You did not say if your house had a basement or crawlspace, or sits on a slab.
Basically, Radon is a gas created as radioactive material decays deep under the ground. Like any other lighter-than-air gas, it seeks to rise to the surface of the earth, then continues its rise to the upper atmospheres. The gas is basically everywhere. With the invention of house, and more so basements and crawlspaces, radon has a better path to vent. When it builds to higher levels, it can be deadly if exposure is long-term. It can be selective. I once lived in a semi-detached home. The other house attached to mine had a basement level of 8.5. I tested mine, it was 0.02. It can also change over time, with levels in the home rising or lowering as the earth below shifts and the radioactive material decays. Itís an unavoidable thorn in the side of home ownership..
Generally, radon levels that average 5.0 and above require some sort of correction. However, any test which measured radon levels over a period of less than 6 months really is not a good test. You see, radon gas emission levels change over the year as well, and depending on climate conditions could be different. Also affecting the reading is moisture in the soil. For example, if you had a 1-day (24-hour) test done (often all that is required by local ordinances), and during that time it rained, your number may be much higher than itís average really is. Thatís because rain saturates the soil, and the radon gas, still looking to vent to the surface, goes where the moisture is less, which is right under your home.
As a general rule, when the level is 5.0 or higher, the 6-month test is recommended. Often in this case the seller will have to escrow moneys to pay for the correction, if so warranted. Real estate agents Iíve spoken to will often recommend that their sellers just go ahead and correct the problem if the 24-hour test shows anything over 5.7 because the odds are, even though the 6-month test might be lower, it probably will still be above 5. In this way, the sellers save the cost of the 6-month test.
This is a very fresh topic because the house we just purchased registered a radon level of 5.2. The sellers will be paying for the 6-month test ($150.00), and escrow the cost of correction. Estimates seem to be around $3500.00, but the size and shape of your home will affect the price. This is not really a DYI project.
At a high-level, controlling marginally-high radon levels consists of sealing the basement,/crawlspace/slab (seams, cracks, etc.), then installing a small vent pipe which runs up along a gutter downspout (usually at the rear of the house) then above the roofline somewhere. This allows the radon gas a safe escape. At a slightly higher test level, or depending on the configuration of the basement, a small fan is added to the vent pipe to create a vacuum under the basement,/crawlspace/slab, essentially sucking up all venting gases under the house. As the test levels increase, more sealing and venting is added. Sometimes, in crawlspace situations where there is no potential living space under the house, simply adding ventilating fans in the crawlspace is sufficient to lover the levels in the house to within safe values.
Depending on when you plan to sell, if you have not done the 6-month test, consider doing it. Also, seek the advice of your municipality. Their advice is invaluable. They can give names of companies who specialize in the work, plus you may need permits.