If you put your own mask on, your seatmates may pass out but will revive as soon as you have placed their masks on their faces. Thus, your attempt to help them will only succeed if you help yourself first. It is marvellous to think that the danger of a human being assisting another before herself is so great that the FAA felt the need to warn against it on every airplane flight. Such altruism in the face of danger reveals great compassion, for it is performed in a moment of terrible risk, at great potential cost to oneself, when no-one knows what the future holds. Altruism and compassion are not synonyms.
Compassion is an emotion, sharing the suffering of another.
Altruism involves acting to help another. Not all compassion leads to altruism, and not all altruistic acts are performed because of compassion. Altruism and compassion are linked to one another. One could argue, with Kant, that this last individual would be the most moral of beings, feeling no immediate pleasure from her acts. Acts of altruism are easier to study than feelings of compassion.