The Green Guide through its use of the Environmental Profiles Methodology 2008 assesses impact using the thirteen categories of environmental damage.
Environmental impacts in one category can be caused by many different emission substances (inventory flows), and one substance can contribute to several impact categories. The step of characterisation assesses all the different substances contributing to an impact category relative to one another to give an overall measure of the level of environmental damage in that category.
This is undertaken by using a reference substance or unit, whereby the contribution of each measured emission is calculated by converting the amount of emission into the equivalent amount of the reference substance or unit. This conversion is done by using what are called characterisation factors. For example, for the impact category of climate change, the reference substance CO2 (carbon dioxide) is used.
The use of a characterisation factor can be examined by taking methane as an example. We know that methane also contributes to climate change, but is measured to be 23 times more damaging that CO2 over a 100 year timescale. So, through the step of characterisation the effects of say 1 kg of methane are converted into the amount of CO2 needed to cause the same effect. This equates to applying the climate change characterisation factor of 23 to the measured methane amount of 1 kg which gives a figure of 23 kg CO2 equivalent.
The Environmental Profiles Methodology uses characterisation factors to cover the full range emissions and environmental impacts caused by the manufacture, use and disposal of construction materials examined in this Guide.
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