Environmental problems caused by atmospheric aerosols are well documented in the specialized literature. Studies reporting on the role of dense clouds of soil particles in past mass extinctions of life on Earth and, more recently (Turco et al., 1983), on calculations of potential global effects of dust and aerosol clouds (generated in a nuclear war) have received the attention of the public through oral and written reports by the news media. Because of the importance played by particulate elemental carbon (PEC) on climatic impact, a study was conducted on the size distribution of PEC in samples collected: (i) in an urban area, (ii) inside the air duct of an urban tunnel, and (iii) next to the take-off runway of a busy airport. Besides providing needed data to improve the precision of calculations of potential local and global environmental effects, the PEC size distributions reported in this study help understand the mechanism of association of PEC and polycyclic aromatic hydrocarbons (PAHs), a class of organic compounds that include several mutagens.