if after swimming in salt water or being in salt air it could be due to dilation of the sinus blood vessels due to the salt. First, do not lean the head back if the bleeding is heavy. The blood will just go down into your stomach and make you nauseated. Leave your head in the "sniffing position" and take a piece of ice wrapped in a wet rag or a paper towel. Place the piece of ice in your mouth, between your upper lip and your front upper gums. The cold ice will cause the nearby blood vessels in the sinuses to contract and the bleeding will stop rather quickly.