Combustion and incineration often produce noxious and toxic gasses that must be treated or neutralized before being released into the atmosphere. Car exhaust produces carbon monoxide and nitrogen oxides as off gasses. That is why car exhaust is passed through a catalytic converter which contains various transition metal catalysts to convert the carbon monoxide to carbon dioxide and the nitrogen oxides to nitrogen gas and water vapor. In a similar manner, exhaust from incinerators must pass through scrubbers to remove toxic gasses and particulates. One particular area of concern here is that halogenated compounds when burned can produce acidic gasses like HCl and HBr. These gasses must pass through wet scrubbers where the acidic gasses are neutralized by reacting with bases.