The risk attributed to exposure to low levels of air pollution for human pathogenesis using only epidemiological tools requires long-term studies and the participation of a large number of individuals exposed to pollutants. This type of study has disadvantages such as the high operational cost and logistical complexity and because if the association is established, a considerable number of participants will already have developed the disease. Although plant mutagenesis and aggravated human health cannot be compared, the genotoxicity assay with T. pallida can be used as a screening tool to assess human risk under unfavorable environmental conditions .
With respect to environmental variables, studies by Alves et al. , Isidori et al. , Klumpp et al.  and Savóia et al.  show that air temperature may act as a possible confounding factor, promoting the increase in micronucleus frequency. In our study, this confounding factor was minimized by virtue of the climatological homogeneity of the points. This can be observed by the fact that the ratio between the temperature of each test station and that obtained in the control station ranged between 0.9 and 1.0. The micronucleus frequency found in the actively and passively exposed plants in the control area of Chapada dos Guimarães was similar to that observed in the control group of studies that also assessed the genotoxicity potential of compounds present in air pollution, conducted by Batalha et al.  and Guimarães et al.  in Brazil, Carreras et al.  in Argentina, and Prajapati & Tripathi  in India.
The comparative results between the environmental and meteorological variables (Tab. 1) at the different points and exposure periods corroborate those reported by Rosa et al.  about the delimitation of two well-defined climatic periods (dry and rainy). This fact interferes significantly in the number of biomass burning points and pollutant dispersion, given that the pollution generated by biomass burning in the Amazon reaches its peak during the dry season . Studies conducted by Andrade Júnior et al. , Savóia et al.  and Souza Lima et al.  to assess the genotoxicity of pollutants present in the atmosphere, using the Trad-MCN test also adopted a similar exposure sampling period. Although the literature contains studies that were conducted with much shorter exposure periods, the results remained significant [43, 47, 49]. In relation to micronucleus frequency, the results show that the response obtained in the dry season was higher than those in the rainy season at 92% of the points and statistically significant at most of them, irrespective of exposure type. These findings may be explained by the fact that in the dry season there are a larger number of hotspots generated by biomass and sugar cane straw burning and thus higher levels of air pollutants, especially fine particulate matter. Regardless of the type of exposure, when we compare the results obtained at the test points compared to control, we observe that, unlike the dry season, there are no significant differences in the rainy period. This may be owing to the lower number of burnings in this period , which generates a lower concentration of pollutants emitted into the troposphere and, associated to greater pollutant dispersion  during the rainy period, produced a reduction in %MCN at all the test points, in both exposures (active and passive).
When we analyze only %MCN of the dry season, we observe that stations DE, T-1 and T-2 showed the most important results, given that they were all statistically significant in both types of exposure. The response exhibited at station DE may be directly related to the proximity of the largest sugar cane plant of the region and to the fact that the municipality is the second largest sugar cane producer in the area, which contains large plantations surrounding the city limits where non-mechanized harvesting is still practiced . The results of stations T (T-1 and T-2), both for micronuclei and pollutant levels, corroborate Ignotti et al.  and Rosa et al. , whose studies show that Tangará da Serra exhibits one of the worst indicators of morbidity and mortality from respiratory diseases in the southern region of Brazilian Amazon.
A comparison of %MCN obtained in the two types of exposure shows a very strong and statistically significant correlation (r = 0.794 p = 0.006), although there is an underestimation of passive compared to active response, in relation to the genotoxicity potential generated by air pollution. Other studies that used these two types of exposure simultaneously also obtained similar results when they evaluated the genotoxicity potential generated by pollutants emitted by the burning of fossil fuels in the cities of Feira de Santana/Brazil  and São Paulo/Brazil  using T. pallida. Meireles et al.  suggested that the difference between the two types of exposure may be due to an adaptation process of the plant, under passive exposure, to air pollutants at the exposure site. This likely adaptation of Tradescantia to air pollution was explained by Alves et al. , in a study conducted in São Paulo, Brazil, in which the reduction in stomata size on the abaxial surface of the leaves (region sensitive to air pollutants) after exposure to air pollutants, which provokes a reduction in gas exchange capacity and consequently in the possible effects that pollution has on the plant.
In terms of public health, the particulate matter is an important pollutant emitted into the atmosphere. Although the concentrations of fine particulate matter obtained in this study have been below the level suggested by the World Health Organization who is 25 μg/m3/24 h, it is important to report that WHO in its Air Quality Guidelines show that the continuous exposure PM2.5 concentrations even below the level suggested may increase the risks to the population exposed . Studies conducted by Annesi-Maesano et al.  in France, Braga et al.  and Cançado et al.  in Brazil, Casas et al.  in Chile and Rojas-Martinez et al.  in México report a significant positive relationship between exposure to air pollutants and the increasing incidence of aggravated health in children. The main risk factor from this exposure is the capacity that these pollutants have to compromise pulmonary development , which causes impaired lung growth in children, increasing the risk of developing chronic obstructive pulmonary disease at adult age, as well as increased morbidity and mortality from cardiovascular diseases .
When we correlated the values of PM2.5 during the entire period with the %MCN of active exposure, we observed a positive and extremely significant correlation (r = 0.818 p = 0.004), corroborating the results obtained by Batalha et al. , Carvalho-Oliveira et al. , Carreras et al.  and Prajapati & Tripathi . All these results show the importance of this biomarker in the evaluating genotoxic damage caused by exposure to particulate matter generated by burning biomass. The fact that we observed no significant correlation between %MCN and air temperature (r = -0.30 p = 0.934), contrasts with work published by Isidori et al. , Klumpp et al. and Savóia et al. . This may be owing to the similarity in temperature data from all the points, regardless of the period, a situation that reduces the confounding factor. When we correlated rainfall with PM2.5 data (r = -0.806 p = 0.005), we found a significant negative relationship, which shows that rain has a strong influence of biomass burning in the region. Indirectly, we observed that rainfall also has an important influence on micronucleus indices (r = -0.636 p = 0.048), since it reduces the number of burning points and pollutants emitted into the air.
The fact that we observed no statistically significant correlation between RD-C and the fine particulate matter (r = 0.467 p = 0.174) does not rule out the capacity of the latter to compromise the health of sensitive individuals. Mazzoli-Rocha et al. , in a study conducted with BALB/c mice exposed to particulate matter from sugar cane burning in Araraquara, Brazil, observed that the particulate matter collected in filters may induce alterations in the mechanics and pulmonary histology of the exposed mice, causing an increase in polymorphonuclear (PMN) cells and a reduction in mononuclear (MN) cells in the pulmonary parenchyma compared to the control.
A study carried out by Mariani et al.  in São José dos Campos, Brazil showed that micronucleus frequency in T. pallida exposed to air pollution was significantly associated to the mortality rate (deaths/10,000 inhabitants) adjusted for cardiovascular diseases (r = 0.841 p = 0.036) and cancer (r = 0.890 p = 0.018) in the general population. The authors showed that there was a substantial relationship, albeit not significant, between micronucleus indices and mortality from chronic obstructive pulmonary disease (r = 0.640 p = 0.170). These data corroborate the result obtained for the relationship between the standardized rate (hospital morbidity/1000 inhabitants) in children (RD-C) and %MCN (r = 0.721 p = 0.019), showing a strong, statistically significant positive correlation between the two variables.
A factor that could be considered a limitation of this study was the number of vases with T. pallida at each point of the active exposure, which limited the number of inflorescences analyzed by point and that may have contributed to the large standard deviation in some stations. Another limitation was the analysis of all Chapter X/ICD-10 for construction of the adjusted morbidity rate in children since not all diseases in this chapter are related to exposure to air pollutants. However, this measure was necessary to minimize failures in the procedures for notification of diseases by the health system observed by Rosa et al.  in a study that analyzed the magnitude of the hospital admissions for respiratory diseases in the region. This study observed that the increase in notifications due to pneumonia can be explained by the higher financial remuneration of this type of procedure as compared with hospitalizations for asthma. Another important factor noted by Rosa et al.  and that influencing the clinical diagnosis of asthma in the region was the prejudice of the parents in accepting that the child has the disease.