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This part of ISO 11665 gives guidelines for estimating the radon-222 surface exhalation rate over a short period (a few hours), at a given place, at the interface of the medium (soil, rock, laid building material, walls, etc.) and the atmosphere. This estimation is based on measuring the radon activity concentration emanating from the surface under investigation and accumulated in a container of a known volume for a known duration.This method is estimative only, as it is diffcult to quantify the infuence of many parameters in environmental conditions. This part of ISO 11665 is particularly applicable, however, in case of an investigation, a search for sources or a comparative study of exhalation rates at the same site. This part of ISO 11665 does not cover calibration conditions for the rate estimation devices.The measurement method described is applicable for radon exhalation rates greater than 5 mBq/m2/s.NOTE The uncertainty relating to the estimation of the result obtained by applying this part of ISO 11665 cannot guarantee that the true fux value is included in the uncertainty domain.