Hydrogen is collected by downward displacement of water as it is less denser than water.Hence it comes out at the surface of water as it is insoluble in water.In case of displacement with air,it is not possible even if hydrogen is lighter than air.This method is dangerous as the mixture of hydrogen  and air is explosive. The hydrogen gas is highly combustible and gets oxidised when it comes in contact with oxygen it burns with a pale blue flame.thus we do not collect hydrogen by downward displacement