efficiency for submicrometer particles) for long duration large air volume viral sampling (Thomas, 1970a) . He showed that 23% of naturally airborne rabbit pox particles were ≤2.5 µm and 71% were between 2.5 and 10 µm (Thomas, 1970b) . Both Thomas and Westwood et al. (1966) in a room supplied with 10 ACH containing 7-9 infected rabbits. Westwood et al. probably obtained higher concentrations because they used an electrostatic precipitator allowing higher efficiency collection of submicrometer particles compared with Thomas's slit sampler. Thomas also studied convalescent cases of variola minor (Thomas, 1974) . One patient with relatively active lesions produced an average concentration of approximately 1 PFU/m 3 . Unfortunately the samples were collected late in the disease when the patient was probably minimally infectious, based on comparison with epidemiological data (Rao et al., 1968; Eichner and Dietz, 2003) . The airborne virus observed appears to have been due to resuspension and is unlikely to be representative of the airborne concentration of respirable variola earlier in the course of the infection. The method used would also not have been able to collect submicrometer viral aerosol particles.