While it's present in trace amounts, it's incredibly rare because:

* Radioactive decay: Radon is a product of the radioactive decay of uranium and thorium in rocks and soil. This means its concentration varies depending on the geological makeup of an area.

* Short half-life: Radon has a short half-life, meaning it decays quickly, making it hard to find in significant quantities.

* Noble gas: It's a noble gas, meaning it's chemically inert and doesn't readily form compounds, further reducing its presence in the atmosphere.

Although rare, radon poses health risks due to its radioactivity. It can accumulate in buildings, especially basements, and can lead to lung cancer if inhaled in high concentrations.