Your planet is kicking out huge quantities of radon gas.
Radon is a chemical element with the symbol Rn and atomic number 86.
It is a radioactive, colorless, odorless, tasteless noble gas. It
occurs naturally in minute quantities as an intermediate step in the
normal radioactive decay chains through which thorium and uranium
slowly decay into lead and various other short-lived radioactive
elements; radon itself is the immediate decay product of radium.
Radon can be problematic here on earth, even with the very low qualities of thorium and uranium in the crust. On a planet with much more of these heavy elements in the crust there would a lot more radon generated. It is heavy and so it will not be lost. As on Earth it will preferentially accumulate in low areas because it is heavy, but wind could carry it upward.
Airborne particulate heavy metals.
Potential Human Health Effects of Uranium Mining, Processing, and Reclamation
Radon and its alpha-emitting radioactive decay products are generally
the most important, but are not the only radionuclides of health
concern associated with uranium mining and processing. Workers are
also at risk from exposure to other radionuclides, including uranium
itself, which undergo radioactive decay by alpha, beta, or gamma
emission. In particular, radium-226 and its decay products (e.g.,
bismuth-214 and lead-214) present alpha and gamma radiation hazards to
uranium miners and processors. Radiation exposures to the general
population resulting from off-site releases of radionuclides (e.g.,
airborne radon decay products, airborne thorium-230 (230Th) or
radium-226 (226Ra) particles, 226Ra in water supplies) present some
If your world had enough uranium in the crust to make radon problematic, you could make volcanoes that spew uranium-containing ash high into the air. These particulates are long lived and can lodge in the lungs, and so are potentially more dangerous than radon. Volcanic ash particles can remain airbone for long periods and travel great distances. You could assert that the mid to upper atmosphere was contaminated by such particles.