If a men ejaculates insides the mouth / deep-throat, the semen may enter the air-track and the woman giving the blow-job may die of it. Apart from this, uncleaniliness may cause kidney-infections or even failure if done frequently over a long period. Blow-job should only be used momentarily for arousing a man or just to show love-for-phallus by a devoted woman to her man.
Besides this, it may cause harm to a man causing infection in prostrate glands. It can reduce a man's erectile-function in later stage of life.
- An Anonymous Concerned Man.
Why do you worry about it? It does not have any problem if you are doing it with a healthy partner. Enjoy it and stop worrying.