ACHIEVING ACTION LEVELS: 4.0, 2.7, 2.0
Achieving below the USEPA action level of 4.0 pCi/L can be accomplished for virtually any home. Older homes can present challenges due to construction practices when they were built and in special cases it can become a question of expense versus benefit.
The World Health Organization (WHO) has recently modified their recommended action level from about 5.3 pCi/L to about 2.7 pCi/L. This reflects recommendations regarding life-long, direct exposure. The change reflects recent population studies for statistical evidence of radon induced lung cancer.
The USEPA recommends to consider action between 2 and 4 pCi/L since no exposure is safe. However, around about 2.0 pCi/L and lower, the lower limits of sensitivity for test devices has been reached and errors up to 300% have been seen to occur. Hence, verifying less than 2.0 pCi/L is not truly feasible (though some contractors knowingly or unknowingly make such claims or warranties).
Outside air anywhere on earth. This is so low that even the best equipment scientists use can only estimate between 0.4 to 0.8 pCi/L.
EXPECTATIONS AFTER RADON SYSTEM INSTALLATION:
One can expect reductions to below 4 pCi/L. More often retests show close to outside air (or below 2.0 pCi/L) in the worst case basement areas. (Note however that readings around 2.0 or lower are below the range most devices are capable of for accurate measurement.) Radon Concentrations in areas above a basement where people may spend more time are often 1/3rd to 1/2 the basement concentrations.
Note, about one in ten systems need adjustment. Retest quickly after installation with a continuous monitor where hourly readings can help identify problems and verify success. Thereafter, home test kits may be recommended as an independent, inexpensive and effective method for continued verification.