The filter system consists of a custom designed housing containing a blend of naturally occurring minerals. Different minerals provide optimum affinity for the wide range of toxic molecules in the air. Thus the blend of minerals is carefully determined from the data collected during the EUROFILTA survey. Each granule is composed of a honeycomb of tiny cavities. These cavities contain an ionic or covalent charge, which attracts and bonds to the toxic gas molecules as the air moves over the filter. These cavities effectively give the filter an enormous surface area enhancing its absorptive capabilities. Once bonded to the filter, the toxic molecules are effectively locked-in and cannot be released back into the air stream. At regular intervals to maintain their efficacy EUROFILTA personnel replace the filters.