# Carbon Nanoparticles and Their Biomedical Applications
## 13 Nanotoxicity  
When we discuss the possibility of using CNPs in medicine, we must not forget the potentially toxic effects of individual nanoparticles. The toxicity of CNPs depends on numerous factors, including physiochemical properties, such as size, shape, charge, functionalization (functionalized nanoparticles may exhibit significantly lower toxicity than pristine ones), corona formation, the duration of exposure, and the dose of CNPs. Nanotoxicity is often time- and dose-dependent. Experimental and epidemiological studies have shown that CNPs have cytotoxic and genotoxic potential against numerous tissue types and systems (respiratory, cardiovascular, nervous, gastro-hepatic, and reproductive systems). They also show toxicity against the kidneys, immune system, skin, and eyes; in addition, developmental nanotoxicity was confirmed (Figure 6) [3,146]. Damage is often induced by the interaction of CNPs with the cell membrane, nucleus, or mitochondria that is associated with increased production of reactive oxygen species (ROS) and the activation of signal pathways responsible for inflammation, accompanied by increased production of proinflammatory cytokines (TNF-\(\alpha\), IL-6) [147]. Inflammation, ROS, and direct interaction of CNPs with the nucleus can damage DNA and disrupt DNA reparative processes, which may lead to the accumulation of mutations and malignant transformation [148]. The presence of CNPs can also induce cell death (apoptosis, pyroptosis, or necrosis); in particular, necrosis and pyroptosis induce an inflammatory response [149]. In vivo studies detected changes in various tissues, inflammation with the accumulation of immune cells, and fibrotic changes. Importantly, reproductive and developmental changes are documented; the alteration of hormonal levels, decreased sperm production and functions (malformation of sperm and decreased mobility), developmental delays, damage to the nervous or cardiovascular systems, inflammation, and skeletal malformation of the fetus [146]. Toxicity also depends on the route of administration of the CNPs. However, they can also penetrate tissues, enter the bloodstream, and reach distant tissues. Intratracheal instillation results mainly in damage to the lung tissue and the cardiovascular system, whereas per os administration is mainly associated with the accumulation of CNPs in the gastrointestinal tract [150]. Epidemiological studies in workplaces where workers have been exposed to inhaled CNPs for long periods of time also have demonstrated potential health risks of exposure to CNPs. In these persons (workers), changes in the expressions of genes associated with various biological pathways (cell cycle control, carcinogenesis, apoptosis, and cell proliferation), increases in inflammatory cytokines, profibrotic factors, oxidative stress (decreased activity of antioxidant enzymes, such as superoxide dismutase, glutathione peroxidase, and increased ROS production), endothelial activation, and associations with hypersensitivity reactions (allergies) have been described [151]. Single- or short-term exposure to nanoparticles usually does not cause temporary or irreversible tissue damage (as opposed to long-term chronic exposure). When CNPs are used in medical investigation methods, the exposure is short-term, usually a one-off, and targeted with a well-defined dose and route of administration, which can significantly eliminate health risks to the patient. However, these medical procedures (exposures) may increase the health risks for staff exposed long-term to CNPs. The toxicity of nanoparticles can be modulated by their functionalization (polyethylene glycol, poly-l-glutamic acid, etc. ), which could reduce the risk of toxicity; on the other hand, the medical functions of CNPs, their distribution in the body, penetration into tissues, response to illumination, etc., could be negatively affected [152]. The risk of toxicity of CNPs in medicine appears to be low, especially for patients; however, it is important to keep it in mind and weigh the benefits and risks.