Page 131 - Design for Environment A Guide to Sustainable Product Development

110 Cha pte r Se v e n

Relative metrics are those that are defined with respect to another
metric or variable. A common approach is to use time-based relative
metrics, i.e. those which compute the change in a particular quantita-
tive metric over a given time period; for example “percent reduction
from 2006 to 2007 in total hazardous waste generated per unit pro-
duced.” Another common approach is to use “intensity” metrics,
where flows of resources or emissions are normalized by total sales or
production volume; for example, “total hazardous waste generated
per unit produced.”
Many stakeholder groups advocate the use of absolute metrics
because intensity metrics reflect efficiency of production rather than
actual environmental impacts. On the other hand, the use of absolute
metrics may lead to inappropriate comparisons, whereas relative
metrics are generally less biased by differences in the organization
characteristics. For example, the largest companies in a given indus-
try will typically be the largest emitters of airborne pollutants, even
though their emission intensity may be significantly lower than oth-
ers. Also, if sales happen to decline due to cyclical changes, absolute
environmental performance appears falsely to improve. The exam-
ples in Part 3 show that companies use a mixture of absolute and
relative metrics and, in some cases, report both types of metrics.

Source vs. Impact
With regard to environmental performance, source metrics are those
that address the presumed root causes or origins of environmental
consequences associated with an organization’s activities. An exam-
ple is the Toxic Release Inventory (TRI) mentioned in Chapter 3, which
measures the quantity of toxic materials released at a given site. An
advantage of source metrics is that they are both readily observable
and controllable. A disadvantage is that they are an indirect indicator
of potential impacts and generally ignore differences in fate, transport,
exposure, and effect pathways among different organizations.
Impact metrics are those that address the actual environmental
consequences which may result from an organization’s activities. An
example is the use of exposure assessment and dose-response assess-
ment to calculate the “increased cancer risk in the exposed popula-
tion.” While impact metrics have the obvious advantage of directly
addressing the impacts of concern, the development of environmen-
tal impact metrics is generally challenging due to the technical and
statistical uncertainties involved in both assessing impacts and attrib-
uting them to specific sources (see Chapter 9).
In practice, the most efficient means of performance measurement
is to select company-specific indicators (typically source-oriented)
which are believed to be correlated with broad categories of environ-
mental impacts. A common example is the measurement of airborne
emissions of suspected toxic substances. Although the magnitude (or