A co-benefit approach is one where research on the effects of environmental policies is presented on the basis of indirect effects, as opposed to direct measured reduction of pollutants.
For eg. A policy aimed to reduce Green House gas emissions, could have co benefits of number of lives saved, due to a reduction in premature deaths from air pollution. However, no research can be exactly sure about how this approach will play out in the future.
A measured approach is one where the quantity of pollutants can be measured but the number of premature deaths cannot be predicted. Currently, most researchers use the measured approach.