The purpose of the exercise is to measure the activity of radon and radon progeny in an indoor environment. Radon is one of the products of the 238U decay chain and it is the only one in gaseous form, and therefore the only one that can escape the out of the Earth’s crust. Radon may represent a significant contribution to natural radiation exposure, especially in karstic terrain (for which Slovenia is renowned), and after smoking, is the second most important cause of lung cancer.