Personally knowing four people with GBM and not being an oncologist is exceptional and worrying. That feels like it should be raised to… someone. It is very possible something bad is happening and a commonality needs to be tracked down urgently.
It's anecdotal, of course, but I've always thought that there could have been a connection.
Looks like benzene, some pesticides, and formaldehyde are the common workplace exposures that can trigger leukemia. But some of those can turn up near housing.
That rings a bell. I remember that someone mentioned a recent repainting of the building. The incident happened at least a decade ago, so I can't remember all details.