Tobacco and e-cigarettes exposure
Exposure to tobacco has been associated with oxidative imbalance and stress, which may change barrier function of the epithelial layer by reducing ciliary beating and enhancing mucus production, epithelial to mesenchymal transition, and immune dysfunction [73]. Moreover, some cigarette smoke derivates may disrupt epithelial cellular junctions, allowing a deeper penetration of toxins and allergens [73]. Oxidate stress related to exposure to tobacco may activate the transcription of factors involved in inflammatory responses, which are responsible for sustained immune cell recruitment and activation. Furthermore, prolonged exposure to tobacco may enhances both Th2- and Th1-mediated inflammatory responses, being associated with a more severe asthma phenotype [74]. In utero exposure to nicotine may affect lung development in the fetus, consequently leading to impaired lung function and an increased risk of asthma [75]. Nicotine has been also associated with epigenetic changes, such as DNA methylation or changed microRNA expression, which may modulate epithelial differentiation, immune cell differentiation, or both, in a permanent way [76].
The chemical compounds, including reactive carbonyls, resulting from heating in e-cigarettes are also known for their toxic and irritant effect on the airways, causing airway construction, direct damage to airway epithelium and changes in gene expression [77]. Similar to exposure to cigarettes, acute exposure to SHS to e-cigarettes was also associated with altered lung cells, including the small airway epithelium, alveolar macrophages, and circulating endothelial microparticles [78].

Prevention and mitigation of the effects of air pollution on childhood asthma

Actions against the well-known childhood asthma risk factors, industrial activities, TRAP, cleaning and consumer products, and smoking, should be taken simultaneously. Global, national, local, and individual action plans should be implemented considering each situation and need to be planned and implemented by engaging policymakers, governments, civil society, and each individual (Figure 3 ).