The aerodynamic diameters of particles and fibers greatly influence their site of deposition and toxicity in the respiratory tract. To characterize cellulose acetate fibers released from the filter end of cigarettes, we designed a multistage impactor to determine the aerodynamic diameters of large fibers ranging from 20-50 µm in diameter and 75-1000 µm in length. One hundred cigarettes were puffed directly into the impactor; 2-10 fibers/cigarette were released and their aerodynamic diameter determined. In two studies, we found that the aerodynamic diameters were always greater than 22 µm. Using standard lung deposition models, we concluded that the fibers are non-respirable, would deposit primarily in the oropharynx and are not likely to penetrate into the tracheobronchial region. Based on physical characteristics, shapes and aerodynamic diameters, there is a very low probability of penetration to the distal respiratory tract. Our findings, demonstrating release of only a small numbers of fibers with an extremely low likelihood of reaching the distal lung, indicate that the fibers should not represent a health concern.