Radon is a radioactive gas which is formed as part of the natural radioactive decay of uranium in rock, soil, and water. Uranium is pervasive across the planet, existing generally, in low levels both at the surface and deep underground. You can’t see, smell or taste radon and while it is radioactive it is, being one of the noble gasses, chemically inert, so it can’t be tested for chemically. Unless you test for the radioactivity produced when radon decay it, there is no way of telling how much is present in the air.
As for why it is so important at this time of year, the simple answer is, it’s not. It’s important all year long because radon is produced constantly. It will be for millions of years to come. The reason it gets so much press at this time of year is that, in North America, November is, by recent tradition, “Radon Awareness Month”.
The reason behind this is complex and is partly due to the apathy of people towards the risks posed, and partly due to the fact that in North America it gets cold in the winter months so people are less likely to open windows and patio doors. This can help to increase the build-up of radon in the home leading to higher risks for residents.
The problem is, this leads to the perception that radon only enters the home in the winter. This is untrue. If radon can get into the home, it can get in at any time of the year. It is also not guaranteed that the levels will be higher in winter.
The only way to know is to test.