The planet has naturally occurring uranium throughout it. Radon is unique from other uranium products as it is a gas. As a gas, it can migrate from the source of the uranium into surrounding soils. Radon has always been around and humans have been breathing it forever. The question is not if radon is dangerous, but rather at what levels is it dangerous.
When uranium decays, one of the elements it produces is radon, which is Radon 222. The Radon 222 decays into more sub-particles, creating polonium 218, which then decays into lead 214, which then decays into bismuth 214, then again decays into polonium 214. Each decay process takes a different time, which is measured by half-life’s. The half-life of radon 222 is 3.8 days. This means that if you had a full jar of radon, in 3.8 days it would decay into one half of a jar and after 7.6 days it would have decayed into one quarter of a jar. The half-life of the polonium 218, lead 214 and bismuth 214 is about 30 minutes while the half-life of the polonium 214 is .00016 seconds.
When the radon gas is airborne, breathing it means that one of these isotopes can become “lodged” in the lungs. While the EPA has set limits for what is considered to be high radon levels, the complexity and unknown factors make these numbers somewhat arbitrary.