This is the fallacy of counting the set of things that don't happen (I don't know if there's a technical term for it). The rivers aren't on fire anymore, the air is clean, the skies are safe - things are just about as good as they've ever been (despite a few outliers). That's all thanks to voting and common-sense regulations.
So to answer your question directly, "pretty much constantly but it's easy to miss since by definition they're successfully being prevented."
So to answer your question directly, "pretty much constantly but it's easy to miss since by definition they're successfully being prevented."