What is the cause of radon gas in home?
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In some parts of the country radon exists "naturally" in the soil. It seeps from underground into the house,
Radon gas is a decay product from Uranium. Uranium is commonly found in granite, so areas with a lot of granite in the area are likely to have radon gas. Radon gas is heavier than air so tends to settle in basements and crawl spaces. It is seldom an issue for homes that are slab on grade.
I have to disagree about slab-on-grade. Google "radon, slab-on-grade" for some opinions on the subject. CDC and the EPA both have several articles on the subject.
The basic principles are that:
A) Radon can be present almost anywhere- just because one house has low readings, doesn't mean the one next door will.
B) The radon has to go somewhere. There's no slab that doesn't have holes and cracks in it and the stack effect often depressurizes crawl spaces and sub-slab fill.
The only way to be sure about radon is a long-term test (at least 6 months) that will average your exposure over different seasons, weather and living habits, preferably starting in summer or winter and extending into the opposite season. The EPA and CDC say radon is second only to smoking as a cause of lung cancer, involved in around 20,000 cases a year, so it shouldn't be taken lightly.
The likelihood of radon problems varies widely, based on your local geology. Some areas have no difficulty with it, while in others you can just assume that radon mitigation will be required.
FWIW, I recommend doing a DIY test for 6 months or so. It's pretty simple- you just place the detector in the living area of your home, someplace where it won't be disturbed, and leave it there , preferably for 90-365 days, then use the post-paid mailer to send it to the lab for analysis. There are several national labs offering that service- just Google "long term radon test kit." The price should be less than $40. Asking a remediation company to test can lead to a conflict of interest.
As DanH indicates, testing in your own house is essential- in the area where I live, Portland, OR, radon ranges from negligible (parts of Tigard) to real hot spots (Scappoose, Alameda Ridge.)
Radon is definitely something your shouldn't mess around with. Here's an article by Martin Holladay which outlines a number of options for reducing or eliminating the radon in your home:
https://www.finehomebuilding.com/2014/05/14/radon-mitigation-systems
The EPA also has a very comprehensive section on their website about radon health risk and mitigation strategies:
https://www.epa.gov/radon