Considering the likelihood of contributions of various indoor air pollutants to detrimental health effects, the Community-COST Concertation Committee of the Concerted Action "Indoor Air Quality and its lmpact on Man" (COST Project 61 3) decided that indoor radon is a well studied indoor pollutant both in terms of occurring concentrations and expected adverse health effects. In July 1985 the Article 31 Euratom Treaty Group of Experts set up a Working Party to study and report on this matter. Their investigations were published in May 1987 as the report "Exposure to Natural Radiation in Dwellings of the European Communities". The following text is largely based on the above report but also includes other recent evaluations of this problem. The Community-COST Concertation Committee by publishing this text would like to provide further support to the work of the General Directorate XI (Environment, Consumer Protection and Nuclear Safety) in this matter and offer it to the Commission of the European Communities for its consideration.