Precision of environmental data

A warning! The engineering properties of materials – their mechanical, thermal, and electrical attributes – are well characterized. They are measured with sophisticated equipment according to internationally accepted Standards and are reported in widely accessible handbooks and databases. They are not exact, but their precision – when it matters – is reported; many are known to 3-figure accuracy, some to more.

The eco-properties of materials are not like that. There are no sophisticated test-machines to measure embodied energies or CO₂ footprints. International standards, detailed in ISO 14040, lay out procedures, but these are vague and not easily applied. The differences in the process routes by which materials are made in different production facilities, the difficulty in setting system boundaries and the procedural problems in assessing energy, CO₂ and the other eco-attributes all contribute to the imprecision. This leads to wide variations in quoted values.

So just how far can values for eco-properties be trusted? An analysis suggests a standard deviation of ±10% at best. To be significantly different, values of eco-properties must differ by at least 20%. The difference between materials with really large and really small values of embodied energy or CO₂ footprint is a factor of 1000 or more, so the imprecision still allows firm distinctions to be drawn. But when the differences are small, other factors such as the recycle content of the material, its durability (and thus life-time), and the ability to recycle it at end of life are far more significant in making the selection.