The field of Green Chemistry has evolved over the decades. In the 1980s, the focus was on pollution control, with the publication of the 12 principles of green chemistry  in the late 1990s encouraging a more holistic approach, through to the 2010s, which has seen an explosion in the number of publications claiming to include green reaction processes . Answering the question as to whether a reaction is genuinely ‘green’ is not straightforward and requires an objective method of measuring its environmental impact.
In order to assess the greenness of a reaction, a number of ‘green metrics’ have been developed, which allow chemists to quantify and qualify the environmental impact of their reactions and assist with the comparison of different routes/methods. Early green metrics focused largely on efficiencies in terms of mass inputs and outputs, but this has since expanded to incorporate a much more comprehensive and holistic approach.
The following references provide an excellent overview of available green chemistry metrics:
Metrics to ‘green’ chemistry-which are the best?, Green Chem., 2002, 4, 521-527.,
Evaluating the “Greenness” of chemical processes and products in the pharmaceutical industry-a green metrics primer, Chem. Soc. Rev., 2012, 41, 1485-1498.,
Towards a holistic approach to metrics for the 21st century pharmaceutical industry, Green Chem., 2015, 17, 3111-3121.,
To study this area in more depth, see Metrics
The CHEM21 project has developed a unified metrics toolkit to comprehensively evaluate the sustainability of chemical and bio-chemical reactions based on a series of key parameters (see Figure 1). Moving beyond the use of ‘mass based metrics’ alone, the toolkit uses a blend of both qualitative and quantitative criteria to assess how green a reaction is, as well as considering factors both upstream and downstream of the reaction itself. This ensures a truly holistic approach.
The rationale behind the toolkit and the methodology employed is now available in an open access publication . As part of the background to this work CHEM21 performed a survey of available Green Chemistry metrics, which is accessible within the supplementary information of this paper.
To study this area in more depth, see CHEM21 metrics toolkit