It means that independent labs will test anything that is sent to them for a fee.
Potheads are paranoid and send in samples to be tested for THC levels, heavy metals and pesticides.
These tests almost always come up negative.
Do you have some citations about that. I know a few, and have been exposed to a few more, and none have ever mentioned that. In fact, because of the media, most of them don’t think there could be any negative effects of marijuana. Governments and the media promote it as a panacea.