General Flying Public and Nuclear Fallout While Flying

For the aviation experts:

With the possibility of North Korea and the US using nuclear weapons against each other, what if any impacts would there be to the general flying public and those people and flights transporting cargo, especially frequent flyers? Doing a Google search, I have recently seen articles posted about what occurs at ground level that have noted that radiation disperses at high altitudes but, without reading in the weeds, the articles don’t indicate what “high altitude” means (ie. does “high altitude” mean above 30000-40000 feet OR at OR below?).