This is a 'double dagger'. It is not the symbol for the American Lung Association. That looks more like a telephone pole than a 'double dagger'. Although I'm not positive what a real double dagger actually looks like. I mean, I've never seen a double dagger. There isn't even one in D&D.

Whatever it is, you don't use it to kill people. You use it to footnote things. It's an extended HTML character, which you can display by typing an ampersand, followed by the word Dagger, and then a semicolon. Like this:

See? Make sure you capitalize Dagger, though, or you'll only get a single one. A single dagger is just plain silly. I mean, look at it:

It looks like an uncurved lowercase T. An uncurved lowercase T! It's not the symbol of the Protestant Church. That's a cross. This is just a dagger, and a single one at that. I urge you to use the double dagger instead - it will make all of our lives more worthwhile.