to describe particle size and to describe potential transmission routes (Box 2-1). As research efforts move forward, agreement is needed on terminology to be used so that studies can be compared. Box 2-1 provides the definitions used by the committee throughout the report, including in describing earlier studies. The terms and definitions of the transmission routes were developed at a recent Centers for Disease Control and Prevention (CDC) workshop (David Weissman, personal communication, CDC, November 2010) and are provided as a starting point. These are operational

BOX 2-1

Terminology—Particle Size and Transmission Routes

As noted above, the terms and definitions here are used to frame the discussion, and efforts are needed to reach consensus agreement among the many relevant areas of research and clinical care.


Particle Size:

  • Respirable particles—particles with da ≤ 10 μm that can be inhaled and penetrate to the alveolar region; although a substantial fraction deposit in the alveolar region, they deposit throughout the respiratory tract. These are the equivalent of “droplet nuclei.”

  • Inspirable particles—particles with 10 μm ≤ da ≤ 100 μm, which can be inhaled but cannot penetrate to the alveolar region; nearly all deposit in the head airways region.

Transmission Routes:

  • Contact transmission:

    • Direct contact transmission occurs when the virus is transferred by contact from an infected person to another person without a contaminated intermediate object.

    • Indirect contact transmission involves the transfer of viral agents by contact with a contaminated intermediate object.

  • Droplet spray transmission: Person-to-person transmission of the virus through the air by droplet sprays. A key feature is deposition by impaction on exposed mucous membranes.

  • Aerosol transmission: Person-to-person transmission of influenza or other respiratory viruses through the air by aerosols in the inspirable (inhalable) size range or smaller. Particles are small enough to be inhaled into the oronasopharynx and distally into the trachea and lung.

NOTE: da= aerodynamic diameter. Terminology regarding particles with da > 100 μm is needed.

SOURCES: Nicas and Jones (2009); Personal communication, D. Weissman, November 2010.



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