For NIOSH 7400 PCM collection, what is the minimum total liters required?

• A. 390 liters
• B. 490 liters
• C. 590 liters
• D. 690 liters

Answer: (B)

In PCM analysis, the number of particles you can count on the filter determines how precisely you can estimate the airborne concentration. The sampling volume sets how many particles are collected; too few particles means high variability and unreliable results. NIOSH 7400 specifies a minimum total air volume to ensure a sufficient particle count on the filter for a meaningful PCM examination. The 490 liters threshold is this practical minimum, giving enough particles to obtain a reliable count without requiring an impractically long sampling time. At typical sampling flow rates, reaching about half a cubic meter of air usually corresponds to a few hours of collection, which is a reasonable balance between accuracy and practicality. Volumes smaller than this would lead to greater counting uncertainty, while larger volumes aren’t required by the method and would just extend sampling time.