A common example of a negative externality is pollution. For example, a steel producing firm might pump pollutants into the air. While the firm has to pay for electricity, materials, etc., the individuals living around the factory will pay for the pollution since it will cause them to have higher medical expenses, poorer quality of life, reduced aestetic appeal of the air, etc. Thus the production of steel by the firm has a negative cost to the people surrounding the factory--a cost that the steel firm doesn't have to pay.