How does a per-unit tax affect producers' output decisions in the presence of negative externalities?

A per-unit tax is a fee that producers have to pay for each unit of a product they sell. When negative externalities, like pollution, occur, it means that the production of a good can harm others who are not involved in the transaction. The tax is intended to make producers consider these harmful effects by increasing their costs, which often leads them to produce less. For example, if a factory that produces candy also releases smoke that causes air pollution, a per-unit tax on each candy bar sold may encourage the factory to find cleaner ways to produce candy or reduce the number of candy bars they make. This way, the tax helps reduce the negative impact on the environment while also influencing producers to make better decisions.