A side effect of breaking the strong molecular bonds of dioxin is the potential for breaking the bonds of nitrogen gas (N2) and oxygen gas (O2) in the supply air. As the exhaust flow cools, these highly reactive detached atoms spontaneously reform bonds into reactive oxides such as NOx in the flue gas, which can result in smog formation and acid rain if they were released directly into the local environment. These reactive oxides must be further neutralized with selective catalytic reduction (SCR) or selective non-catalytic reduction (see below).