90
inputs and outputs of the secondary co-products are calculated and subtracted from
those of the primary product. In the simplified wooden table example, the utility a
table provides is different than that of a toy or sawdust. The subprocesses not
directly associated with the table are placed outside the system boundary or the
table, and its co-products are grouped together as one expanded system that considers a set of wood products together with all additional functions they provide.
If neither of these approaches is appropriate to the system under study and allocation cannot be avoided, Step 2 partitions system inputs and outputs to reflect
physical relationships such as each co-product’s mass percentage relative to the
total mass or each co-product’s proportional financial market value relative to a
total. If it is not possible to establish comparable physical or economic units, other
value choices can be used as a last resort. Natural sciences provide the most reliable
allocation approaches. Clearly documented links that exist between emissions, for
example, relative to increasing or decreasing amounts of co-products create a firm
basis for proportional allocation.
The closer a study’s method for linking relationships between the objects of
study is to the natural sciences, the more grounded it is in the natural world and the
easier it is to express its conclusions in physical terms. The well-established economic proxies used in LCAs create reasonable and defensible equivalencies; but
they are at least once removed from directly describing the physical impacts attributable to a physical process or system. It is, therefore, always preferable to allocate
on a physical basis when causal links exist (Jolliet et al. 2015). This same preference
hierarchy holds for designers and others reviewing LCA studies about various product system alternatives to support one choice over another.
5.3.7 What Are a Design’s Most Important Physical Impacts
to the Environment, and How Are They Indicated
and Modeled?
Each of the assembled materials specified in a design will impact the environment.
Life cycle impact assessment (LCIA) is one of the four major iterative stages in the
overall LCA framework (ISO 14040 2006a) but also appears as an initial consideration in the goal and scope phase. Some materials used to execute a design in physical form will negatively affect some environmental impact categories more
than others.
There are numerous impact categories to be considered and modelled (Jolliet
et al. 2015), and some will be more relevant than others depending on the system
being studied. For instance, all metals must be refined from ore and processed into
billet and then shaped into finished goods. This high-temperature energy-intensive
process emits carbon dioxide from burning fossil fuel. Wherever metals are used,
greenhouse gas emissions will likely be a primary consideration. Higher
5 Life Cycle Assessment
inputs and outputs of the secondary co-products are calculated and subtracted from
those of the primary product. In the simplified wooden table example, the utility a
table provides is different than that of a toy or sawdust. The subprocesses not
directly associated with the table are placed outside the system boundary or the
table, and its co-products are grouped together as one expanded system that considers a set of wood products together with all additional functions they provide.
If neither of these approaches is appropriate to the system under study and allocation cannot be avoided, Step 2 partitions system inputs and outputs to reflect
physical relationships such as each co-product’s mass percentage relative to the
total mass or each co-product’s proportional financial market value relative to a
total. If it is not possible to establish comparable physical or economic units, other
value choices can be used as a last resort. Natural sciences provide the most reliable
allocation approaches. Clearly documented links that exist between emissions, for
example, relative to increasing or decreasing amounts of co-products create a firm
basis for proportional allocation.
The closer a study’s method for linking relationships between the objects of
study is to the natural sciences, the more grounded it is in the natural world and the
easier it is to express its conclusions in physical terms. The well-established economic proxies used in LCAs create reasonable and defensible equivalencies; but
they are at least once removed from directly describing the physical impacts attributable to a physical process or system. It is, therefore, always preferable to allocate
on a physical basis when causal links exist (Jolliet et al. 2015). This same preference
hierarchy holds for designers and others reviewing LCA studies about various product system alternatives to support one choice over another.
5.3.7 What Are a Design’s Most Important Physical Impacts
to the Environment, and How Are They Indicated
and Modeled?
Each of the assembled materials specified in a design will impact the environment.
Life cycle impact assessment (LCIA) is one of the four major iterative stages in the
overall LCA framework (ISO 14040 2006a) but also appears as an initial consideration in the goal and scope phase. Some materials used to execute a design in physical form will negatively affect some environmental impact categories more
than others.
There are numerous impact categories to be considered and modelled (Jolliet
et al. 2015), and some will be more relevant than others depending on the system
being studied. For instance, all metals must be refined from ore and processed into
billet and then shaped into finished goods. This high-temperature energy-intensive
process emits carbon dioxide from burning fossil fuel. Wherever metals are used,
greenhouse gas emissions will likely be a primary consideration. Higher
5 Life Cycle Assessment