Radon is a radioactive gas that you can't see or smell or taste. It is found all over the entire United States and can build up in homes. The radon gas enters a home through cracks in the foundation or slab and then gets trapped inside homes. This build-up of radioactive gas can be measured by short term tests or long term tests. A radon test that inspectors do is usually a 2-day test with a Continuous Radon Monitor, which can give a result at the end of the 2-day period. This immediate result is what is usually used in the inspection phase of the purchase agreement.
Radon has recently become a well-known issue in 2014, and there is quite a push to have radon tests done. You can have a radon test done in the home as part of your inspection time frame. The general home inspector can do a radon test with their inspection, but it takes a minimum of 48 hours for measurement. They usually go to the home 2 days in advance and leave a detector at the house, and come back to pick it up in a couple of days.
The EPA publishes a range of values to look for in radon test results. They mention in their publications that if a house has a reading greater than 4 pCi/L, the house should be mitigated. If the reading is between 2 pCi/L and 3.9 pCi/L, mitigation should be considered.
Radon mitigation costs can vary depending on the house. These costs usually vary between $500 to $2000. I’ve had a few come in at $1400 - $1700 or so and would predict this to be the average cost. But since there is a big push for radon testing right now, there are lots of homes getting mitigated from radon, and this should push up the cost of radon mitigation in the short run.
Radon inspectors should be National Radon Proficiency Program (NRPP) certified, a National Radon Safety Board (NRSB) member, or Midwest Universities Radon Consortium (MURC) certified.
Minnesota added a law in 2015 that required personnel that deal with radon to be licensed. This would include the Radon tester person, the radon inspector, the radon mitigator, etc.