Radon is today considered to be the main source of human exposure to natural radiation. The UNSCEAR (2006) report suggests that, at the worldwide level, radon accounts for around 52 % of global average exposure to natural radiation. The radiological impact of isotope 222 (48 %) is far more significant than isotope 220 (4 %), while isotope 219 is considered negligible. Exposure to radon and its decay products varies tremendously from one area to another. It depends firstly on the amount of radon emitted by the soil and, secondly, on the degree of containment and weather conditions in the areas where exposure takes place. The radon atoms in the soil are produced by the disintegration of the radium-226 contained in the mineral grains. Some of these atoms reach the interstitial spaces between the grains (emanation). Some of the atoms produced by emanation reach the soil's surface by diffusion and convection (exhalation). These mechanisms are also brought into play in materials (building materials, walls, etcetera). The quantity of radon-222 reaching the open air per unit of time and per unit of surface is called the radon-222 surface exhalation rate. This part of the DIN ISO 11665 series 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, etcetera) and the atmosphere. This method is estimative only, as it is difficult to quantify the influence of many parameters in environmental conditions. It is however of particular interest in the case of an investigation, a search for sources or a comparative study of exhalation rates at the same site. The responsible committee is GUK 967.2 "Aktivitätsmessgeräte für den Strahlenschutz" ("Activity measuring instruments for radiation protection") of the DKE (German Commission for Electrical, Electronic and Information Technologies) at DIN and VDE.