Respiratory diseases are the most common cause of death in the world. The respiratory system has a large surface area exposed to blood and the surrounding environment, which is susceptible to the penetration of infectious and non-infectious factors such as tobacco smoke, exhaust gases, and inhalational allergens. It is possible to analyze inflammatory mediators of plasma and urine, but these data reflect systemic inflammation rather than inflammation located in the respiratory tract. Because of the inflow of mediators from other organic systems and the resulting background noise, the sensitivity and specificity of the data obtained is reduced. One of the methods for assessing inflammation locally in the lungs is an analysis of condensate of exhausted air (EBC). EBC provides insight into the reduction-oxidation reactions within the lung, the acid status, and the degree and type of inflammation in chronic obstructive pulmonary disease, idiopathic pulmonary fibrosis, and other respiratory disease. The disadvantages of the EBC method are insufficient standardization of sampling as well as lack of evidence on the origin of the aerosol particles. There is little known benefit of markers not related to oxidative stress, which is generally a small number of studies. EBC is used for research purposes and is not available in everyday clinical practice. For more precise information on the respiratory system, methods such as sputum induction or methods such as bronchoalveolar lavage (BAL) and biopsy should be applied. Precisely due to the above mentioned drawbacks of all methods there was a need to develop new ways of studying the local pathological process in the airways, such as local pH analysis in the lungs as a possible reliable marker. Idiopathic pulmonary fibrosis (IPF), previously known as cryptogenic fibrous alveolitis (CFA), is a disease of unknown cause. This is the most common type of idiopathic interstitial pneumonia (IIP). IIPs are spontaneous (i.e., idiopathic) diffuse parenchymal lung diseases. The American Thoracic Association (ATS) has expressed consensus in defining IPF as a spontaneous (idiopathic) specific form of chronic fibrotic interstitial pneumonia restricted to the lungs and associated with UIP in computerized high resolution tomography or histologic appearance on the surgical (thoracoscopic or open) biopsies of the lungs. The pathogenesis of idiopathic pulmonary fibrosis (IPF) / common interstitial pneumonia (UIP) is complex and probably involves cell injury and unregulated repair cycles. Chronic obstructive pulmonary disease (COPD) is the fourth cause of mortality in the world and is considered a major public health challenge. The disease is preventable, and when it develops, it requires chronic healing. COPD is considered organ-specific disease and is characterized by chronic inflammation of the respiratory tract with systemic manifestations such as malnutrition, cardiovascular diseases, and weakening of the immune system with frequent respiratory infections. Typical characteristics of the disease are episodes of exacerbation. COPD acute exacerbation (AECOPD) is characterized by worsening of sputum, cough and sputum production compared to daytime variation, which leads to the need for additional treatment and / or hospitalization. Study I Several studies have suggested that idiopathic pulmonary fibrosis (IPF) may be related to repeated aspiration of gastric contents over long periods of time. We aimed to investigate differences between pH measured directly in the lung, and biomarkers of acute inflammation in patients with newly diagnosed IPF and in patients with newly diagnosed GERD. All subjects (N=61) underwent collection of medical history, physical examination, pulmonary function testing, bronchoscopy, endoscopy, arterial blood gas analyses, and biochemical testing. Previously diagnosed GERD was found in 56.7%, typical symptoms of reflux in 80%, and Helicobacter pylori in gastric biopsy specimens in 76.6% of the cases. pH in peripheral branches of bronchi in the cases was 5.32±0.44 and was 6.27±0.31 (p<0.001) in the control group. The average values of LDH, ALP, and CRP in bronchoalveolar aspirate and in serum, as well as TNF-α in bronchoalveolar aspirate, were significantly higher in IPF patients. The more acidic environment in the bronchoalveolar aspirate of the IPF subjects could contribute to the development or progression of IPF, possibly via changes in local metabolism or by damaging local cells and tissue. However, further studies with larger numbers of patients are required to clarify the role of gastric fluid aspiration in IPF pathogenesis. Our preliminary work has identified inflammatory biomarkers LDH, ALP, and TNF-α as potentially important in the pathologic processes in IPF. Further research is needed to determine their importance in clinical intervention and patient care. Study II This study aimed to directly measure pH in the lungs, determine lactate dehydrogenase (LDH), C-reactive protein (CRP), and glucose levels in serum and bronchoalveolar aspirate, and identify bacterial pathogens from bronchoalveolar fluid during acute exacerbation of chronic obstructive pulmonary disease (AECOPD). We performed an observational, analytical case–control study from February 2015 to March 2017. We included 84 patients with AECOPD and 42 with stable chronic obstructive pulmonary disease (COPD). All participants underwent detailed medical anamnesis, a clinical examination, chest radiography, spirometry, an arterial blood gas test, bronchoscopy, bacterial culture, and serum/bronchiolar aspirate laboratory testing. The mean pH of bronchoalveolar fluid was significantly higher in patients with AECOPD than in patients with stable COPD. The mean lung pH value, bronchoalveolar and serum LDH levels, and serum CRP levels in patients with isolated bacteria were higher than those in patients without isolated bacteria in the AECOPD patient group. Lung pH values in patients with AECOPD were significantly correlated with bronchoalveolar LDH and glucose levels. AECOPD is associated with local cell and tissue injury in the lungs, especially in the presence of bacterial pathogens, which is accompanied by a low systemic inflammatory response.