Air-quality metrology and calibration NABL 138 Air Quality Monitoring Equipment Calibration 2026: Gas Analyzers, PM Monitors and Traceability Updated: 13 September 2026 Short answer: NABL 138 is the specific-criteria document for calibration laboratories working on air-quality monitoring equipment. The current NABL register lists Issue 01 with Amendment 02 dated 3 November 2025, which remains the current published version in September 2026. It covers gas analyzers such as NOx, CO, SO2, O3, NH3 and benzene analyzers, along with PM2.5 and PM10 analyzers. The criteria require traceable reference standards, controlled dilution or calibration systems, suitable reference equipment and ISO/IEC 17025 laboratory competence. Air-quality data can influence public-health information, pollution-control decisions, industrial compliance and environmental policy. But an air-quality monitoring station does not measure pollution simply because a number a...