An empirical model for estimating the collection efficiency of dust-mist respirators

Our plain-English summary of An empirical model for estimating the collection efficiency of dust-mist respirators, by L.M. Brosseau, J.S. Evans, M.J. Ellenbecker (The Annals of Occupational Hygiene, 1993).

The study

• Authors: L.M. Brosseau, J.S. Evans, M.J. Ellenbecker
• Journal: The Annals of Occupational Hygiene
• Published: 1993;37(2):135-50
• Cited by: 3 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

This paper compares experimental measurements of filter performance with predictions of aerosol collection by dust-mist respirators using the single fibre efficiency model and an empirical model developed by the authors. The former, a theoretically-based semi-empirical model, indicates that mechanical phenomena such as impaction, interception and diffusion are responsible for only a fraction of the collection efficiency of dust-mist respirators, i.e. 30-70% depending on particle size. The difference between these relatively low predicted efficiencies and experimentally observed values, which were greater than 95%, can be attributed to electrostatic forces. Because it was not possible to determine filter fibre charge the single fibre efficiency model could not be used to predict electrostatic collection. An empirical model developed from experimental data with monodisperse latex aerosols was used to predict silica and asbestos collection. The empirical model adequately described silica collection, but predicted much higher asbestos penetration than was observed experimentally.

In plain English

This laboratory modelling study tested how well two kinds of calculation predict how much airborne dust passes through the filters of dust-mist respirators from three manufacturers. The predictions were compared with earlier experiments in which these respirators, at a steady airflow and without leakage around the face seal, were challenged with a silica test dust and an amosite asbestos aerosol. A physics-based model that left out electrical charge suggested that mechanical capture accounted for only 30-70% of collection, depending on particle size. The authors attributed the gap to observed efficiencies above 95% to electrostatic forces, which they could not model because filter charge was unknown. A second, empirical model built from latex-sphere tests matched silica results well for two manufacturers but underestimated penetration about 10-fold for the third, possibly because of exhalation valve leakage. For amosite, predictions based on fibre aerodynamic diameter overestimated measured fibre penetration by 10 to 100 times. Using fibre length improved agreement, and differences in microscopy methods explained only part of the gap. The authors concluded the empirical model was reasonably accurate for silica but could not predict collection of a fibrous aerosol.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory modelling study comparing predictions from a theoretical model (single fibre efficiency model) and an empirical model (built from latex-sphere filter tests) with experimental measurements of silica and asbestos penetration through respirator filters
• Who was studied: Dust-mist respirators (filters) from three manufacturers, tested at a steady flow of 533 cubic centimetres per second with no face-seal leakage. Manufacturer 1's filters were corona-treated melt-blown polypropylene. Manufacturers 2 and 3 used a material of 45% wool and 55% polypropylene or acrylic carded with colophony resin. No people were studied.
• Exposure: Filters were challenged with the silica test aerosol used by NIOSH in respirator certification (mass median aerodynamic diameter 3 to 4 micrometres) and a fibrous amosite asbestos aerosol (median fibre length about 5 micrometres, median width 0.2 micrometres, by transmission electron microscopy). The empirical model was built from monodisperse latex sphere data.
• Main results:
  • The single fibre efficiency model, without electrostatic effects, indicated that mechanical mechanisms (impaction, interception, diffusion) accounted for only 30-70% of collection efficiency, depending on particle size.
  • Experimentally observed collection efficiencies were greater than 95%; the authors attributed the difference from the mechanical predictions to electrostatic forces.
  • For silica, agreement between the empirical model's predictions and measurements was excellent for two of the three manufacturers' filters.
  • For manufacturer 3's filters, the model underestimated observed silica penetration by a factor of about 10. Earlier experiments suggested exhalation valve leakage may have contributed.
  • Predictions of amosite asbestos penetration based on aerodynamic diameter overestimated measured fibre penetration by a factor of 10-100.
  • Using the distribution of fibre length instead of aerodynamic diameter to characterize the amosite aerosol improved agreement between predictions and measurements appreciably.
  • Accounting for fibres too thin to be seen by phase-contrast microscopy lowered predicted penetration dramatically, but not enough to explain the difference between observed and predicted amosite penetration.
  • Sensitivity analyses showed the silica predictions were relatively insensitive to assumptions about particles larger than 2 micrometres.
• The authors' conclusion: The authors concluded that their empirical model gave reasonably accurate estimates for relatively isometric particles such as silica, but that collection of a fibrous aerosol could not be predicted from latex sphere experiments. They added that fibre length may be a better predictor than aerodynamic diameter, but further experimentation would be needed.
• Limitations the authors state:
  • Because of the limited range of particle sizes tested, the empirical model's predictions must be interpreted cautiously.
  • Filter fibre charge could not be determined, so the theoretical model could not predict electrostatic collection.
  • The experimental results reflect valve leakage but not leakage around the facepiece seal.
  • The amosite size distribution used for predictions came from transmission electron microscopy, while observed penetrations were based on phase-contrast microscopy fibre counts, which miss thin fibres.
  • The equations for fibre aerodynamic diameter were developed under conditions not necessarily representative of flow inside a filter.
  • The suggestion that fibre length predicts collection better needs further experimentation.

Topics

Medical terms in this study, explained in our Lexicon: Respirator.

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