an assessment of the evidence - Council of Canadian Academies

Influenza Transmission and the Role of PPRE: An Assessment of the Evidence 3Traditional infection control terminology has categorized influenza transmissionas occurring either by “contact,” “droplet” or “airborne” modes. Since both dropletand airborne transmission involve the inhalation of infectious particles into therespiratory tract, for the purpose of this report these two modes have been groupedtogether under the term “inhalation transmission.”A person emits respiratory particles in a wide range of sizes. Expelled particles can becategorized into two groups depending on how they travel – “ballistic” particles and“inhalable” particles. Ballistic particles are those with a mean aerodynamic diametreof greater than approximately 100 µm 1 and are predominantly affected by gravity,as opposed to air resistance. Their infectious range lies very close to the originalpoint of departure – generally less than a metre. Inhalable particles are those withaerodynamic diametres falling approximately in the range of 0.1 to 100 µm and,depending on size and shape, they may remain in the air from seconds to days.Where particles are deposited in the respiratory tract of the potential host dependsprimarily on their size. Ballistic particles can be deposited directly onto mucousmembranes but have a low probability of being inhaled. The inhalable particlescan be classified into three size categories that have different deposition behaviour.Nasopharyngeal-sized particles range from approximately 20 to 100 µm in diametreand tend to travel no further than the upper respiratory tract. Tracheobronchialsizedparticles have a diametre ranging approximately from 10 to 20 µm and arecapable of depositing as far down as the tracheobronchial region. Alveolar-sizedparticles are less than approximately 10 µm in diametre. They are the only particlescapable of reaching the alveolar region but can be deposited anywhere in therespiratory tract.Long-range and Short-range Transmission via Inhalation: There is accumulatingevidence that, while the risk of acquisition of respiratory pathogens decreaseswith increasing distance, transmission of infection across distances of greater thanone metre may occur. The U.S. Centers for Disease Control and Prevention (CDC)have recently reconsidered the traditional “short-range” distance benchmark (oftenreferred to as the “three-foot rule”) and expanded it to two metres. In this report,short-range transmission is defined as infection occurring within about two metres ofthe source, and long-range transmission as infection at distances greater than abouttwo metres.The persistent survival of influenza virus in ambient air under common environmentalconditions suggests that long-range inhalation transmission of influenza is1 A micrometer, also called a micron, denoted “µm”, is 10 -6 metre (m).