Radon Gas movements ?

Most central air systems leak. But the suction vents are on the inside. As a result, most homes have negative pressure on the interior.

Foundations, even concrete ones, are porous, and air can come up through them. The constant negative pressure thus sucks air up out of the soil, which is where radon hangs out.

This is why commercial buildings are generally required by code to have systems that produce positive pressure, so that they blow on you as you enter from outside.

It’s not only heavier than air, it’s heavier than practically every other (naturally occurring) gas on the periodic table, which causes it to accumulate on the lowest levels of dwellings, generally in the basement. If you change the wording of your sentence, the behavior of radon becomes a bit easier to understand. It’s not “invading” so much as “seeping.” In a poorly insulated home or one with a cracked foundation or basement, radon finds an easy pathway from the soil to a dwelling. A second-story bedroom, on the other hand, in unlikely to be very contaminated.

Incidentally, if you’re curious about whether or not your home has radon present, an easy way to test is to turn your thermostat down to -96 degrees Fahrenheit, at which point it begins emitting so much radiation, it glows yellow. Wear a sweater.

Radon is a short lived gas. It’s most stable isotope has a half-life of only 3.8 days. Radon is produced from the radioactive decay of other radioactive elements that are often trace elements in rock and building materials. Radon gas is produced constantly anywhere the source elements exist. It is only a danger where the amounts are sufficient and it is allowed to collect.