<em>A person living on the sixth floor of an aparment probably should not be concerned about radon</em>. In the conditions of the Earth's atmosphere (temperature and pressure), radon exists as a gas. This gas has a density that is approximately 8 times higher than the density of air (9.73 g/L compared to 1.22 g/L). <em>This means that radon gas would not rise, and instead remain close to the ground</em>, meaning that an apartment on a sixth floor is too far away from the ground for radon gas to reach there.
A gas being denser than air is also the reason why if you blow into a balloon, it will fall to the ground, because CO₂ is denser than air.
