The large spray of particles from sneezing visualized by high speed photography consists of particles down to about 10 µm in diameter (Papineni and Rosenthal, 1997) . Smaller particles may also be dislodged from the upper airways by the turbulence of sneezing, coughing, and talking, but will mostly be larger than 2.5 µm in diameter. Recent studies, however, show that the healthy lung generates abundant fine particles (100-1000/l with size <0.3 µm diameter) during normal breathing (Fairchild and Stampfer, 1987 ) that do not arise from the oropharynx; condensates of these particles are the subject of recent reviews (Mutlu et al., 2001; Hunt, 2002) . Such particles could carry variola virus (0.2-0.3 µm diameter), would remain airborne in indoor air for many hours, and would be deposited primarily in the lower airways after inhalation.