what is created when combustion happens without enough oxygen

A tighter built home means less outside air infiltrating in the home, which means less oxygen to use for combustion within the furnace. As the furnace burns fuel and oxygen for combustion, it "eats up" surrounding air, which has to be "made up" with fresh air. If air cannot type in the home as easily anymore, this can cause serious issues. As the surrounding air is utilized for combustion, rather than substituted with oxygen, a negative pressure inside the space can happen. This could create combustion problems for the furnace, plus burn up fresh air for anybody inside the home. This picture amazingly captures the think layer of clouds in the valley at the south of Lake Como, northern Italy. In the above image, the dense cloud hides almost the valley and instead gives off some artificial lights which make town such as an opaque blanket. The village over distance is clearly seen out from the blanket.