In 170 experiments relationships between 31 different materials used in indoor premises and ionization of the air in their proximity were assessed. The influence of temperature and relative humidity on ionization of air was studied. It was observed that the materials tested could be divided into four groups according to their behaviour towards ambient light negative ions. From the results obtained it follows that the more humid and thus more conductive the material in question, the greater is its influence on the ionization of the air. The most important criterion influencing the relationship of the material and ambient air ionization is the magnitude of the contacting surface of the material that is given by its ruggedness and structure. In natural materials considerable porosity may be expected. In contrast smooth surfaces without pores can be expected in materials produced by melting. Other physico-chemical properties of the materials are manifested in a more discrete way.

        Key words: indoor environment, air ionization, material