**4. Exhaled Breath Condensate**

Examination of the exhaled breath condensate (EBC) is a new, non-invasive method that allows us to assess the inflammatory process within the respiratory tract and may be successfully used in children. The material is gathered by calm breathing for 10–15 min, and then cooling and accumulating the air exhaled into the capacitor. Thanks to the obtained condensate, it is possible to measure inflammatory markers in the collected material, as well as miRNA. In this way, a potentially useful, non-invasive technique was obtained for diagnosing and controlling the course of the disease and assessing the effectiveness of asthma treatment. The presence of miRNAs in EBC was confirmed for the first time in 2013. The difference in the expression of 11 miRNAs between asthmatics and healthy people was proven, as well as the greater stability of miRNA molecules in EBC compared to the material obtained from blood serum due to the difference in the number of miRNAs enclosed in exosomes [41,42].

It was found that the concentration of miRNA contained in EBC is related to the functional parameters of the respiratory tract. In the first study conducted among the pediatric population with diagnosed asthma in which the exhaled breath condensate was analyzed, the influence of a number of miRNAs (e.g., miRNA-155, miR-126-3p, miR-133a-3p, miR-145-5p, 3p) on lung function parameters and reversibility of airway obstruction was proven [39]. In subsequent studies, the relationship between miRNA-570-3p in EBC and spirometry results was also found through an inverse relationship between the expression of this molecule and FEV1 values. A similar relationship was obtained after examining miRNA-1248. With regard to the intensity of the inflammatory process, it was found that miRNA-570-3p affects the diverse expression of many cytokines, chemokines, and the HuR protein (it binds RNA and regulates post-transcriptional processes). Thus, this biomolecule becomes a potential regulator of inflammation in asthma [43]. In addition, attention was paid to miRNA-423, the relationship of which was proven with obesity. Obesity is known to be one of the factors predisposing a higher risk of developing asthma. Since asthma and obesity are characterized by chronic inflammation, it is likely that miRNAs may be misregulated in these diseases by modulating the immune cells found in EBCs. Interestingly, an unobvious relationship between the increase in the amount of fatty acids in the food consumed and the increase in the level of miRNA-133a-3p in the EBC affecting the inflammatory response in the condensate of exhaled air among children diagnosed with asthma was also noted [40].

The limitation of our work is the fact that some studies have been based on relatively small pediatric populations or studies with no control group. To present the trends and potential perspectives for future studies, we have presented studies conducted on adult populations or animal models in case of a lack of data on children.
