Nitrous oxides (NOx) are one product of incomplete combustion of fossil fuels, for example in cars. They can lower the pH of precipitation.

Sulphur dioxide results from combustion of an impure hydrocarbon, it can also lead to acid rain.

Carbon monoxide is produced from incomplete combustion of hydrocarbons, this means that even completely pure fuels may produce some carbon monoxide on combustion.

Hydrocarbons derived from crude oil are not a renewable source of energy, this means that if crude oil runs out we can't make more. In order to sustain energy production for future generations renewable sources of energy are researched and used where possible.