According to the International Agency for Research on Cancer (IARC), radon and its daughters are human carcinogens, and there is no threshold.
The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) estimates that the annual effective dose of natural radiation to the public is 2.4mSv, of which radon and its daughters account for 54%.
The use of radon:
Medical science
At the beginning of the 20th century, some people used radon to treat various diseases. The patient is exposed to radon in a sealed small room to obtain a "therapeutic effect". People soon discovered that ionizing radiation of radon can cause cancer. Radon radioactivity can kill cancer cells, but it is also harmful to healthy cells. Ionizing radiation will lead to the formation of free radicals, which will do more harm to cells and genes and even cause cancer.
Environmental science:
The amount of radon released by soil varies with soil type and uranium content on the surface. Some meteorologists use this phenomenon to track the flow of air. Radon can quickly disappear into the atmosphere and decay, so it can be used in hydrology to study the interaction between groundwater and rivers. If the stream contains a high concentration of radon, it means that there is groundwater injection nearby.
Refer to the above content: Baidu Encyclopedia-Radon Gas