“The methods from this category offer information on the upstream energy needs of UMs. They usually consider the network of components in the UM to model the flows of energy. The period covered by those methods is relatively long because it ascends the stream up to natural resources. Their main advantage is their capacity to simplify the aggregation of different types of flows (e.g. mass, energy, area, volume, money) by defining specific energy equivalents with the use of pre-calculated conversion factors for all components/processes. This single type of information is however difficult to tie with environmental impacts. For example, it is almost impossible to correlate the energy content of emissions and their specific toxicity levels. Emergy and Exergy are the two types of methods from this category that have been used in UM studies.” (Beloin-Saint-Pierre et al. 2016)