hydrogen is cheaper to produce than industrially than helium . A nitrogen/oxygen mixture comes free in the air . So why do we fill pary ballons with helium instead of cheaper hydrogen and fill deep sea divers' tanks with an expensive helium/oxygen mixture instead of cheaper compressed air ?
Both hydrogen and helium are lighter than air gases but helium is unreactive , whereas hydrogen could catch fire/explode . Nitrogen in air dissolves in the blood and causes health problems . Insoluble helium doesn't