Controversy: Respiratory Protection for Healthcare Workers

Kathleen H. Harriman, PhD, MPH, RN; Lisa M. Brosseau, ScD


April 28, 2011

In This Article

How Does Aerosol Transmission Occur?

A second area of controversy is whether pathogens can be aerosolized and transmitted by inhalation. NIOSH defines aerosols as a suspension of tiny particles or droplets in the air.[22] Aerosol transmission has been defined as person-to-person transmission of pathogens through the air by means of inhalation of infectious particles. Particles up to 100 μm in size are considered inhalable (inspirable). These aerosolized particles are small enough to be inhaled into the oronasopharynx, with the smaller, respirable size ranges (eg, < 10 μm) penetrating deeper into the trachea and lung (Figure).[23,24] Aerosols are emitted not only by "aerosol-generating procedures,"[19] but may also be transmitted whenever an infected person coughs, sneezes, talks, or exhales. Pathogens transmitted by respiratory aerosols can travel short or long range from the source depending on the size and shape of the particles, the initial velocity (eg, cough vs exhalation), and environmental conditions (eg, humidity, airflow).[25,26,27]

Figure. Deposition regions of the respiratory tract for the various particle sizes. From Roy CJ, Milton DK. N Engl J Med. 2004;350:1710-1712. Copyright Massachusetts Medical Society. Used with permission.[28]

Whether influenza viruses can be transmitted by respiratory aerosols has been a central issue in the argument about appropriate PPE for influenza. The evidence will not be reviewed in this commentary; however, the CDC, the Institute of Medicine, and other researchers have found that influenza can be transmitted through the inhalation of infectious aerosols.[19,23,24,29,30,31,32,33]

Droplet vs Airborne Transmission

The 2007 updated CDC Healthcare Infection Control Practices Advisory Committee (HICPAC) infection control guidance defines droplet transmission as a form of contact transmission in which respiratory droplets carrying infectious pathogens transmit infection when they travel directly from the respiratory tract of the infectious individual to susceptible mucosal surfaces (nasal mucosa, conjunctivae, and less frequently, the mouth) of a recipient, "generally over short distances, necessitating facial protection."[34]

For diseases classified as being transmitted by the droplet route, surgical masks are recommended to protect the mouth and nose. The guidance also states that although 5 µm has traditionally been defined as the particle size break point distinguishing between larger particles (droplet transmission) and smaller particles (airborne transmission), observations of particle dynamics have demonstrated that a range of droplet sizes, including those with diameters of 30 μm or greater, can remain suspended in the air. Influenza virus is an example of a pathogen transmitted by the droplet route.

The HICPAC guidance defines airborne transmission as dissemination of either airborne droplet nuclei or small particles in the respirable size range containing infectious agents that remain infective over time and distance. It states that microorganisms carried in this manner may be dispersed over long distances by air currents and may be inhaled by susceptible individuals who have not had face-to-face contact with (or been in the same room with) the infectious individual.

Furthermore, preventing the spread of pathogens by the airborne route requires the use of special air handling and ventilation systems, such as airborne infection isolation rooms (AIIRs) to contain and then safely remove the infectious agents. In addition to AIIRs, respiratory protection with a NIOSH-certified N95 or higher level respirator is recommended for HCP entering the AIIR to prevent acquisition of airborne infectious agents. Mycobacterium tuberculosis is cited as an example of a pathogen transmitted by the airborne route.

In spite of the distinction made between droplet and airborne transmission, current knowledge of aerosols indicates that there is no clear line differentiating droplet and airborne transmission, as currently defined, on the basis of particle size. Coughing, sneezing, talking, exhalation, and certain medical procedures generate respiratory particles in a wide range of sizes -- not just very large droplets that launch directly to the mucosal surfaces or drop to the floor. In addition, particles begin to evaporate and become smaller immediately upon emission, and particles ranging from very small up to 100 μm can be inhaled by persons in the near vicinity of the source (Figure).[28]

In the current infection control paradigm, airborne transmission is synonymous with long-range transmission of pathogens that can be inhaled and require special air handling to contain. This contrasts with droplet transmission, in which infectious particles are thought to be deposited on a mucous membrane, are not inhaled, and do not require special air handling. However, the association of droplet exposure with infection is confounded by inhalation exposure because close contact with infectious people permits droplet exposure but also maximizes inhalation exposure. Therefore, it is incorrect to conclude that because long-range transmission of infection is not observed, a pathogen is transmitted only by the droplet route.

Absence of long-range transmission, as demonstrated for tuberculosis and measles, does not mean that a pathogen cannot be transmitted by inhalation. As currently defined, the terms "droplet" and "airborne transmission" are inadequate to describe aerosol transmission by inhalation at short range.


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