Children oxygen mask VS adult oxygen mask - difference in capacity? in Education and Training Posted August 19, 2012 I was wondering that because I saw a medic gives a children mask to an adult, who admittedly had a relatively small face. I was wondering whether he did right. Don't those masks have different capacities appropriate to certain ages, or is the choice purely based on the size of the person face? Personally, I thought the mask looked a bit too small on her face.