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LCA part3

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hardwood somewhere in this life cycle as well. Perhaps the office of the accountant of the aluminium
smelter has tropical hardwood window frames. Of course, the amount of tropical hardwood per functional
Part 3: Scientific background 30 May 2001
unit of window frame is much lower for aluminium window frames than it is for those made of tropical
hardwood. If we wish to take this into account – and obviously we do – we must enter the domain of
quantitative analysis. We conclude that qualitative information like ‘contains hardwood’ is difficult to
employ in a comparative analysis covering more than a single aspect.
A more sophisticated case is the desire to include qualitative attributes of the quantified information. An
example is given by comparison of tropical hardwood window frames with and without an FSC certificate.
One could argue that this calls for a method that is capable of handling qualitative information. Better
still though, we could also recognise a parallel with distinguishing between emissions of copper from
mercury, and distinguish extractions of FSC-certified wood from non-certified wood. In other words, the
primacy of quantitative information does not imply that we stop using language to indicate the identity of
the flows modeled. Technically, every distinction within a class of flows means that these flows are
regarded as different. Thus, if we distinguish certified from non-certified tropical hardwood, we actually
have two different flows for hardwood. Likewise, distinguishing atmospheric and aquatic releases of lead
effectively yields two different flows that have the term ‘lead’ in common. And the same applies to every
form of spatial and temporal differentiation. It is a matter of personal preference whether we mentally
consider Iberian SO2 and Scandinavian SO2 as two different flows or as one flow that comes in two
varieties. In terms of modeling, however, they are two different flows, with separate entries in data tables.
The general structure for indicating quantifiable entities in any science is a tripartite one (see, for
example, the Handbook of Chemistry and Physics):
‘quantity’ = ‘value’ ´ ‘unit’
(not to be read as an equation), as in
the temperature is 23 degrees Celsius.
We thus have the following elements:
- quantity: the quantifiable entity of interest, like temperature;
- value: the numerical magnitude, like 23;
- unit1: the yardstick used to express the value, like degrees Celsius.
This same general structure is, in principle, applicable to the LCA model, including its input and output
data. However, several remarks are in order.
- ‘Quantity’ is a confusing word, because it also indicates ‘amount’ (‘quantity of product required for
the functional unit’). Other terms are ‘variable’, ‘parameter’ and ‘flow type’, and, in specific contexts,
‘observable’, ‘data’, and ‘coefficient’. All these terms also have disadvantages. In the context of LCA
modeling the term ‘variable’ seems to be the most appropriate. Observe that the quantity may be a
simple one, like temperature or emission of SO2, but it may also be more complex, like sea
temperature or emission of SO2 in Sweden.
- ‘Value’ is probably an even more confusing term than ‘quantity’, because it may easily suggest a
focus on economics, welfare or utility. The term ‘numerical value’ is a rather clumsy solution.
‘Magnitude’ may also indicate the type of variable rather than its size. ‘Size’ and ‘amount’ are
probably more unequivocal and neutral terms.
1 The terms ‘unit’ and ‘dimension’ are sometimes used incorrectly as synonyms. Every quantity has a unique
dimension. Speed, for instance, has the dimension length over time. Dimensions therefore provide no
yardstick. Units are specific cases of yardsticks. The dimension ‘length’ can be expressed by the units meter,
inch, mile and so on. Moreover, different quantities may share the same dimension. For instance, length,
distance, height and thickness all have the dimension length. Finally, certain quantities can have a composite
dimension with a non-composite unit. Energy has the dimension mass times length squared over time
squared, and is usually expressed in joule (J), although the composite unit kg times square metre over square
second is identical with joule.
Part 3: Scientific background 31 May 2001
- Even the term ‘unit’ is not free of contextual differentiation, as in, for instance, ‘unit processes’ and
‘functional unit’. Furthermore, ‘units’ in LCA are often used loosely. The Système International des
Unités lists basic and derived units, like kg, m and m/s. In published LCAs, guidebooks and
software, one often sees unconventional units, like ‘kg/functional unit’, ‘GWP’ and ‘mPts’, and units
may even be lacking altogether.1
As one terminology for all the disciplines involved in LCA seems too much to ask for, we here simply
state our first preference, with what appear to be relevant synonyms:
Variable: parameter, quantity, entity, flow type, flow
Value: amount, size (possibly magnitude)
Unit : [no synonyms]
Finally, variables are often abbreviated to user-defined symbols, while units are abbreviated using
conventional symbols. Speed, a variable, is written as v, u, s, or some other symbol. Attributes may be
indicated using other symbols, subscripts or diacritical symbols, like v versus u, vinside versus voutside, or v
versus v'. It is conventional practice to italicise symbols. A possible unit for speed is metre per second,
abbreviated to non-italic m/s. Values are not abbreviated, although the unit may be subdivided or
expanded using standard prefixes, as when 1200 m is written as 1.2 km. Given the prominent usage of
mathematical models for analysing environmental impacts, it seems only natural that the conventions of
mathematical notation developed in physics should be adopted in LCA, too.
In addition to qualitative attributes that can be regarded as part of or an attribute to the flow name, it may
be the case that we have real qualitative data. An example is a unit process which is known to disperse
an ‘awful odour’. If such information is not quantifiable and we wish to keep this information available
throughout the LCA, we must add it to the other qualitative and quantitative aspects of the unit process,
and carry it over to the inventory table and subsequent Impact assessment and Interpretation. In the
example with which we opened this section (the phrase ‘contains tropical hardwood’) we showed,
however, that this type of purely qualitative information is of very limited value in the course of an LCA.
1 What units would we recommend in these cases? “Kg/functional unit” is sometimes seen in inventory tables
for the aggregated emissions of one functional unit. Almost all calculations are about a functional unit, so
adding this is superfluous, just as it is superfluous to say that a can of paint contains 0.75 litre per can. A mere
“kg” suffices here. In the “GWP” example more is involved. The GWP (global warming potential; see section
4.3.5) is a quantity (or variable) and it is an inherent (or intensive) property of a substance that does not depend
on the amount of substance. One can speak of the GWP of methane, just as one can speak of the density of
stone. The GWP of methane is a dimensionless 21, and the multiplication of inventory results (in kg) by their
respective GWP yields a quantity that is again expressed in kg. “mPts” or “millipoints” is a unit that is
sometimes used for the weighting result after characterisation, normalisation and weighting. If
characterisation yields category results in kg and normalisation totals are in kg/yr, normalisation results are in
yr. Finally, if weighting factors are dimensionless, the weighting result is again in yr. If the reference flow of the
functional unit is expressed in terms of “function years”, normalisation and weighting results are
Part 3: Scientific background 32 May