Which expression represents fiber density in this method?

• A. (Sample count - Blank count)(Filter area) / (Field area)(Flow rate)(Sample time)(1000)
• B. (Sample count - Blank count) / (Field area)
• C. (Field area) / (Sample count)
• D. (Sample count - Blank count)(Filter area) / (Field area)

Answer: (A)

Fiber density is found by turning the counted fibers into a density per unit air volume, which means correcting for background, scaling counts to the whole filter, and dividing by the air volume sampled. Start with the net count (sample minus blank) to remove background fibers. Then scale from what you counted in one microscope field to the entire filter by multiplying by the ratio of filter area to field area. Finally, divide by the air volume that was drawn through the filter, which is flow rate times sampling time; the 1000 accounts for converting the sampled volume (often in liters) to the desired density units (per cubic meter). This combination—net count, area scaling, and air-volume normalization with the unit conversion—accurately represents the fiber density for this method. Other expressions miss one of these essential steps, such as not correcting for blanks, not scaling to the full filter, or not including the correct volume conversion.