I hear you, I am a skeptic myself. Radon is everywhere, but so is the sun when it's shining...and we now know (hopefully) it's a good idea wear sunscreen to protect against UV rays, which cause skin cancer.
Scientists conduct epidemiological studies to determine the linkage between exposure and cancer (and other diseases). For radon, the studies began after noticing high rates of cancer in uranium miners.
well yeah but correlation =/= causation . There are so many radioactive things around us it is impossible to determine causality exclusively for one
Modifying effects from other factors are usually taken into account. Excerpt from an article published by the National Academy of Sciences in 1999:
Don't get me wrong, there are a lot of pollutants as well as radioactive material in the enviroment, and not everyone will die of cancer. But if a strong correlation is found and there are low-cost ways to increase your chances of not getting cancer, might as well give it a shot, right?