Which is lighter: hot air or cold air?

ok I guess hot water is lighter than cold water but what about hot air and cold air?
we can breathe cold air but not hot air so how does that work? is it because hot air is heavier than cold air? and if you put the covers over your head you can't stay...

under for long because it gets too hot to breathe

Hot air rises, therefore is lighter.

Most things expand when heated, and if a given volume expands it's density will decrease (I.e. it will get lighter.)

Your head under the covers 'problem' is due to a build-up of CO2. (CO2 is also the reason that you can't hold your breath longer. When you HAVE to take a gulp of fresh air, your lungs have only extracted a small part of the oxygen.)

thank you

oh ok thanx