Uhh, being hit with an alpha particle randomly somewhere where something radioactive is exposed to air? I think Patrick Blackett proved that's what happens when that happens, back in 1925 or so.

It absolutely is an incredibly insignificant amount, but I didn't want to say "the number of oxygens on Earth is perfectly fixed and they're just cycled through being part of different particles", because even the number of oxygen atoms on Earth goes up or down. Probably more down as we send it into space? I don't know.

Water is made of hydrogen and oxygen atoms. There's not exactly a constant amount of these, because for instance radiation can convert nitrogen atoms into oxygen at some very slow pace, but it doesn't change very much even over a long time. Chemical reactions generally can't change what element an atom is, but they can take particles apart to build something else out of them. So as long as water particles have their hydrogens taken off to build hydrocarbons out of them, that reduces the amount of water, but the amount of hydrogens and oxygens remains the same.

And, contrarywise, when a hydrocarbon burns, the hydrogens from it rejoin with oxygen to make water again.

Titanium is a chemical element. Out of chemical elements, all of them from americium onwards are solely synthetically created in labs and such, while technetium and all elements from polonium onwards are rare naturally and radioactive (except for thorium and uranium, which aren't that rare.)

Titanium doesn't belong to either category.