Other types of cells, like red blood cells, only produce lactic acid because they don't have the machinery the mitochondria needed to actually regenerate those NAD plus coenzymes that we must have if we want to actually use glycolysis to form those ATP molecules. Now, notice that if the cells continually use lactic acid fermentation, there's going to be a build up in the H plus concentration in the cell and the surrounding tissue and that will decrease the PH and increase the acidity. And that can be very dangerous. Why? Well, because all different types of structures, the proteins and Euclide acids and DNA molecules are held together by what types of bonds will electric bonds. And these electric bonds can essentially be disrupted as a result of the increase in the ion concentration of the H plus ions.