**3. Respiratory system**

There are few studies on the effects of boron and its compounds on the lung. In some studies, it has been reported that exposure to boron may cause damage to the lung cellular layer. It causes symptoms such as irritation in the nasal epithelium, nosebleeds, cough, and shortness of breath in people working in boron mines [18]. Boron nitride nanoparticles (BNNTs) appear as a nanotechnological product with a wide range of applications in engineering and biomedical fields. Therefore, there is an increasing interest in studies on the risks these products may pose for health. It has been demonstrated that BNTTs cause cytotoxicity in human lung adenocarcinoma epithelial (A549) cells and murine alveolar macrophage cells (RAW 264.7), being concentration dependent (0.02, 0.2, and 2 μg/mL concentrations). In addition, cytomorphological examinations revealed serious cell morphology disorders in both cell types [19]. In another study, it was shown that high boron application may cause an increase in the capacity to transform in non-tumorogenic cells. In total, 5–50,000 μM concentration range of boric acid is applied to human nontumorigenic lung epithelial (BEAS-2B) cells and human lung epithelial carcinoma (A549) cells. It has been determined that 5000–50,000 μM concentrations significantly suppressed the anchorage-dependent growth of cells. This effect could have occurred via an important pathway that plays key role in cell transformation such as SRC and PI3K/ AKT and MEK/ERK signaling pathway [20]. Diborane gas (B2H4) is a strong toxic substance for the respiratory tract. Mice were exposed to diborane gas at a concentration of 5 ppm diborane (1.7 mg boron/m3) for 2 weeks, which caused severe damage to their lungs, including pulmonary congestion, hemorrhage, and edema. Mild changes such as infiltration of polymorphous neutrophils in the peribronchiolar region were observed in the group given 0.7 ppm diborane (0.2 mg boron/m3). Case reports have stated that boron can be lethal after short-term oral exposure at high doses and can be quite large despite variability in human responses to acute exposure. It has been reported that the minimum lethal dose of boron (as boric acid) taken is 2–3 g in infants, 5–6 g in children, and 15–20 g in adults. However, 784 cases with boric acid (10–88 g) reported no deaths, of which 88% of the reviewed cases were asymptomatic. Liver, kidney, central nervous system and gastrointestinal effects, and skin lesions have been found in fatal cases following boron ingestion, but death has been attributed to respiratory failure [21].
