The spraying was Operation LAC, a nationwide informal experiment to learn something about how wind disperses fine powders. Cadmium zinc sulfide is florescent, which made it easier to see where it landed. Some of it went all the way to Canada.
Powdered cadmium is poisonous, but it's not clear whether the quantities people may have inhaled or ingested were high enough or enough different from background levels to cause cancer or other diseases.
The original link starts off with cancers that were earlier than Operation LAC.
I would be curious to know what neighborhoods were chosen for getting sprayed with cadmium zinc sulfide.
 It isn't exactly an experiment, but I'm not sure what to call it. An effort at meticulous exploration?
There some discussion of these matters here, starting with comment 15.
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