88
A flow diagram shows the relationships between inputs and the intermediary
components in the processes that produce outputs. These diagrams feature bounding perimeters. Parts of the processes used to produce the system are inside, and the
adjacent parts that fall outside indicate the extent of the assessment. Essentially
what is in and what’s out determines whether the LCA is a cradle to grave/cradle,
cradle to gate, gate to gate, or gate to grave/cradle study.
Unlike vast regions of terra incognita marked at the edges of ancient maps with
warnings to navigators that “here be dragons,” the parts of the process that fall
immediately outside the system boundary are typically knowable. The data and
modeling needed to describe them are purposely excluded if they are simply not
part of the stated object of the study. Initial boundaries can be iteratively expanded
or contracted at a later point as required by an LCA’s evolving goals and scope.
5.3.5 How Little of Something Is Too Little to Include?
Cutoff criteria set the quantitative threshold for what is included and excluded in the
study. As unit processes and product systems and subsystems are evaluated, the
associated material and energy flows or environmental impacts that fall below the
established threshold are purposefully left out. Without cutoff criteria, the interconnectedness of everything quickly overwhelms. Recognizing that, for example,
delivering a can of soda to a consumer requires an array of machinery to make, fill,
transport, and track the can and those machines were built by machines which were
built by other machines and so on back to the start of the first industrial revolution,
it is reasonable that a large number of these subprocesses must be excluded from
any calculations. The impacts from these secondary, tertiary, and lower-order systems typically fall below the cutoff criteria threshold since they are not easily quantified (due to a lack of reliable data about them) nor do they contribute a material
percentage to the ultimate system impact assessments.
LCA experts acknowledge an apparent paradox: accurately setting appropriate
cutoff criteria in a study requires the results provided only by a full and complete
study. The chicken and egg problem is typically addressed through first taking a
rough cutoff for mass, energy, or cost flows below some stated fraction of the total.
Appropriate estimates range from a high 1% (Curran 2012) to an order of magnitude lower 0.1% (Hauschild et al. 2018)—but with the caveat that high environmental impacts can be caused by miniscule quantities of some substances that may fall
below a cutoff threshold. LCA’s iterative nature allows for the inclusion or exclusion of smaller quantities of certain high-impact elements or compounds. Sensitivity
analyses and other statistical evaluation methods refine the criteria throughout the
LCA process. Examples of low-quantity high-impact substances include radioactive isotopes, toxic chemicals, or rare metals mined and processed using environmentally hazardous methods. While nonexistent in the soda can example, these
materials may be critical in the products or components in which they are used and
be difficult if not impossible to substitute in a design. Iteratively qualifying their
5 Life Cycle Assessment
A flow diagram shows the relationships between inputs and the intermediary
components in the processes that produce outputs. These diagrams feature bounding perimeters. Parts of the processes used to produce the system are inside, and the
adjacent parts that fall outside indicate the extent of the assessment. Essentially
what is in and what’s out determines whether the LCA is a cradle to grave/cradle,
cradle to gate, gate to gate, or gate to grave/cradle study.
Unlike vast regions of terra incognita marked at the edges of ancient maps with
warnings to navigators that “here be dragons,” the parts of the process that fall
immediately outside the system boundary are typically knowable. The data and
modeling needed to describe them are purposely excluded if they are simply not
part of the stated object of the study. Initial boundaries can be iteratively expanded
or contracted at a later point as required by an LCA’s evolving goals and scope.
5.3.5 How Little of Something Is Too Little to Include?
Cutoff criteria set the quantitative threshold for what is included and excluded in the
study. As unit processes and product systems and subsystems are evaluated, the
associated material and energy flows or environmental impacts that fall below the
established threshold are purposefully left out. Without cutoff criteria, the interconnectedness of everything quickly overwhelms. Recognizing that, for example,
delivering a can of soda to a consumer requires an array of machinery to make, fill,
transport, and track the can and those machines were built by machines which were
built by other machines and so on back to the start of the first industrial revolution,
it is reasonable that a large number of these subprocesses must be excluded from
any calculations. The impacts from these secondary, tertiary, and lower-order systems typically fall below the cutoff criteria threshold since they are not easily quantified (due to a lack of reliable data about them) nor do they contribute a material
percentage to the ultimate system impact assessments.
LCA experts acknowledge an apparent paradox: accurately setting appropriate
cutoff criteria in a study requires the results provided only by a full and complete
study. The chicken and egg problem is typically addressed through first taking a
rough cutoff for mass, energy, or cost flows below some stated fraction of the total.
Appropriate estimates range from a high 1% (Curran 2012) to an order of magnitude lower 0.1% (Hauschild et al. 2018)—but with the caveat that high environmental impacts can be caused by miniscule quantities of some substances that may fall
below a cutoff threshold. LCA’s iterative nature allows for the inclusion or exclusion of smaller quantities of certain high-impact elements or compounds. Sensitivity
analyses and other statistical evaluation methods refine the criteria throughout the
LCA process. Examples of low-quantity high-impact substances include radioactive isotopes, toxic chemicals, or rare metals mined and processed using environmentally hazardous methods. While nonexistent in the soda can example, these
materials may be critical in the products or components in which they are used and
be difficult if not impossible to substitute in a design. Iteratively qualifying their
5 Life Cycle Assessment