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ذرات بسیار ریز سنگها و کانیهاکه برای مدتی کم یا زیاد در هوا معلق مانده و ابعاد آنها از یک میلیمتر تا کسری از میکرون متغیر میباشد. یک ذره کوارتز با وزن مخصوص 2/5 تن بر متر مکعب و قطر یک میکرون ظرف مدت یک ساعت از فاصله یک متری روی زمین سقوط میکند در صورتی که اگر قطر آن به ۱۰۰ میکرون برسد ظرف مدت 1/3 ثانیه همان مسیر را طی می کند. حداکثر غلظت مجاز...
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