an assessment of the evidence - Council of Canadian Academies

More documents

Recommendations

Info

Influenza Transmission and the Role of PPRE: An Assessment of the Evidence 37classes: N-series (for “not resistant to oil”), R-series (for “resistant to oil”), and P-series(for “oil proof ”). Each of these three classes of filters is also certified according to itslevel of filter efficiency (rated as 95 per cent, 99 per cent or 99.97 per cent efficientat removing particles 0.3 µm in diametre – the most penetrating particle size). Forexample, a filter marked N95 means that the filter is not resistant to oil and is at least95 per cent efficient at removing test particulates at the most penetrating particlesize. The specifics regarding filter efficiency are discussed in a subsequent section.The level of protection afforded by a particular class of respirators for use by properlyfitted and trained users is based on its “assigned protection factor” (APF). An APFis the minimum anticipated protection provided by a fit tested and properlyfunctioning respirator. A NIOSH-certified N95 respirator is approved to provide aten-fold level of protection – i.e., it carries an APF of 10 (OSHA, 2006). This meansthat the user will be exposed to no more than one-tenth of the outsideconcentration of airborne particles. A respirator with an APF of 50 (full facepiece,air-purifying respirator) means that a user could expect to inhale no more thanone-fiftieth of the airborne contaminant. There are two methods by whichrespirator fit is assessed – qualitative fit tests and quantitative fit tests. APFs arebased on data obtained through quantitative fit testing of respirators under actualuse conditions in the workplace or under simulated conditions in a laboratorysetting. These are termed workplace protection factors and simulated workplaceprotection factors, respectively (NIOSH, 2004).Properties and Roles of RespiratorsFor particulate respirators, filters remove the contaminant particles from the air asthey pass into the respirator. A common misconception regarding filter materials isthat they act as sieves thereby eliminating particles based on particle size alone. Infact, filters are a “mat” of fibers. The air moves through the mat in a non-linearfashion, bringing the particles into contact with the fibers. Large particles will becollected either by inertial impaction – since large or heavy particles cannot followthe air streamlines – or by interception – i.e., particles brought into contact with thefibers. Small particles will be collected by diffusion, or random movements aroundthe streamlines. Studies of filter efficiency have shown the most penetrating particlesize to be between 0.1 and 0.3 µm depending on the air velocity through thefilter (Figure 5). Electrostatic filters contain charged fibers that collect both largeand small particles and offer advantages such as less breathing resistance and amore effective collection mechanism. Nearly all commercially available N95respirators are constructed of electrostatically charged filter material.It should be noted that when conducting certification tests, non-idealized(worst-case) conditions are used. This means that penetration is measured for neutral
38 Influenza Transmission and the Role of PPRE: An Assessment of the EvidenceE100.00Collection efficiency98.096.094.092.090.0High efficiencyfilterFace velocity, cm/s25102088.00.01 0.1 1.0 10Particle diameter (micrometers)Figure 5Effect of face velocity on collection efficiency andmost penetrating particle size (NIOSH, 2006)particles with the most penetrating particle size diametre. Thus an efficiency of95 per cent means that for particles greater than or less than the most penetratingparticle size, nearly 100 per cent of the particulate matter is collected by the filter.A recent study by Balazy et al. (2006) explored the filter penetration in the nanoparticlerange. The study raised the possibility that some N95 respirators would not filter atthe required level particles in the 40 nm to 80 nm (going down to an efficiency ofapproximately 94 per cent instead of the required 95 per cent at a flow rate of85L/min). Since influenza viruses are above that size range (80 nm to 120 nm) it appearsto have little bearing on influenza control, but the study will have to be kept in mindwhen dealing with small viruses. Further studies will be required to clarify this issue.Particle Release by FiltersTransmission of infectious material could possibly occur as a result of release of thetrapped particles back into the surrounding environment or the improper handlingof respirators. There are very few studies that examine particle release from the filter(Qian, 1997; Richardson, 2006). In one study, heavily loaded filters(10 5 particles/cm 2 ) showed extremely low release (0.002 per cent) at the low velocitythat is characteristic of normal breathing. At higher velocities (sneezing or coughing),one per cent of particles 3 µm in size and six per cent of particles 5 µm in size werereported to be released. In general, it was shown that the percentage release oftrapped particles increases in a way that is roughly proportional to the square of
Page 7 and 8: Influenza Transmission and the Role
Page 9 and 10: 2 Influenza Transmission and the Ro
Page 11 and 12: 4 Influenza Transmission and the Ro

an assessment of the evidence - Council of Canadian Academies

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?