Significant temporal variations of radon and other air pollutants are always observed in any room, even one with permanently closed windows and doors. Therefore, the following question remains relevant: how can one assess the conformity of a room with a normative and make a reliable decision if the test lasts <1 year (days or months)? The measurement protocol fundamentally differs between Europe with its long‐term testing tradition lasting several months, and the US where short‐term tests of several days are more common. Neither the European nor the American protocols considers the temporal uncertainty of indoor radon, a factor that usually exceeds the instrumental uncertainty (including in long‐term tests) and is 2–3 times higher the coefficient of variation (COV) commonly used to estimate temporal variations. This problem significantly complicates the creation of a rational and harmonized ISO standard. At the same time, strict adhering to the fundamental ISO/IEC rules within such concepts as “measurement uncertainty” and “conformity assessment” allows to control the coverage probability or reliability of decision making. Within ISO/IEC, proposed are a criterion of conformity assessment of a room with a normative for both short‐ and long‐term measurements, as well as a statistical algorithm for determining the temporal uncertainty considering mode and measurements duration.