Mainstream cigarette smoke generated using a Walton smoking machine and Kentucky 2R1 research cigarettes was studied. Results ~howed that puff volume and total particulate matter were consistent after the first puff, with average values of 35.6 cm' and 3.37 mg, respectively. The particle size distribution, measured with a multijet cascade impactor, was not related to butt length or relative humidity ( 1 95%), but was strongly dependent on the aging time. Based on simple monodisperse coagulation, the mass median aerodynamic diameter was calculated to be 0.45 pm at a dilution ratio of 21.7. Using a technique based on the dimensional change of collected droplet particles at various viewing angles of a scanning electron microscope, the count median diameter was