Distinguish between direct-reading instruments and laboratory analysis in industrial hygiene practice.

• A. Direct-reading instruments provide real-time field measurements; laboratory analysis provides definitive, validated concentrations after sample processing.
• B. Direct-reading instruments require sample processing before results are produced.
• C. Laboratory analysis gives real-time measurements in the field.
• D. Both methods yield identical results and can be used interchangeably.

Answer: (A)

Direct-reading instruments give instant readings right in the field as sampling occurs, which makes them great for screening, immediate hazard assessment, and guiding quick control decisions. But those readings can be influenced by environmental conditions, interference, and calibration drift, so they’re not usually treated as definitive exposure values. Laboratory analysis, on the other hand, involves collecting samples and sending them to a lab where validated analytical methods are used to determine concentrations after processing. The results are definitive and defensible, with proper QA/QC and chain-of-custody. In practice, direct readings provide real-time information, while lab results provide validated concentrations after processing.