Explain why the incomplete combustion of methane is dangerous.

Incomplete combustion occurs when the supply of air or oxygen is poor. Water is still produced, but carbon monoxide and carbon are produced instead of carbon dioxide. The carbon is released as soot. Carbon monoxide is a poisonous gas, which is one reason why complete combustion is preferred to incomplete combustion.

Carbon monoxide is produced.
In some cases when oxygen is too much less than soot (Carbon) is produced

1) according to the law of conservation of mass ,in all chemical reactions total mass is unchanged