PCM counting requires a minimum of how many fields counted?

• A. 10 fields
• B. 15 fields
• C. 20 fields
• D. 25 fields

Answer: (C)

In PCM counting, you need enough sampling units to get a reliable estimate of fiber concentration. Each microscopic field is a small sample, and the number of fibers seen in a field can vary by chance. By counting across multiple fields, you average out that randomness and reduce sampling error. The standard practice sets the minimum at twenty fields, which provides a practical balance between effort and statistical reliability for routine air-monitoring results. Counting fewer fields would lead to higher variability and less confidence in the result, while counting more fields would increase precision but isn’t required for the minimum. So, twenty fields is the minimum required.