Concrete block walls can be vented by sucking air from the hollow spaces in the wall and venting it away from the house to prevent radon from re-entering. Lastly, there are methods for decreasing negative pressures within the house by bringing air into the house in proportion to losses from chimneys, dryers, etc., or by positive pressure including basement pressurization by blowing air from upper floors into the sealed basement. These ideas involve using paints or surface sealers, epoxy or rubber-based. Experts warned against using nails, because as radon concentration builds up underneath the paint, the holes would let radon in. EPA has evaluated surface paints or sealers as non-effective.
Moisture naturally brings in chemicals from inside the concrete, which attack the paints.
Necessary cracking, initially microscopic, and eventual peeling of the paint or surface sealer allows radon in. Covering walls with gypsum, plaster or wallpaper does not reduce the radon level. Covering floors with carpeting or linoleum does not work either. Remember that even several inches of concrete with only tiny pores do not stop radon. Covering the concrete surface merely leads to a local build-up of radon, which will find its way in, and the overall level indoors will not change at all.