This part of ISO 11665 outlines guidance for measuring radon-222 activity concentration and the potential alpha energy concentration of its short-lived decay products in the air. The measurement methods fall into three categories:a) spot measurement methods;b) continuous measurement methods;c) integrated measurement methods.This part of ISO 11665 provides several methods commonly used for measuring radon-222 and its short-lived decay products in air.This part of ISO 11665 also provides guidance on the determination of the inherent uncertainty linked to the measurement methods described in its different parts.