an assessment of the evidence - Council of Canadian Academies

22 Influenza Transmission and the Role of PPRE: An Assessment of the EvidenceThe thoracic region is defined as the tracheobronchial and alveolar regionscombined (Figure 3).Upper Respiratory TractPharynx (throat)Larynx (voicebox)Trachea (windpipe)Right lungBronchusBronchiolePleuraDiaphragm(breathing muscle)TracheobronchialRegionImage not availableHeartBranch from thepulmonary vienLeft lungBronchioleBranch from thepulmonary arteryBlood CapillariesAlveoliPulmonary RegionFigure 3Anatomical regions of the respiratory tract(Fig. 38.22 p.839 from BIOLOGY, 3rd ed. byNeil A. Campbell. Copyright © 1987, 1990, 1993 by the Benjamin/CummingsPublishing Company, Inc. Reprinted by permission of Pearson Education, Inc.)A person emits respiratory particles in a wide range of sizes. Where particles aredeposited in the respiratory tract of the potential host depends primarily on theirsize (Nicas, 1995; Vincent, 1990; Sattar, 1987; Lippmann, 1980; Stuart, 1976).While the size delineations for each category serve as effective discussion tools forplanning purposes, it is important to note that, in actuality, particle behaviors fallalong a continuum.• Alveolar-sized particles are less than approximately 10 µm in diametre. Theyare the only particles capable of reaching the alveolar region but can bedeposited anywhere in the respiratory tract.• Tracheobronchial-sized particles have a diametre of approximately 10 to 20 µm.They are capable of depositing as far down as the tracheobronchial region.• Nasopharyngeal-sized particles are from approximately 20 to 100 µm in diametre.They tend to travel no further than the upper respiratory tract.• Ballistic particles have a diametre greater than approximately 100 µm. They canbe deposited directly onto mucous membranes but have a low probability ofbeing inhaled.