (lower concentration of air pollution). The association of resting HRV with sex, with higher vagal modulation at rest among women, has been observed in several studies,^{10 , 12 , 35} corroborating our finding. This may be due to a higher resting HRV and greater vagal modulation in women than men.^{10 , 12 , 13} However, pollution causes a decline in parasympathetic modulation and an increment in sympathetic activity,^{36 , 37} suggesting that a greater exposure to pollution did not equally affect the expected responses in HRV in men and women.

Concerning parasympathetic modulation, the within-sex analysis revealed an impact of air pollution exposure on females, as women in group 2 showed greater modulation than those in group 1. Thus, exposure to a higher concentration of pollution had an unfavorable impact on cardiac autonomic modulation in the study women. This indicates that females are more susceptible to the damage caused by pollution, as the parasympathetic modulation of men in the two cities was not significantly different. Other studies have also reported a reduction in cardiac vagal tone in individuals exposed to pollution, but they did not carry out an analysis by sex, which makes it difficult to make a comparison with the findings of the present study.^{8 , 20}

As previously stated, we observed different HRV between sexes and, therefore, men and women may suffer different effects of the long-term exposure to air pollution on the ANS. This finding may guide future studies regarding the association among these variables.

Regarding the complexity of HRV, assessed by the conditional entropy, no statistical differences were found between the groups (São Paulo vs. São Carlos residents) or between men and women. This is in accordance with the findings of Catai et al.,¹¹ who assessed various age groups and, for the age range similar to our study, there were no differences between sexes, as women showed lower HRV complexity only after menopause, due to the sharp decline in the estrogen hormone, while men showed a linear decrease throughout the aging process. On the other hand, Voss et al.,³⁸ who also assessed different age groups, found that in the 25–34-year age range, the complexity indices varied according to sex and were higher in women than in men.

If, on the one hand, the impact of sex on the variables related to HRV is well documented in the literature, on the other hand, the relationship of these same variables with exposure to atmospheric pollution is still poorly known. Existing studies mainly cover short-term pollution,^{3 , 4 , 6 , 7 , 9 , 36 , 37} making it difficult to compare their results with ours. In a meta-analysis of epidemiological studies carried out by Pieters et al.,³⁹ the authors found a reduction in HRV with an increase in air pollution exposure. However, when the literature search encompassed HRV, sex and long-term air pollution, we did not find any studies.

This study has some limitations. The first is that it involved only two cities in São Paulo State and, therefore, the results cannot be generalized; although there are differences in the concentrations of smoke pollution between the cities of São Paulo and São Carlos, these may not have been enough to cause other significant changes in HRV. It is important to consider that São Carlos is a medium-sized city, with many industries and a large vehicle fleet. If the comparison had been made between São Paulo and a smaller city (and thus with lower air pollution concentrations), perhaps greater differences would have been observed; however, smaller cities usually do not have air quality control, which could lead to spurious conclusions. Also, even though the municipality of São Carlos performs the measurement of smoke indices, these represents only a fraction of the particulate-matter air pollution. Another potential limitation is the small sample size, which may have influenced the results in some of the analyses, such as of HRV complexity (conditional entropy).

Therefore, our study suggests that long-term exposure to air pollution reduces HRV in healthy young adults, especially females, with a reduction in cardiac parasympathetic modulation. Thus, our findings can be considered as a springboard for further studies with a larger sample size, focusing on the assessment of data from different cities and age groups and the relationship between HRV, long-term pollution and sex, in order to confirm and expand our findings.

The results suggest that long-term exposure to pollutants, specifically to smoke, has an unfavorable impact on HRV, with a reduction in cardiac vagal autonomic modulation, in healthy young adults, mainly females.