Metrics to 'green' chemistry - which are the best?

A considerable amount has been written about the use of metrics to drive business, government and communities towards more sustainable practices. A number of metrics have also been proposed over the past 5-10 years to make chemists aware of the need to change the methods used for chemical syntheses and chemical processes. This paper explores several metrics commonly used by chemists and compares and contrasts these metrics with a new metric known as reaction mass efficiency. The paper also uses an economic analysis of four commercial pharmaceutical processes to understand the relationship between metrics and the most important cost drivers in these processes.