A study of the effect of moisture from exhaled air on the filtration performance of N95 respirators in a controlled environment

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A study of the effect of moisture from exhaled air on the filtration performance of N95 respirators in a controlled environment

 

Author: Chung, Kwai-cheung
Title: A study of the effect of moisture from exhaled air on the filtration performance of N95 respirators in a controlled environment
Degree: M.Sc.
Year: 2007
Subject: Hong Kong Polytechnic University -- Dissertations.
Respirators (Medical equipment) -- Testing.
Department: School of Nursing
Pages: xiii, 75 leaves : col. ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2115927
URI: http://theses.lib.polyu.edu.hk/handle/200/1533
Abstract: Objectives: 1. To investigate whether the moisture that is exhaled in air during breathing affects the filtration performance of N95 respirators. 2. To explore whether the designed filter testing system is feasible in a laboratory setting. Setting: The experiment was conducted in the Nursing Laboratory at the School of Nursing of the Hong Kong Polytechnic University. The controlled environment was achieved by (1) maintaining the room temperature at 21oC and (2) maintaining a constant relative humidity by air-conditioning with a low fan. Entry to the room during experiment was prohibited so as to maintain minimal disturbance to the air flow in the room. Sample: Five certified disposable air-purifying respirators (3M1860) were used in this study. Method: The study consisted of the following three phases. (1) Preparatory phase: The prototype was developed and the filtration efficiency of the respirator filters was measured by a count of penetrating particles using the particle size range as the baseline measurement at two pre-determined flow rates and at room temperature. (2) Simulation phase: The selected filter was ventilated with a mechanical ventilator to simulate the normal breathing of an adult healthcare worker. The duration of the ventilation was four hours to simulate the length of time for which the respirator might be worn by a healthcare worker. (3) Measurement phase: At the end of the first and fourth hours of ventilation the filtration efficiency and particle size range of the filter were measured at two pre-determined flow rates that corresponded to the rate of inhalation under a light workload (30L/min) and a heavy workload (85L/min). Results: The study showed that the penetration of particles increased with a decreasing size of ambient particles. In a size range of 0.3 um to 5 um, the maximum penetration occurred in the range of 0.3 to 0.4 um. There was a less than 1% increase in the average particle penetration at the end of the fourth hour, and a less than 1% increase in the average particle penetration at a flow rate of 85L/min. There was almost no penetration of particles larger than 0.65 um. There was a decreasing trend in the filtration efficiency of the filter, with the trend being sharper at the flow rate of 85L/min than at 30L/min. Analysis using an independent T-test showed a statistically significant difference in particle count between the two flow rates in the downstream area (t = -11.762, df = 1198, P < 0.000) and a statistically significant difference in the fit factor at the two flow rates (t = 14.465, df = 1198, P < 0.000). Discussion: A filtration efficiency of 95% or greater, which is similar to the minimum regulation efficiency of N95 respirators, was found for the most penetrating particle size of 0.3 um at a flow rate at 85L/min. The reproducibility of the filtration efficiency of 95% supports the development of this novel testing system. Conclusion: The design of the filter testing system is feasible and workable for the measurement of the filtration performance of N95 respirators. Although this type of respirator provides excellent protection against ambient particles larger than 0.3 um, healthcare workers must be aware that the duration of use and inhalation flow rate affects its filtration performance.

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