When the SSD system is impractical, as is the case where there is an inaccessible crawl space, a Heat Recovery Ventilator (HRV) can be used. These systems work by changing the air in the basement with fresh outside air, which is heated by transferring the heat from the warm inside air to the incoming outside air.
Additionally these systems can be adjusted to provide a slight increase in pressure in the basement. The radon reduction thus obtained will be twofold, one due to the dilution of the radon and the other by the retarding effect of the pressure increase.