Calibration checks are performed to verify instrument accuracy and are conducted how often?

• A. Only at the start of the sampling period.
• B. Before, during, and after sampling to ensure instrument accuracy.
• C. Only at the end of the sampling period.
• D. Only when a measurement seems off.

Answer: (B)

Calibration checks verify instrument accuracy throughout the measurement process. They should be performed before sampling to establish a valid baseline, during sampling for longer or variable campaigns to catch drift or faults, and after sampling to confirm the final readings and document performance. If checks are done only at the start or only at the end, drift or sensor problems that occur during the measurement can go unnoticed, leading to biased data. Regular checks help ensure data quality and reliability in exposure assessments. In practice, this often means a zero and span check before use, a mid-session check if feasible, and a post-session verification.