an assessment of the evidence - Council of Canadian Academies

18 Influenza Transmission and the Role of PPRE: An Assessment of the EvidenceIf the concentration of virus in the particles is constant, the same volume of particleswill have the same amount of virus. However, since the volume of a particle isproportional to the cube of its radius, a single 100 µm particle will have 1000 timesthe amount of virus as a single 10 µm particle. Thus, given a similar number of 10 µmand 100 µm particles the 100 µm particles would contain 1000 times more virus.Data presented by Duguid et al. (1946) showed that particles ranging in diametrefrom 2 to 24 µm accounted for almost 90 per cent of the number of particles emitted.However, in terms of total volume, these particles accounted for a very small fraction.It is not known how influenza virus is distributed in particles emitted from infectedpersons. The viral concentration in emitted particles is expected to vary both amongindividuals, and over the course of their infection.Effect of Particle SizeThe route of an expelled particle depends on the physical and environmentalfactors that surround it. Expelled particles can be categorized into two groupsdepending on how they travel: “ballistic” particles and “inhalable” particles.Ballistic particles – with a mean aerodynamic diametre of greater than approximately100 µm – are predominantly affected by gravity (as opposed to air resistance)and follow so-called “ballistic trajectories” (Xie, 2007; Embil, 2003;Teleman, 2004; Langley, 2005; Wells, 1934; Duguid, 1946). These ballistic particlessettle out of the air in seconds. Their infectious range lies very close to the originalpoint of departure – generally less than a metre. Ballistic particles are capable oflanding on any nearby surfaces, including people, and are thought to play a role inthe contact mode of influenza transmission. However, at close range, these particlescould also land on the mucosa of nearby individuals.Inhalable particles are particles with aerodynamic diametres falling in the 0.1 to100 µm range. The time during which a particle is likely to remain in the air isrelated to its overall size and ranges from seconds to days (Table 1). Some havediametres sufficiently small to allow them to be carried considerable distancesdepending on air currents and other factors (Evans, 2000). Particles near 100 µmin size deposit exclusively in the nasopharynx with thoracic and alveolar depositionoccurring as the particles become smaller, especially less than 20 µm.