Hydrogen is a cleaner fuel than CNG. This is because the burning of hydrogen produces only water, which is totally harmless. On the Other hand, burning of CNG produces carbon dioxide gas and heating of the environment in the long run.

Some of the environment consequence of the increasing demand for energy are the following:

a) The combustion of fossil is producing acid rain and damaging plants (crops), soil and aquatic life.

b) The burning of fossil is increasing the amount of greenhouse gas carbon dioxide in the atmosphere.

c) The cutting down of trees from the forest (deforestation) for obtaining fir-wood is causing soil erosion and destroying wild life.

d) The construction of the hydro-power plants is disturbing ecological balance.

e) Nuclear power plants are increasing radioactivity in the environment.

The various factors which we should keep in mind while choosing a sources of energy are :

a) The ease of extracting energy from that source

b) The cost of extracting energy from the sources

c) The efficiency of technology available to extract energy from that sources.

d) The damage to environment which will be caused by using that sources.