2828735 He-O2 helium-20% oxygen 13 2828739 HeO2 He-20% O2 2 2828736 HeO2 helium + 21% O2 2 2828734 HeO2 helium--21% oxygen gas mixture 2 2828738 HeO2 He/30% O2 by mask 1 2828737 HeO2 helium and 20 percent oxygen 1