[en] In uranium mines, radon is present with its daughter products which may be fixed on different particles, the form of which is described. It is well-known that uranium miners may contract some pulmonary diseases when radon-daughter concentrations are too high during long periods. The origin of lung cancer is confirmed by animal experiments. To have a good protection, practical limits for radon and radon-daughter concentrations are established and individual monitoring is initiated. This monitoring must be well-fitted because the actual levels are almost up to maximum permissible levels. Ventilation is the most important technique to control radon and radon-daughter concentrations. Some ventilation investigations in an experimental mine are described