It is a popular perception that narghile (water pipe) smoking is less harmful to the lungs than cigarettes in both active and passive smokers.Using a sinusoidal pump we simulated water pipe smoking in conditions close to users' habits. The particle sizes and concentrations in the smoke streams were measured with an electrical low pressure impactor (ELPI), sorting particle numbers and size into 12 classes ranging from 0.028 to 10 microm in diameter.Water pipe smoke contains microparticles and nanoparticles. The main inhaled smoke stream (C1) contains particles in a concentration of 3.55 x 10(6) ml(-1) with a median particle diameter (D50) of 0.34microm before bubbling through water. After bubbling, it retains 1.20 x 10(6) ml(-1) particles with a D50 of 0.27 mm, indicating that 2/3 of the particles are retained in the water, the smallest being trapped the less. Compared with C1, the D50 of the side stream smoke (C2) is smaller (0.11mm) while the D50 of the expired stream (C3) is similar (0.25mm).After bubbling, C1 particle sizes are similar to those measured in cigarette smoke but the volumes inhaled by a water pipe smoker are higher allowing prediction of greater deposition in the respiratory tract than with cigarette smoking.