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Study on the Aerosol Quantitative Fit Testing
Quantitative fit testing,Ambient aerosol,Filtering facepiece respirators,Respiratory tract deposition effect,Face-seal leakage,
|Publication Year :||2019|
|Abstract:||為了更客觀的判斷呼吸防護具與使用者臉部是否緊密貼合，美國職業安全衛生署(Occupational Safety and Health Administration, OSHA) 於2006年公布定量密合度測試法。其中最為廣泛應用的環境氣膠凝核計數法(Ambient Aerosol Condensation Nuclei Counter, CNC) 乃利用TSI公司所生產的PortacountTM，針對不同動作下口罩外與口罩內濃度進行採樣，計算出該呼吸防護具之密合係數(FFportacount)。然而，研究結果發現，因微粒濃度本身於面體內無法均勻混合，Portacount將抽到更多乾淨空氣；並且一部份微粒受到呼吸道沉積作用，使得測試的密合度值結果會高估真實密合的情形。因此，本研究將針對不同條件下讀值的高估情形以及相對應可能的修正方向提出相關建議。
本研究在定流量情形下(1 - 50 lpm)，量測洩漏管道與面體壓降對流量的關係，並藉由總流量與毛細管流量之比值，推算出只考慮吸氣階段氣流分配所計算出的真實密合度值(FFtrue)，因不受微粒沉積的影響，應較能反映真實洩漏的情況。實際進行密合度量測時，將使用100ppi海綿以模擬人體肺部微粒沉積情形，面體後端連接呼吸模擬器，前端則連接PortacountTM微粒採樣儀器，用以比較儀器量測值相對於真實密合度之準確度。在調整不同測試參數如呼吸模擬器潮氣容積(0.5 – 1.25 L)、呼吸頻率(3 - 30 bpm) 、面體洩漏管道直徑(1.0 - 1.5 mm)與肺部沉積(有無置入海綿)，結果發現PortacountTM讀值皆會高估真實密合度值約2 - 3倍，其原因為呼吸道沉積與面體內微粒混合不均勻作用，導致採樣管採到更多經過濾材的乾淨空氣，進而稀釋微粒濃度，使密合度上升；另外，在呼吸過程中，部分微粒也會沉積於呼吸管道內，同樣使得密合度上升。因此，從實驗結果而言，由於上述兩種作用影響，導致PortacountTM測試結果並無法反應面體真實佩帶時的洩漏情形。
The OSHA promulgated a quantitative fit test (QNFT) to ensure the respirator provides a satisfactory seal between the contaminated area and the wearer, and to check if the respirator properly be donned prior to initial use. The fit factor (FF) is determined by the ratio of particle concentration outside (Cout) and inside (Cin) the respirator. However, the result was found that the Portacount measurement might overestimate compared to the real fitting condition due to the effect of incomplete air/agent mixing in the respirator facepiece.
In this work, investigation of the fit factors was divided into three phases: (1) foam penetration test, (2) experimental testing using constant flow rate, and (3) simulation tests using a breathing machine (combination of tidal volume and breathing frequency). To simulate leakage on the respirator, capillaries with 1.5 mm in length with different diameters (1.0 -1.5 mm), were inserted at the place of nasal between the respirators the wearer’s face. The ratio of total flow to leak flow was considered the “true fit factor”.
In Phase 1, foam penetration test was conducted in order to fit the International Commission on Radiological Protection (ICRP) deposition model to simulate the lung deposition effect. A scanning mobility particle sizer (SMPS, model 3080, TSI Inc.) and an aerodynamic particle sizer (APS, model 3321, TSI Inc.) were used in this study. In Phase 2 of the study the pressure drop corresponding to the leakage and filter flow rates at the constant flow rates of 1-50 L/min were measured, which then used to assess the amount of leakage during different breathing pattern. In Phase 3, the effects of breathing pattern on in-mask particle concentration during fit testing were investigated. The values and correlation for FFt were obtained by the experiment results in Phase 2 and Phase 3.
The result shows the foam (α = 0.03, df = 36µm) with the thickness equals to 50 mm and the face velocity equals to 100 cm/s was good-fitted to the ICRP deposition model. For the breathing simulation experiment, it was shown that the fit factor of N95 and N100 were found to be overestimated compared to the FFt because the filtered air was partly sampled during inspiration phase by sampling tube which might dilute the particle concentration inside the facepiece. In addition, the fit factor might be more overestimated by Portacount when in the high breathing flow rate because of the flow distribution ratio through filter to the leakage increases. Therefore, it was concluded that the FF measured by Portacount might not represent the worst cases.
In this study, the suggestion for the current quantitative fit test method is proposed: the FFmin calculated from the highest particle concentration during the sampling time, which the value is closer to the FFt at the low ventilation breathing flow, will be better and more accurate to represent the real leakage condition compared to the Portacount measurement.
|Appears in Collections:||職業醫學與工業衛生研究所|
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