. Burning these fuels in insufficient air, harmful carbon-mono-oxide gas is also produced with carbon-di-oxide gas which is dangerous for health.
On the other hand, if fuel contains particularly sulfur and nitrogen containing compounds, then on burning of fuel various oxides of nitrogen and sulfur harmful to the health and environment is produced. Sulfur-di-oxide combines with atmospheric water vapor and produce sulfuric acids which causes acid rain.
So, it is very important to ensure the use of pure fuels for safety of health and environment.