My experience with cesium is that everyone thinks its radioactive. It isn't-- like many common elements there is a radioactive isotope, but that is not what would have been used here.
I speculate this worked by the highly reactive cesium ionizing the air. Similar things have been done with doping rocket fuels with metals to stimulate lightning strikes.
Presumably most of the cesium would end up reacted very quickly, I don't know what the products would be but plenty of obvious candidates like cesium carbonate appear to be non-toxic.
Well, you are correct in just some cases. Caesium has only one stable isotope, so beside the actual Caesium and Caesium-133 everything else has ß-decay. Unless you are burning 100% refined Caesium, there is always a chance that you are also burning its radioactive isotopes.
All the isotopes of Cs other than 133 exist only in negligible amounts in nature. So sure, perhaps there would be a trace amount of something else-- but that is true of anything.
I speculate this worked by the highly reactive cesium ionizing the air. Similar things have been done with doping rocket fuels with metals to stimulate lightning strikes.
Presumably most of the cesium would end up reacted very quickly, I don't know what the products would be but plenty of obvious candidates like cesium carbonate appear to be non-toxic.