As a Minnesota home inspector who does a lot of radon testing, I hear the same myths about radon repeated over and over again. I've actually heard other home inspectors perpetuate a few of these radon myths as well.
Myth: vacant houses have high levels of radon. The idea behind this myth is that radon will build up in a house while it's sitting vacant, so a radon test on a vacant house won't be accurate. This simply isn't true. Radon has a very short half-life; as radon particles die off, they're replaced with new ones. A radon test conducted on a vacant house will be just as accurate as a test conducted at an occupied home, all things being equal.
Myth: radon is mostly found in older houses. In reality, the radon doesn't care how old the house is. Both new and old houses can have radon problems; we have found zero correlation between radon levels and the age of the home.
The one bit of unintentional truth to this myth is that starting in June of 2009, Minnesota began requiring passive radon mitigation systems in all new construction homes. We've performed many radon tests on these new homes, and most of them have very low levels of radon - not all though.
Myth: opening the second story windows shouldn't affect the radon test, because the test is located in the basement. The problem with this myth is that houses act like chimneys. Opening the windows on the second floor might actually increase the radon levels in the home. For a valid test, the windows in the house need to be kept closed.
Myth: radon tests should always be placed in the lowest level of the home. If the home has a crawl space or a basement that nobody will be spending any time in, why in the world would you want to know what the radon level is down there? If you're testing your own home, put the test in the lowest level of the home that you use regularly. For a real estate transaction, the radon test should be placed in the lowest level of the home that could be used regularly.
Myth: radon tests aren't needed for homes with walkout basements. While we've found that radon levels in homes with walkout basements tend to generally be lower, this is certainly no guarantee that the radon levels will be low. The highest radon level we've found at a home with a walkout basement was nearly four times higher than the EPA action level.
Myth: granite countertops have an effect on radon levels in a home. This myth gained popularity in 2008 because of a media scare. You don't need to worry about granite countertops. They're fine. You can read more about this myth here - radon in granite.
Myth: you need to hire a professional to test for radon. As long as the test isn't part of a real estate transaction, the do-it-yourself radon test kits that you buy online or at a home improvement store will work just fine. It takes a little longer to get the results, but these kits are far less expensive than hiring a professional to test your home for radon.
Myth: short term radon testing is worthless. Short term tests are actually quite accurate; certainly not as accurate as a long term test, but they're right in line with long term tests 90% of the time. If a short term test comes up low, it's almost a sure thing that a long term test will come up less than the EPA action level. If a short term test falls in the marginal range, a long term test is recommended. When a short term test is over the EPA action level, it's time for a mitigation system.
Myth: holy water will keep radon from entering a home. Ok, I made that last one up... but on the other hand, I don't have any proof that it doesn't...