An aerosol particle containing enriched uranium encountered in the remote
We describe a submicron aerosol particle sampled at an altitude of 7 km near the Aleutian Islands that contained in a small percentage of enriched uranium oxide. 235U was 3.1 ± 0.5% of 238U. During twenty years of aircraft sampling of millions of particles in the global atmosphere, we have rarely encountered a particle with a similarly high content of 238U and never a particle with enriched 235U.
Via Journal of Environmental Radioactivity