Radon gas emits radiation in the form of an alpha particle. Alpha particles do not penetrate paper. As you know the HEPA filter are the most efficiency filters with a ratio of 99.97%. The test with HEPA filter was done on single room during 8 days. In the first 5 days the reading show a long term average at 73 and short term (1 Day) at 56 units. Leaving HEPA filter to operate for almost 18 hours in the same room the reading on the right picture show the radon gas reduced with more than an half during 1 day.
Long time exposure to Radon Gas cause lung cancer. More info