When complete combustion does not occur, some of the things partially broken down react with others to produce new compounds, some of which are as dangerous as the original ones. So, incomplete combustion causes as many, if not more, problems as it solves. It makes new dangerous compounds and it allows them to be spread over large areas. To understand why complete combustion is not always achieved, look back at the four conditions that must be satisfied if all of the bonds are to broken. Imagine a furnace the size of your house.
Garbage is dumped in from the top, onto a fire.
If the garbage is not spread out uniformly, it will pile up in places. The water content of the garbage is also important. The temperature will drop until the water evaporates, and the amount of that drop will depend on how much water is there. So, for some unknown period of time, the temperature will probably be below the 900 degree figure, which is the minimum required, and incomplete combustion will occur. By the time the temperature gets back above 900 degrees, some of the products of incomplete combustion will have been released to the atmosphere.