The condition for including countries in the analysis was at least 50 publications on nanotoxicology. The first cluster analysis was performed to identify groups of co-authorship (Figure ). The analysis identified the top-14 countries distributed in three clusters. The "green" cluster included countries with the most cited scientists, as demonstrated in Table . These countries are the USA, China, UK, and Australia. The "blue" cluster was formed by Asia countries: India, Saudi Arabia, South Korea, due to the geographic proximity. The "red" cluster includes the remaining countries: Brazil, Canada, France, Germany, Iran, Italy, Spain.