Radon is constantly being generated by the radium in rocks, soil, water and materials derived from rocks and soils. The radon generated in rocks or water usually stays trapped in that material unless the rocks are highly fractured or the water is mixed with the air. In most soils radon travels only a few feet before decaying. Thus, the radon that gets into your home comes from the soil immediately around and underneath your house. In water, rocks, or other dense materials radon doesn't even travel a few inches.
Radon travels into houses generally by a combination of diffusion and advection.
Radon escapes from the ground into the atmosphere by diffusion. Continuous electronic radon monitors generally produce the most accurate radon measurements. However, they are expensive and can be difficult to operate. This means that if you want to assess your long-term exposure to radon, you need to measure over a period of a year or more. ATDs can readily supply that information at a reasonable cost.