Multiple riddles with the same solution

What is as light as a feather but not even the strongest man in the world can hold it for more than a few minutes?

I cost no money to use, or conscious effort to take part of. And as far as you can see, there is nothing to me. But without me, you are dead.

Die without me, never thank me. Walk right through me, never feel me. Always watching, never speaking. Always lurking, never seen.

What is used by man, tossed by trees, everywhere but unseen?

I am always around you but often forgotten. I am pure and clean most time, but occasionally rotten. What am I?

For all the above riddles there is a single answer. If you only find one of them you would have the answer for the others. If not, here below is the hidden answer.