石綿鏡檢效率受粉塵干擾影響之研究

Masking Effect on the Asbestos Counting

為了在石綿等纖維在計數時能夠達到統計上的精確與準確，對於纖維形的氣懸微粒採樣有其密度的限制，從以往的一些採樣經驗得知：計數結果在統計上變異性的大小和薄膜濾紙上的纖維濃度有關。本研究以電腦模擬產生雙變項對數常態分布的纖維形氣懸微粒，改變纖維的物理特性，如：纖維直徑、長度及其分布幾何標準偏差（Geometric Standard Deviation, GSD）、濾紙表面的纖維密度以及背景非纖維形氣懸微粒分布，如微粒的中位數粒徑（Count Medium Diameter, CMD）、其分布幾何標準偏差和計數鑑別率，探討在這些參數下遮蔽效應對纖維計數的影響，並比較NIOSH METHOD 7400 兩種計數方法的差異，另外也對纖維採樣時加裝前分徑器對後續計數效率的影響做一探討。結果發現計數的精確度會隨著濾紙表面的纖維密度的增加而降低。A規則計數下計數效率隨纖維長度增加而增加，但以B規則計數則纖維長度對計數效率沒有明顯影響。另外在非纖維形氣懸微粒的影響方面，當其粒徑或GSD增加時，都會降低纖維的計數效率，而其與纖維的濃度比例增加也會降低計數效率。在模擬加裝分徑器之後，不論是以A或B規則計數，纖維的計數效率皆明顯的上升。在建議上限值部分，若要求纖維計數效率達95%，以直徑中位數0.125 ~ 2μm、長度中位數5~10μm，GSD為1.5、長寬關係係數為0.48 的纖維為例，在背景非纖維形氣懸微粒中位數粒徑為2μm、GSD為1.5、濃度為纖維的30 倍時，其採樣上限值建議為12 count/field （1530 fiber/mm2）。

Fibrous aerosols are of great importance to industrial hygienists because of the severe health risks that are associated with inhaling such particles. Studies show that 100-1300 fiber/mm2 filter area is the most appropriate fiber density range to avoid or reduce bias in fiber counts due to overcrowding of the fibers and non-fibrous particles during counting by phase contrast microscopy have concluded. No study has considered the effect of distributions of both fibers and particles on fiber counting. In this study, computer-generated points on a Cartesian coordinate system were used to simulate the surface area of a standard diameter 100 μm field of view. The resolution of 2000×2000 was found accurate enough to distinguish the actual size of the fibers and particles. The width and length distributions of fibrous aerosols were described by a bivariate lognormal distribution. The capability of distinguishing overlapped fibers, the coverage of the field, the surface densities and size distributions of both fibers and particles, and the concentration ratio of fiber to particle were the primary parameters in this study. The counting efficiency was found to consistently decrease with increasing surface density and decreasing resolution index. The recommended upper limit of fiber density on membrane filter depended not only on the numerical concentration ratio but also the surface densities and size distributions of both fibers and particles. The fiber counting efficiency was greatly improved if a size-selective preseparator (thoracic sampler) was added to remove non-fibrous particles larger than 10 μm in aerodynamic diameter. The enhancement in counting efficiency is particularly notable when the CMD and GSD of the non-fibrous particles, and the ratio of particle to fiber increase. To have a minimum counting efficiency of 95%, the fiber density should be less than 12 count/filed-of-view (equivalent to 1530 fiber/mm2), with the following conditions: fiber-to-particle concentration ratio 1/30, correlation coefficient between fiber diameter and fiber length 0.48, fiber diameter 0.125~2 μm, fiber length 5~10μm, particle diameter 2μm, all with a geometric standard deviation of 1.5.