Offsetting and risk imposition

Suppose one of your actions imposes a risk of harm that, on its own, would be excessive, but a second reduces the risk of harm by a corresponding amount. By pairing the two actions together to form a set of actions that is risk neutral, can you thereby make your overall course of conduct permissible? We argue that the answer to this is sometimes yes and sometimes no. We propose a criterion, the Principle of Aggregate Risk Imposition, that distinguishes the two kinds of cases and apply it to the offsetting of personal greenhouse gas emissions.