A friend of mine is convinced that chemtrails are a primary cause of her son's asthma. This is a conspiracy theory that asserts governments are intentionally spraying chemicals out of airplanes for various unclear but nefarious reasons; see Wikipedia for more details.
Now I'm generally a live-and-let-live kind of guy, but her belief has reached the point where she won't take her son outside if chemtrail "monitoring" sites (which I won't dignify with a link) are indicating high chemtrail activity. I'm aware that this is going to be an uphill battle, but how do I get her to realize that chemtrails don't exist, and she's better off focusing on things that are actually known to trigger asthma?
(And oh, before somebody tells me to call CPS, she does fortunately also believe in modern medicine and uses inhalers etc for management.)