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Natural gas which is non pollution gas and petroleum gas is the pollution gas.
Natural gas
Consists mainly of methane.  The large proportion of methane is one of the reasons natural gas has such a good safety record.
Methane is lighter than air and will disperse relatively easily if there is a gas leak.
Petroleum Gas
Is a hydrocarbon fuel comprised primarily of propane or butane, either separately or as a mixture. 
non-toxic, non-corrosive and free of lead.
 mainly stored in tanks in liquid form and is heavier than air.
more prone to catch fire or explode if a leak occurs.