This document specifies a continuous measurement method for the determination of the concentration of carbon monoxide present in ambient air based on the non-dispersive infrared measuring principle (NDIR). This document describes the performance characteristics and sets the relevant minimum criteria required to select an appropriate non-dispersive infrared carbon monoxide analyser by means of type approval tests. It also includes the evaluation of the suitablibility of an analyser for use in a specific fixed site so as to meet the Directives data quality requirements and requirements during sampling, calibration and quality assurance.