Hello. All balloons in this set (for Latex Balloons: 12"inch-30cm and Foil Balloons: 18 inch- 40cm size and above) are helium compatible. Both models of balloons require HELIUM gas to make them fly. No balloon can stay in the air if it is not filled with helium. Apart from filling your balloons with helium, you can also inflate them by blowing (Balloons inflated by blowing or with a hand pump do not fly). Note: Please fill it with helium at reliable places. In places with air mixing systems, the balloons that are inflated lose their ability to fly after a few hours. It stays up for 1 week if used with pure helium.