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UNCERTAINTIES IN LCA
Analyzing uncertainty in a comparative life cycle assessment
of hand drying systems
Jeremy R. Gregory &Trisha M. Montalbo &Randolph E. Kirchain
Received: 10 July 2012 /Accepted: 26 May 2013 /Published online: 19 June 2013
#Springer-Verlag Berlin Heidelberg 2013
Abstract
Purpose The goal of this study is to evaluate and compare the
environmental impact (with a focus on global warming po-
tential) of five hand drying systems: hands-under (HU) dryers,
high-speed hands-under (HSHU) dryers, high-speed hands-in
(HSHI) dryers, cotton roll towels, and paper towels. Another
objective is to incorporate uncertainty into this comparative
life cycle assessment (LCA) as a means of understanding the
statistical robustness of the difference between the environ-
mental impacts of the hand drying systems.
Methods We conducted a life cycle assessment in accor-
dance with the ISO 14040/14044 standards using data pri-
marily from publicly available reports. As part of the study,
we performed a parameter uncertainty analysis for multiple
scenarios to evaluate the impact of uncertainty in input data
on the relative performance of products. In addition, we
conducted a probabilistic scenario analysis of key drying
system parameters in order to understand the implications
of changing assumptions on the outcomes of the analyses.
Results and discussion The scope of the analyses enabled us
to draw robust conclusions about the relative environmental
performance of the products. We can say with a high degree
of confidence that the high-speed dryers have a lower impact
than paper towels and cotton roll towels. Differentiating the
performance of the hand dryers requires being more specific
about framing assumptions. Under certain conditions, the
HSHI dryer is expected to have a lower impact than the
HU and HSHU dryers. However, under other conditions,
one cannot say that the HSHI dryer is clearly better than
the other dryers. We cannot differentiate the performance
between the HU dryer, cotton roll towels, and paper towels.
Conclusions This work demonstrates the importance of go-
ing beyond traditional uncertainty analyses for comparative
LCAs that are used for assertions of relative product envi-
ronmental impact. Indeed, we found instances where the
conclusions changed as a result of using the probabilistic
scenario analysis. We outline important elements that should
be included in future guidance on uncertainty analyses in
comparative LCAs, including conducting parameter and
scenario uncertainty analyses together and then using the
outcomes to guide selection of parameters and/or choices
to analyze further.
Keywords Life cycle assessment .Hand drying systems .
Parameter uncertainty .Scenario analysis
1 Introduction
Characterizing the relative environmental impact of everyday
life activities (and the products that enable them) has been a
staple of life cycle assessment since the inception of the field.
The relative environmental impact of hand drying systems is a
clear example of this. Interest by the public at large is made plain
by broad coverage of this topic in the media (Koerner 2008;
Clarren 2007; Skoczen 2009;Adams2007;Watson2007).
Interest within the technical and academic community is clearly
evidenced by no less than nine studies that target this very topic,
including a streamlined life cycle assessment (LCA) conducted
for Airdri Ltd. and Bobrick Washroom Equipment that com-
pares a standard warm air dryer to paper towels (Environmental
Resources Management 2001), a hand dryer–towel comparison
produced by myclimate and commissioned by Dyson in
Switzerland (Wettstein 2009), a comparison between cotton roll
Responsible editor: Andreas Ciroth
Electronic supplementary material The online version of this article
(doi:10.1007/s11367-013-0606-0) contains supplementary material,
which is available to authorized users.
J. R. Gregory (*):T. M. Montalbo :R. E. Kirchain
Materials Systems Laboratory, Engineering Systems Division,
Massachusetts Institute of Technology, 77 Massachusetts Ave, Rm.
E38-424, Cambridge, MA 02139, USA
e-mail: jgregory@mit.edu
Int J Life Cycle Assess (2013) 18:1605–1617
DOI 10.1007/s11367-013-0606-0
towels and paper towels commissioned by Vendor (Schryver
and Vieira 2008), and some calculations made by the Climate
Conservancy for Salon (Clarren 2007). More comprehensive life
cycle assessments that comply with the ISO 14040 and 14044
life cycle assessment standards (International Organisation for
Standardisation 2006) are also available. These include a study
for the European Textile Services Association that compares
cotton roll towels to paper towels (Eberle and Möller 2006),
another investigating multiple types of tissue products for
Kimberly-Clark (Madsen 2007), and a third for Excel Dryer
that compares its XLERATOR® hand dryer to a standard warm
air dryer and paper towels (Dettling 2009). Dyson has also
conducted a life cycle assessment of its Dyson Airblade™
hand dryer in accordance with the PAS 2050 standard (British
Standards Institute 2008) in order to obtain a Carbon Reduction
Label from the Carbon Trust (Dyson 2010).
Among all these studies, the one by myclimate (Wettstein
2009) is the most comprehensive in the scope of hand drying
systems considered—a high-speed hands-in dryer (the Dyson
Airblade™hand dryer), a standard warm air dryer, cotton roll
towels, and paper towels. It does not include the hands-under
variant of high-speed dryers, however. By contrast, the report
conducted for Excel Dryer includes a high-speed hands-under
dryer (the XLERATOR® hand dryer) but does not consider a
high-speed hands-in dryer or cotton roll towels. Because of the
studies’differing functional units, assumptions, and data, life
cycle assessment outcomes cannot be easily compared. We
conducted this study in order to address this gap. Thus, the
primary goal of the study is to evaluate and compare the various
hand drying systems—including both variants of high-speed
hand dryers—from the different studies by placing the systems
on a consistent basis.
A second and equally important objective is to incorpo-
rate uncertainty into this comparative LCA as a means of
understanding the statistical robustness of the difference
between hand drying system environmental impacts. Lloyd
and Ries (2007) provide definitions for the types of uncer-
tainty in their analysis of the LCA uncertainty literature.
Parameter uncertainty is derived from uncertainty in ob-
served or measured values. In an LCA context, it generally
refers to uncertainty in the input data used to create life cycle
inventories. Scenario uncertainty is related to the choices
made in framing the LCA and constructing scenarios. This
may be driven by inherent variability in geographic locations
or situations in the analysis, or it may be due to methodolog-
ical decisions around issues such as scope and allocation for
which there is no clear direction. Lloyd and Ries also discuss
model uncertainty, which relates to the structure and mathe-
matical relationships in models. In addition, there is uncer-
tainty in the impact factors used to translate the life cycle
inventory to life cycle impact.
In the literature, relatively few studies have been reported
that explore the role of uncertainty in comparative life cycle
assessments (Hong et al. 2010;Huijbregts1998;Huijbregts
et al. 2003;deKoningetal.2010). Of these, all have evaluated
the impact of parameter uncertainty. As a representative ex-
ample, Huijbregts (1998) quantified the implication of uncer-
tainty in the mass of the functional unit, the rate of recycling,
and many of the associated inventory items on the ability to
resolve the environmental performance of two alternative roof
gutter systems. Methods typically involve defining a baseline
set of conditions (or scenarios) around which a Monte Carlo
simulation or other analysis is conducted; ratios or differences
of impacts between products being compared are then calcu-
lated from analytical results. Indeed, current international
product carbon footprint standards suggest this type of analy-
sis (World Resources Institute and World Business Council for
Sustainable Development 2011). While these approaches are
important for the consideration of parameter uncertainty, they
only provide insight for a given scenario. Huijbregts first
pointed out this limitation and demonstrated an approach to
address it by examining four specific scenarios (two end-of-
life allocation rules and two future global warming potential
(GWP) reference scenarios). Later, Huijbregts et al. (2003)
demonstrate a more expansive comparative assessment that
includes both parameter and scenario uncertainty. Specifically,
in comparing two insulation alternatives for a Dutch home,
Huijbregts et al. evaluated aggregate parameter, scenario, and
model uncertainty across 32 specific scenario and model con-
ditions. Finally, de Koning et al. explored scenario uncertainty
due to analyst choices, but only insofar as uncertainty due to the
choices of other analysts is embedded in available databases
and secondary data. Instead, the focus of de Koning is on
demonstrating the importance of the decision framing scenario
(e.g., evaluating alternatives within the firm or comparing
products across firms) on the ability to resolve alternatives.
There is limited attention paid in LCA standards on how to
comment on the significance of the difference between prod-
ucts’environmental impacts. The ISO 14044 standard recom-
mends that “An analysis of results for sensitivity and uncertainty
shall be conducted for studies intended to be used in compara-
tive assertions intended to be disclosed to the public”(Interna-
tional Organisation for Standardisation 2006). While this state-
ment is important, there is no further guidance in the ISO 14044
standard on how to conduct sensitivity and/or uncertainty anal-
yses to support comparative assertions. The International Ref-
erence Life Cycle Data System (ILCD) Handbook (European
Commission-Joint Research Centre-Institute for Environment
and Sustainability 2010), the PAS 2050 (British Standards
Institute 2008), and the recently released Product Life Cycle
Accounting and Reporting Standard from the GHG Protocol
(World Resources Institute and World Business Council for
Sustainable Development 2011) each have sections discussing
uncertainty. Although these documents contain definitions of
uncertainty similar to those provided by Lloyd and Ries (2007),
the guidance is limited in that the focus is on qualitative
1606 Int J Life Cycle Assess (2013) 18:1605–1617
characterizations of data quality and quantitative calculations of
uncertainty in input data (i.e., parameter uncertainty) with min-
imal discussion about how to conduct a full uncertainty analysis.
For instance, the ILCD Handbook discusses the importance of
sensitivity analysis (like the ISO 14044 standard) and states that
scenario analysis and uncertainty calculations are the methods
to support the sensitivity analysis. However, there is limited
guidance in the handbook on the differences between sensitivity
analysis, scenario analysis, and uncertainty analysis. This is
almost certainly part of the motivation for calls from the litera-
ture for more guidance on uncertainty analyses in standards
(Draucker et al. 2011).
We will not completely address this gap of meaningful guid-
ance on uncertainty analyses for comparative LCAs in this paper.
However, we believe that the analyses conducted as part of this
case study on hand dryers can illuminate the importance of
elements that should be included in such guidance and will add
to the body of knowledge in this area. We conduct parameter
uncertainty analyses for several scenarios following the approach
proposed by Huijbregts and implied in the standards. In addition,
we go beyond these traditional analyses by conducting further
probabilistic scenario analyses of key parameters in order to
understand the implications of changing key assumptions about
the characterization of uncertainty in the parameters and correla-
tion of parameters among compared alternatives on the outcomes
of the analyses. We use these analyses to make recommendations
for important elements that should be included in future guidance
on uncertainty analyses in comparative LCAs.
2 Methods
2.1 Goal and scope
The goal of this study is to compare the environmental impacts of
a broad range of hand drying systems using a consistent basis and
to analyze uncertainty in order to comment on the statistical
significance of the differences between the systems. The study
was conducted in accordance with the ISO 14040 and 14044
standards (International Organisation for Standardisation 2006),
including critical review. A report that includes the full details of
the study and the critical review comments is available
(Montalbo et al. 2011), but we have included information on
key data and methodological assumptions here and have sum-
marized other important details in the Electronic Supplementary
Material available online. This paper builds on the original report
by providing a more comprehensive uncertainty analysis, pre-
senting a more precise evaluation of the ability to resolve alter-
natives, and using this case study as a platform to discuss the
issues associated with comparative life cycle assessment.
The scope of the study includes five hand drying systems,
detailed in Table 1, which describes the method used by each
product to dry hands and references the primary source of
product material and performance data (the actual data are in
the Electronic Supplementary Material). In addition to the
dryers and towels themselves, packaging is considered in all
cases, as well as dispensers in the case of the towel systems
and a waste bin and bin liners for the paper towel system.
Drying a single pair of hands represents the functional
unit. Since the electric hand dryers (hands-under (HU), high-
speed hands-under (HSHU), and high-speed hands-in
(HSHI)) dry numerous pairs of hands over their lifetimes,
their environmental impacts are allocated across all these
pairs of hands. The same holds true for the cotton roll towels,
towel dispensers, waste bin, bin liners, and packaging used
by these products (see Section 1 in the Electronic Supple-
mentary Material for details on allocations per functional
unit).
The system boundaries encompass all life cycle phases,
from cradle to grave, along with transportation between and
within each phase. Details on the system boundary and the
life cycle stages are included in Section 1 of the Electronic
Supplementary Material. All systems, with the exception of
paper towels, are assumed to be manufactured in China.
Table 1 Overview of hand drying systems evaluated
Hand drying system Drying method Primary source of product
material and performance data
Hands-under (HU) dryer User places hands under the dryer nozzle and
warm air blows onto the hands to dry them
Generic dryer in Dettling (2009)
High-speed hands-under
(HSHU) dryer
Same as HU, but air is blown at high speeds Excel XLERATOR® dryer in
Dettling (2009)
High-speed hands-in (HSHI)
dryer
User places hands into dryer and air blows at high
speeds onto hands to dry them
Dyson Airblade™dryer with
aluminum cover (Blower (2011)
Dyson Ltd., personal communication)
Cotton roll towels (CRT) User pulls cotton towel from roll in dispenser and
dries hands by rubbing on towel
Generic cotton towel in Eberle and
Möller (2006)
Paper towels (PT) User pulls paper towels from dispenser and dries
hands by rubbing on towels
Kimberly-Clark 100 % virgin paper
towel in Madsen (2007)
Int J Life Cycle Assess (2013) 18:1605–1617 1607
Upstream processes such as the mining of ore or the extrac-
tion and refining of petroleum for vehicle fuel are included
within system boundaries. Only the energy required to man-
ufacture dryers or towels is accounted for in the calculation
of manufacturing phase impact—production of capital
equipment is considered outside the scope for this phase
due to limitations on data availability (it is included in
upstream processes that are part of the ecoinvent database).
The use phase takes place in the USA. For the electric
hand dryers, use phase impact is solely due to the production
and distribution of electricity required for operation. The use
phase for cotton roll towels encompasses not only the use of
the towel inside a washroom, but also a cleaning step which
takes place at a laundry facility. Finally, at the end-of-life, all
product types are transported to a nearby waste facility where
they are incinerated or sent to a landfill. With the exception
of the cardboard packaging, there is no clear evidence that
these products are commonly recycled—or in the case of
cotton and paper towels, composted—in the USA.
2.2 Life cycle inventory and impact assessment
We obtained data used to generate drying system life cycle
inventories from a variety of existing sources outlined in
Section 2 in the Electronic Supplementary Material (the pri-
mary sources are summarized in Table 1). Baseline assump-
tions used to generate hand drying system life cycle invento-
ries are listed in Table 2.TheElectronic Supplementary Ma-
terial provides details on the sources for the data, but the hand
drying time data are particularly noteworthy. Hand drying
time is characterized in two ways and will be analyzed in the
uncertainty analyses. The first is drying-driven usage, where a
user dries his hands to either to a defined dryness standard—in
this case NSF Protocol P335 (NSF 2007)(thebaselinedry
times are based on tests run using this standard)—or to a
user’s personal dryness comfort level. This assumes users
are using the products as recommended. The second is time-
driven usage, where users employ the dryer for a prescribed
amount of time, regardless of the dryness of the hands. This
assumes users are not as concerned about having completely
dry hands. The times in Table 2are based on measurements
conducted in accordance with the NSF protocol (representing
drying-driven usage), but the uncertainty analyses will also
present results using drying times reported by hand dryer
manufacturers and time-driven usage scenarios (and are listed
in Section 2.1.3 of the Electronic Supplementary Material).
The only significant difference between the measured and
reported dry times is that the HSHU reported dry time is
12 s as compared to the 20 s measured dry time.
The importance of the hand drying time characterization
has led us to use three scenarios in the uncertainty analyses:
(1) baseline scenario assumptions including hand dryer mea-
sured dry times based on the NSF Protocol P335; (2) the same
baseline scenario assumptions except for dryer dry times
based on reported times from manufacturers (detailed in
Section 2.1.3 in the Electronic Supplementary Material); and
(3) the same baseline scenario assumptions and reported dry
times and a consistent printed wiring board (PWB) unit pro-
cess data used for all three hand dryers. The original data
sources for the three dryers had used different sets of unit
process data: the generic unit process, “electronic component,
active, unspecified,”was used to represent the control and
optics assemblies in original studies on the HSHU and the HU
dryers, whereas a more specific unit process, “printed wiring
board, through-hole, lead-free surface,”was specified for the
HSHI dryer’s inventory; the latter process has a lower impact.
The PWB has a significant impact on the production impact of
the dryers, and thus, the choice of the PWB can be important.
We chose to defer to the judgment of the original LCA
analysts in their selection of inventory data for the PWB and
have used those in our baseline analyses and scenario 2.
However, we believe it is unlikely the PWBs in the three
dryers would be significantly different, which is why we have
analyzed the impact of changing this assumption in scenario 3.
We obtained background inventory data for intermediate
flows from the ecoinvent v2.1 database (Frischknecht et al.
2007). The majority of the data were used directly from the
database, but in a few cases, we needed to make some mod-
ifications to ecoinvent datasets because of a lack of existing
inventory data. These cases and the associated data are listed
in Section 2.2 of the Electronic Supplementary Material.
We conducted all life cycle analyses using a combination of
models in SimaPro LCA software and Microsoft Excel with a
Crystal Ball extension. The specific instances for each software
tool are described in the “Uncertainty analysis methodology”
section below.
In the full report, we used IMPACT 2002+ (Hischier et al.
2010) to calculate life cycle impact. However, due to re-
source limitations, uncertainty analyses were only conducted
using the GWP life cycle impact assessment methodology
(Hischier et al. 2010), which incorporates the impact of
gaseous emissions according to their potential to contribute
to global warming based on the 100-year characterization
factors published in 2007 by the Intergovernmental Panel on
Climate Change (2007). Thus, only results using GWP are
presented here and the conclusions should not be generalized
to other impact measures without further analysis.
2.3 Uncertainty analysis methodology
We conducted two types of analyses to study the impact of
uncertainty on the outcomes of our comparative LCA: a pa-
rameter uncertainty analysis and a probabilistic scenario anal-
ysis. The former is consistent with the approach implied in the
standards, but the latter has not been explicitly outlined in the
literature or standards. A scenario analysis is presumably a
1608 Int J Life Cycle Assess (2013) 18:1605–1617
study of the impact that various scenarios have on the out-
comes of the LCA (it is not explicitly defined in the standards).
This is typically done using analyst-defined, discrete scenarios
(where a scenario is simply a defined set of parameters and
framing assumptions). We have extended this by defining
ranges for specific types of parameters that are particularly
important when defining scenarios and used probabilistic tech-
niques to explore thousands of potential scenarios.
This type of scenario analysis is not to be confused with a
scenario uncertainty analysis. Various sources in the litera-
ture described in the “Introduction”have defined scenario
uncertainty as being about choices made in framing the LCA
(typical examples include allocation methods or boundary
decisions) and hence are not typically analyzed in a param-
eter uncertainty analysis. Our scenario analysis includes
some of these issues, but it also includes analysis of param-
eters that would be studied in a sensitivity or scenario anal-
ysis because of their high impact or high profile.
We did not characterize uncertainty in the models or in the
impact factors due to a lack of information on how the
uncertainty should be defined. Because these are not includ-
ed in our analysis, the method underestimates actual uncer-
tainty and therefore overestimates the ability to resolve dif-
ferences between alternatives. Future work should investi-
gate the significance of this limitation.
2.3.1 Parameter uncertainty analysis
In our uncertainty analyses, parameter uncertainty was char-
acterized using the pedigree matrix approach implemented in
the ecoinvent database (Frischknecht et al. 2007) because it
represents the most widely used methodology in LCA for
characterizing uncertainty. Details on the approach and the
specific scores used in this analysis are provided in Section 3
of the Electronic Supplementary Material. A lognormal dis-
tribution was used for all input data because it always
Table 2 Assumptions used to generate hand drying system life cycle inventories for the baseline analysis
Drying system Hands-under
dryer
High-speed
hands-under dryer
High-speed hands-in
dryer
Cotton roll towels Paper towels
Functional unit 1 pair of dry hands
Lifetime usage 350,000 pairs of dry hands over 5 years (Blower (2011) Dyson Ltd., personal communication; Dyson Airblade
Technical Specifications (2011)
Mass (+ manufacturing scrap)
per dryer or towel
6.4 kg (0.9 kg)
(Dettling 2009)
9.4 kg (1.12 kg)
(Dettling 2009)
14.8 kg (1.43 kg)
(Blower (2011)
Dyson Ltd., personal
communication)
16.2 g (2.2 g) (Eberle
and Möller 2006)
1.98 g (0.08 g)
(Dettling 2009)
Manufacturing location China China China China USA
Manufacturing energy per
dryer or towel
156 MJ electricity
(Dettling 2009)
156 MJ electricity
(Dettling 2009)
146 MJ electricity
(Dettling 2009)
431 kJ electricity,
507 kJ gas (Eberle
and Möller 2006)
14.7 kJ electricity,
24.4 kJ gas
(Dettling 2009)
Use location USA
Use intensity 31 s at 2,300 W 20 s at 1,500 W 12 s at 1,400 W 1 towel (pull) + laundry 2 towels
+ 1.5 s at 1,150 W + 1.5 s at 750 W + 0 s at 0 W
+ 406 s at 0.4 W + 429 s at 1 W + 439 s at 1 W
End-of-life scenario 76.7 % of cardboard recycled
19 % of remaining waste incinerated with energy recovery
81 % of remaining waste landfilled with methane capture and conversion to electricity (Franklin Associates 2011;
United States Environmental Protection Agency 2009)
Transportation
Raw material to plant 250 km via truck
Plant to warehouse 10,500 km via ocean freighter + 2,600 km via freight train + 24 km via truck (excluding paper towels)
Warehouse to washroom 1,760 km via truck
Washroom to laundry and back 100 km via truck (cotton towels only)
Washroom to waste facility 100 km via truck
Additional lifecycles Packaging Packaging Packaging Packaging, dispenser Packaging,
dispenser, waste
bin, bin liners
Packaging per dryer or towel 0.45 kg cardboard
(Dettling 2009)
0.27 kg cardboard
(Dettling 2009)
2.94 kg cardboard
(Blower (2011)
Dyson Ltd., personal
communication)
0.08 g polyethylene
(Eberle and Möller
2006)
0.18 g cardboard
(Dettling 2009)
Int J Life Cycle Assess (2013) 18:1605–1617 1609
remains positive and is consistent with the data available in
the ecoinvent database and the pedigree matrix approach.
We conducted parameter uncertainty analyses using
pairwise Monte Carlo simulations in SimaPro. The difference
in these results was evaluated using a one-way analysis of
variance on the means and through a pairwise comparison
indicator. The latter involved analyzing a pair of products
simultaneously and calculating the ratio of impacts for the
two products in each simulation (the ratio is known as the
comparison indicator (CI) as proposed by Huijbregts (1998)).
The pairwise analysis ensures a correlated analysis. That is,
values selected in each simulation for parameters that are
common within and among the analyzed pair will be the same.
(There may be instances where this correlation constraint is
unrealistic (e.g., it may be reasonable to expect that materials
common to the two products in the analysis come from differ-
ent sources), and thus, the correlation may create an appearance
of more overlap in environmental impact than actually exists.)
A result of the analysis is a cumulative frequency distribution of
comparison indicators (F(CI)) from all simulations. From the
F(CI), we can derive the fraction of simulations in which the
modeled impact of one product exceeded the other. Because the
current analysis excludes some forms of uncertainty (i.e., model
andimpactfactor),wehavechosentoidentifydifferenceinthe
two products at two conservative threshold values of 90 and
95 %. That is, a comparison indicator of greater than or equal to
one is observed in more than 90 or 95 % of the trials.
We conducted pairwise analyses of all product combina-
tions (ten total combinations) for a given set of conditions (or
scenario) using 1,000 simulations in each comparison. These
comparisons were conducted for three different scenarios (de-
scribed in the previous subsection) for a total of 30 analyses.
2.3.2 Probabilistic scenario analysis
A typical uncertainty analysis for a comparative LCA will
include a stochastic analysis of parameter uncertainty for a
selected number of discrete scenarios, which is consistent with
the parameter uncertainty analysis we have described here.
While this is important, the analysis represents a small sample
of the overall scenario space. That is, there will likely be
countless scenarios that can be derived from various combina-
tions of methodological choices and values for parameters that
are evaluated in scenarios. For this paper, we have attempted to
explore the scenario space more broadly by using Monte Carlo
analysis to randomly sample values for a set of drying system
parameters, such as use intensity or electric grid mix, and
repeat this process thousands of times to create a wide range
of scenarios. This enables us to comment on the robustness of a
claim that one product has a lower environmental impact than
another by stating how often a product is observed to have a
lower modeled impact across a wide range of scenarios.
Our probabilistic scenario analysis was implemented using
Microsoft Excel with Crystal Ballfor the Monte Carlo method
and the same inventory and unit process data used in the
parameter uncertainty assessment in SimaPro. Unlike the
parameter uncertainty analyses, all five products can be ana-
lyzed simultaneously for this spreadsheet model. The param-
eters varied in the analysis are shown in Table 3along with
their baseline values, ranges, and distributions; all other pa-
rameter values remained the same as in the baseline scenario.
All of the parameters (with the exception of the composting
assumption which is binary) have a uniform distribution be-
cause we are assuming that all parameter values and scenarios
are equally likely (in the absence of alternative information).
The use of a statistical distribution on the parameter values
and the Monte Carlo analysis enable us to generate numerous
combinations of scenarios across the scenario space.
The set of parameters shown in Table 3was selected because
they are parameters for which the selection of a representative
value was difficult or infeasible due to a lack of solid data or
because they represented variation in geographical scope (i.e.,
use grid mix and municipal solid waste incineration fraction). In
addition, a number of the parameters have a strong influence on
drying system environmental performance, as dictated by sensi-
tivity analyses detailed in the full report. Details on the motiva-
tion for the specific ranges for each parameter are provided in
Section4oftheElectronic Supplementary Material, along with
details about correlation assumptions for the parameters. Corre-
lation assumptions in a scenario analysis are not straightforward,
but can be critical (see de Koning et al. 2010). Due to this
importance, we conducted multiple analyses in which hand dryer
use intensity is either correlated or uncorrelated (correlated use
intensity means that deviations from the use intensity are the
same for all products in a given simulation; uncorrelated means
that deviations for all products are independent). We also
changed the usage pattern to be either drying-driven or time-
driven and whether the PWB unit process is consistent or incon-
sistent in each scenario uncertainty analysis. While we have
attempted to select sensitivity analysis ranges which covered
the majority of reasonable and significant cases, it is possible
that some cases are excluded. As such, the conclusions drawn
here should not be generalized to other cases without further
study.
We conducted six sets ofscenario uncertainty analyseswith
multiple combinations of these three framing assumptions
(dryer use intensity correlation, usage pattern, and PWB unit
process consistency). Each scenario uncertainty analysis in-
volved 20,000 Monte Carlo simulations. The differences in
the means of the resulting simulations were tested using the
Steel–Dwass variant of the Kruskal–Wallis test which exam-
ines significance of difference in sample means while control-
ling for alpha across the entire set of comparisons (details are
provided in Section 5 of the Electronic Supplementary Mate-
rial). Differences in the sample medians were evaluated by a
1610 Int J Life Cycle Assess (2013) 18:1605–1617
sign test on each combination of samples. Additionally, in
each simulation, we calculated a comparison indicator for the
relative impact of two products in the same manner as the
parameter uncertainty analysis. We used the same 90 and
95 % frequency levels as thresholds for evaluating whether
one product has a lower environmental performance than
another across the scenario space.
3 Results
3.1 Deterministic
The deterministic GWP results associated with drying a single
pair of hands for the three scenarios described in Section 2.2 are
showninFig.1. The figures illustrate that there are differences in
overall expected impact among the drying systems, but in some
cases, this can depend on the scenario under consideration. They
also show that the use phase is expected to be the driving factor
for the GWP of electric hand dryers (due to the electricity
consumption during use) and the cotton roll towels (from wash-
ing the towels), whereas manufacturing is expected to be the
driving factor for the paper towel GWP. If reported dry times are
used (scenario 2, results in Fig. 1b), the HU dryer impact
decreases 3 % (relative to the baseline results with measured
dry times in Fig. 1a)to17.3gCO
2
eq (due to a dry time decrease
from 31 s down to 30 s) and the HSHU dryer decreases 34 % to
5.4gCO
2
eq(duetoadrytimedecreasefrom20sdownto12s);
the other three dryer system impacts remain unchanged. If re-
ported dry times and a consistent PWB unit process are used
(scenario 3, results in Fig. 1c), the HU dryer impact decreases
6 % (relative to the baseline results with measured dry times in
Fig. 1a)to16.8gCO
2
eq and the HSHU dryer decreases 40 % to
4.9 g CO
2
eq; once again, the other three dryer system impacts
remain unchanged. Although it may appear that there are clear
differences among the products, with this analysis alone, one
cannot comment on the robustness of comparisons between the
products, particularly when making comparisons across the three
scenarios.
Contribution analyses have highlighted the key drivers of
environmental impact for the hand drying systems for global
warming potential (Montalbo et al. 2011). For hand dryers,
environmental impact is driven by the use phase energy
consumed in the active use of the hand dryer. Within the
production phase (including materials and manufacturing),
key contributors are the housing materials, electricity used in
production, and the printed wiring boards for the controls
and optics assembly.
Table 3 Parameter ranges and distributions for probabilistic scenario analysis
Independent parameters Drying systems Baseline Range Distribution
Lifetime usage High-speed hands-in 350,000 150–550 K pairs of hands over 5 years Uniform
Lifetime usage High-speed hands-under 350,000 150–550 K pairs of hands over 5 years Uniform
Lifetime usage Hands-under 350,000 150–550 K pairs of hands over 5 years Uniform
Lifetime usage Cotton roll towel (dispenser) 350,000 150–550 K pairs of hands over 5 years Uniform
Lifetime usage Paper towel (dispenser and bin) 350,000 150–550 K pairs of hands over 5 years Uniform
Number of reuses Cotton roll towel 103 70–130 launderings and reuses Uniform
Manufacturing grid mix High-speed hands-in CN average 0.019–1.44 kg CO
2
eq/kWh Uniform
Manufacturing grid mix High-speed hands-under CN average 0.019–1.44 kg CO
2
eq/kWh Uniform
Manufacturing grid mix Hands-under CN average 0.019–1.44 kg CO
2
eq/kWh Uniform
Manufacturing grid mix Cotton roll towels CN average 0.019–1.44 kg CO
2
eq/kWh Uniform
Manufacturing grid mix Paper towels US average 0.011–1.22 kg CO
2
eq/kWh Uniform
Use grid mix High-speed hands-in US average 0.016–1.32 kg CO
2
eq/kWh Uniform
High-speed hands-under
Hands-under
Cotton roll towels
Use intensity High-speed hands-in 12 s Drying-driven:−50 to +25 % of measured
baseline; time-driven:5–30 s
Uniform
High-speed hands-under 20 s
Hands-under 31 s
Use intensity Cotton roll towels 1 towel 1–2 towels Uniform
Use intensity Paper towels 2 towels 1–3 towels Uniform
Municipal solid waste
incineration fraction
All 19 % 0–100 % Uniform
Compost Paper towels, cotton roll towels No Yes, No Binary
CN China, US United States
Int J Life Cycle Assess (2013) 18:1605–1617 1611
For cotton roll towels, the use phase (i.e., washing the
towels) accounts for over half of the total impact, followed
by transportation and manufacturing and then by materials.
Between material production (cotton fibers) and processing
(spinning, weaving, and de-sizing), no single step dominates
the GWP production impact of a cotton towel roll. These
results indicate that all life cycle phases, with the exception
of end-of-life, are important to consider when assessing the
life cycle impact of cotton roll towels.
For paper towels, the manufacturing phase makes up over
half of the impact for global warming potential and water
consumption, followed by the materials production phase and
transportation. It is noteworthy that paper towels are the only
product for which product end-of-life has any significant im-
pact—specifically in global warming potential (caused by deg-
radation of paper towels in landfills). Electricity and the natural
gas used in the paper towel production are the primary contrib-
utors to the production GWP, making up approximately three
quarters of the impact, followed by pulp manufacturing.
3.2 Parameter uncertainty
The outcomes of the parameter uncertainty analyses are
shown in Table 4. (Outcomes from the one-way analysis of
variance test, in particular, and the resulting significance of
differences in sample means are reported in Table 28 of
Section 5 of the Electronic Supplementary Material). These
outcomes are consistent with, but generally less stringent than
the comparison indicator results discussed here. Results from
pairwise comparisons are listed for three different scenarios in
Table 4. In some instances, one drying system is better than
another in all simulations and across all three scenarios. For
example, the high-speed hands-in dryer outperformed the
hands-under dryer in every simulation in all three scenarios.
In fact, both high-speed dryers are almost always better than
the other three products (the one exception being the HSHU
and cotton roll towels whose similarity cannot be rejected at
90 % (or 95 %) confidence when the dryer is evaluated at the
measured dry time). When accounting for uncertainty in input
data, we can clearly say with confidence that the high-speed
dryers have a lower GWP than the other drying systems for
these three scenarios. However, the results are much less clear
when comparing the two high-speed dryers (in only one
scenario can the similarity of the two products be rejected at
90 or 95 % confidence) or when comparing cotton roll or
paper towels and the hands-under dryer.
3.3 Probabilistic scenario analysis
The resulting GWP frequency distributions for the 20,000
iterations of the scenario uncertainty analysis are presented
in Fig. 2for both drying-driven (see Fig. 2a) and time-driven
(see Fig. 2b) usage patterns. Dryer GWP distributions associ-
ated with time-driven usage are similar because the dryers
themselves are differentiated only by their respective power
ratings: dry times are the same for all dryers. By contrast, the
distributions of the HSHU and HU dryer systems have a much
wider spread than that of the HSHI dryer system when drying-
driven usage is considered due to the broader range of dry
times for the first two systems. Statistical tests on the signif-
icance of difference for the central tendency of these results
(Kruskal–Wallis test on means and sign test of medians) are
presented in Tables 29 and 30 in Section 5 of the Electronic
Supplementary Material. These results are consistent with, but
0
5
10
15
20
Hands-under
dryer
High-speed
hands-under
dryer
High-speed
hands-in dryer
Cotton roll
towels
Paper towels
Global warming potential [g CO2 eq]
End-of-Life
Use
Transportation
Manufacturing
Materials
(a)
0
5
10
15
20
Hands-under
dryer
High-speed
hands-under
dryer
High-speed
hands-in dryer
Cotton roll
towels
Paper towels
Global warming potential [g CO2 eq]
End-of-Life
Use
Transportation
Manufacturing
Materials
(b)
0
5
10
15
20
Hands-under
dryer
High-speed
hands-under
dr
y
er
High-speed
hands-in dryer
Cotton roll
towels
Paper towels,
virgin
Global warming potential [g CO2 eq]
End-of-Life
Use
Transportation
Manufacturing
Materials
(c)
Fig. 1 Global warming potential for drying a single pair of hands under
three scenarios: ascenario 1: baseline assumptions, measured dry times;
bscenario 2: baseline assumptions, reported dry times; cscenario 3:
consistent PWB unit process, reported dry times
1612 Int J Life Cycle Assess (2013) 18:1605–1617
generally less stringent (i.e., identify more statistically signif-
icant differences) than the comparison indicator results
discussed here.
While the frequency distributions of drying system GWP in
Fig. 2clearly overlap, it is important to look at the comparison
indicator results in order to understand the impact of correla-
tion. Comparison indicator distributions for the GWP of dif-
ferent drying systems relative to that of the HSHI dryer are
shown in Fig. 3given both drying-driven and time-driven
usage patterns. The HSHI dryer was chosen as the point of
comparison because it has the lowest impact in the baseline
deterministic analyses. The results indicate that for drying-
driven dry times, the comparison indicator distribution is
almost entirely above one. By contrast, this distribution is
shifted to the left for time-driven dry times. Overall, however,
the GWP of the high-speed hands-in dryer is still lower than
that of any given drying system in over 92 % of the iterations.
The results in Figs. 2and 3assume correlated use phase grid
mix and use intensity for the hand dryers. The HSHI dryer
system almost always has the lower impact for a given scenario
due to this key assumption of correlated usage (i.e., when the
high-speed hands-in dryer is used at low intensity, the other
dryers are as well, leading the high-speed hands-in dryer to be
consistently favored as it has the shorter dry time (in the case of
drying-driven usage)) and the lower power rating. There are a
few instances, however, when other drying methods have a
lower impact. In scenarios where the HSHU dryer has a high
lifetime usage, the HSHI dryer has a low lifetime usage, and the
use phase takes a low-carbon grid mix, the HSHU system has
the lower impact. In another scenario, the longer HSHI hand
dryer dry times associated with time-based usage in-
crease the frequency at which the other drying systems
will have a lower impact than the HSHI dryer system.
Results from a range of different correlation, use intensity,
and unit process framing assumptions are presented in Table 5
using the comparison indicator metric as the basis for evaluat-
ing the frequency at which one product’s GWP is lower than
that of another. It is particularly important to explore alternative
correlation assumptions because the three hand dryers are
inherently different and may elicit different user behavior.
The six sets of framing assumptions are essentially meta-
scenarios for each set of simulations that explore 20,000 spe-
cific scenarios. (The results in Figs. 2a and 3a were generated
using the meta-scenario 1, whereas the results in Figs. 2b and
3b were generated using the meta-scenario 4. Additionally,
they only represent drying system impact relative to the HSHI
dryer—corresponding to the top four rows in Table 5.)
The results involving the HSHI dryer as a comparator
indicate that removing the correlation between dryer use
intensities increases the frequency at which the GWPs of
the HSHU and HU dryer systems are less than that of the
HSHI dryer system, particularly for time-driven usage. The
comparison indicators for cotton roll towel and paper towel
systems with the HSHI dryer are unaffected by usage corre-
lation. Reconciling the PWB unit process for the HSHU and
the HU dryers also increases the number of Monte Carlo-
generated scenarios that result in their GWPs undercutting
the HSHI dryer GWP; in fact, these lead to comparison
indicator frequencies below the 90 % threshold. Time-
driven usage patterns (which are longer for the HSHI prod-
uct) also decrease the differentiation between the HSHI
product and all other alternatives. In fact, under time-driven
assumptions, the HSHI and HSHU cannot be declared dif-
ferent for a more stringent 95 % threshold (and is only
declared different for one framing assumption (4) for the less
stringent 90 % threshold). For the HSHU system, the use of
Table 4 Results of parameter uncertainty analyses. Numbers in bold are below the 90 % threshold and, therefore, do not meet the prescribed
significance level to be declared different
Drying system A Drying system B Frequency GWP
A
>GWP
B
(F(CI≥1))
1. Baseline assumptions,
measured dry times (%)
2. Baseline assumptions,
reported dry times (%)
3. Consistent PWB unit
process, reported dry
times (%)
High-speed hands-in dryer High-speed hands-
under dryer
0.0 15 35
High-speed hands-in dryer Hands-under dryer 0.0 0.0 0.0
High-speed hands-in dryer Cotton roll towels 0.2 0.2 0.2
High-speed hands-in dryer Paper towels 0.0 0.0 0.0
High-speed hands-under dryer Hands-under dryer 0.0 0.0 0.0
High-speed hands-under dryer Cotton roll towels 13 0.0 0.5
High-speed hands-under dryer Paper towels 1.0 0.0 0.0
Cotton roll towels Hands-under dryer 7.8 10 12
Cotton roll towels Paper towels 1.2 1.2 1.2
Paper towels Hands-under dryer 35 41 46
Int J Life Cycle Assess (2013) 18:1605–1617 1613
the same PWB unit process and time-driven usage assump-
tions impacts the GWP of the alternatives enough so that in
half the Monte Carlo-generated scenarios, its impact is less
than that of the HSHI dryer system. In fact, this comparison
is the only example where significance testing cannot reject
the null hypothesis that the median impacts of these two
products are the same (see Section 5 of the Electronic Sup-
plementary Material). This result is notably different than the
results from the first set of framing assumptions where the
HSHI dryer impact is almost always lower than the HSHU
dryer impact.
The comparison of the HSHU dryer with other dry-
ing systems is consistent with HSHI dryer results: the
HSHU dryer exceeds the 90 % threshold (i.e., can be
declared significantly different) for all sets of framing
assumptions for the cotton roll towels and paper towels
(although it fails a more stringent 95 % threshold for
paper towels). However, it falls below the threshold
(i.e., cannot be declared different) when compared to
the HU dryer in uncorrelated time-driven use intensity
meta-scenarios (numbers 5 and 6 in Table 5). The
cotton roll towels, HU dryer, and paper towels all show
significant overlap in impacts across all sets of framing
assumptions. We cannot say with confidence that any
one of them has a lower impact than another based on
this scenario uncertainty analysis.
4 Discussion
After reviewing the outcomes of the parameter uncertainty
analysis and probabilistic scenario analysis, it is valuable to
assess how much confidence we have in asserting the differ-
ence between product environmental impacts (i.e., the sta-
tistical robustness of the comparison). The deterministic re-
sults in Fig. 1, although temptingly different for the five
products, are clearly insufficient to draw meaningful conclu-
sions because they represent a single scenario and do not take
into account uncertainty in the input data. The outcomes of
the parameter uncertainty analysis listed in Table 4indicate
that for the three scenarios considered, we have statistical
confidence that the high-speed dryers have lower GWPs than
the other drying systems (the slight exception being HSHU
dryer compared to cotton roll towels for measured dry
times), but that the results are inconclusive when comparing
the two high-speed dryers or when comparing paper towels
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0 10203040
Frequency
GWP [g CO2 eq]
(a)
HSHI
HSHU
Cotton Roll
Towels
Paper
Towels
HU
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0 10203040
GWP [g CO2 eq]
High-speed hands-
in dryer
High-speed hands-
under dryer
Hands-under dryer
Cotton roll towels
Paper towels
(b)
Fig. 2 GWP frequency
distributions given (a) drying-
driven and (b) time-driven
usage patterns. Both plots are
based on correlated use
intensities and different printed
wiring boards for the hand
dryers (in accordance with
meta-scenarios 1 and 4,
respectively, from Table 5)
0.00
0.10
0.20
0.30
0.40
0246
Frequency
GWP CI
(a)
HSHU
HU
Cotton Roll
Towels
Paper
Towels
0.00
0.10
0.20
0.30
0.40
0246
GWP CI
High-speed hands-
under dryer
Hands-under dryer
Cotton roll towels
Paper towels
(b)
Fig. 3 GWP comparison
indicator (CI) frequency
distributions (calculated relative
to high-speed hands-in dryer)
given (a) drying-driven and (b)
time-driven usage patterns. The
dashed line indicates a
comparison indicator value of
one as a reference. Both plots
are based on correlated use
intensities and different printed
wiring boards for the hand
dryers (in accordance with
meta-scenarios 1 and 4,
respectively, from Table 5)
1614 Int J Life Cycle Assess (2013) 18:1605–1617
and the hands-under dryer. When we combine this with the
results of the probabilistic scenario analysis in Table 5, the
conclusion that the high-speed dryer impacts are lower than
the impacts of cotton roll towels and paper towels is strength-
ened. However, in two of the six meta-scenarios with time-
driven and uncorrelated use intensity, we cannot say with
confidence that any of the three hand dryer systems have
significantly different impacts because all comparison indi-
cator frequencies are below the 90 % threshold value. Fur-
thermore, we cannot say with confidence that the HU dryer,
cotton roll towels, or paper towels impacts are distinguish-
able from each other based on the fact that all three products
had comparison indicator frequencies below the 90 % thresh-
old across all six meta-scenarios in the scenario uncertainty
analysis when compared with one another.
Based on this information, we can say with a high degree
of statistical confidence that the high-speed dryers have a
lower impact than paper towels and cotton roll towels across
a broad set of scenarios. Differentiating the performance of
the hand dryers compared to one another requires being more
specific about framing assumptions. For drying-driven use
(when consumers use the products as recommended), the
HSHI dryer has a lower impact than the HU dryer in nearly
all cases and a lower impact than the HSHU in the majority
of cases. However, for time-driven use (when consumers are
not as concerned about having equivalently or completely
dry hands), one cannot say that the HSHI dryer is clearly
better than the other dryers. Additionally, we cannot confi-
dently differentiate performance between the HU dryer, cot-
ton roll towels, and paper towels across this scenario space.
This last statement is important to consider. This analysis
does not mean that these products are equivalent in any given
scenario. It only means that when there is considerable
scenario ambiguity and, therefore, the scenario space is
large, it is not possible to reach a single definitive conclu-
sion. If the decision-maker is able to further refine the sce-
nario space (e.g., credibly establish the use location or the
drying habits of the user population), further resolution is
plausible. The case of comparing cotton roll towels and
paper towels is particularly illustrative of this point because
the results of the parameter uncertainty analysis (see Table 4)
indicated that cotton roll towels had a statistically significant
lower impact than paper towels for three specific scenarios.
However, the probabilistic scenario analysis indicated that
there was significant overlap in impacts across all six meta-
scenarios and one could not assert that one product clearly
had a lower impact than the other across a broad scenario
space. This motivates the importance of both going beyond a
typical parameter uncertainty analysis with a few scenarios
to a broader exploration of scenario uncertainty and to al-
ways revisit results with the ultimate decision-maker to
ensure that further information cannot be brought to bear or
that the decision space cannot be further divided.
This work demonstrates the importance of conducting un-
certainty analyses for comparative LCAs that are used for
assertions of relative product environmental impact, as recom-
mended in the several standards. Indeed, there are several
recommendations that are included in the standards and are
supported by this case study. First, broad assertions of relative
impact (e.g., “Product Xhas a lower environmental impact than
Table 5 Probabilistic scenario analysis results for six sets of framing
assumptions. In the descriptions of the six sets of framing assumptions,
“drying-driven”and “time-driven”refer to the ranges of use intensity
times for the hand dryers (see Table 3), “correlated use”and
“uncorrelated use”refer to whether the dryer use intensities are
correlated, and “different PWB”and “same PWB”refer to whether
the unit process used for the printed wiring board in the hand dryers is
the same for all products or different. Numbers in bold are below the
90 % threshold and, therefore, do not meet the prescribed significance
level to be declared different
Drying
system A
Drying
system B
Frequency GWP
A
>GWP
B
(F(CI≥1))
1. Drying-driven,
correlated use,
different PWB
(%)
2. Drying-driven,
uncorrelated use,
different PWB
(%)
3. Drying-driven,
uncorrelated use,
same PWB
(%)
4. Time-driven,
correlated use,
different PWB
(%)
5. Time-driven,
uncorrelated use,
different PWB
(%)
6. Time-driven,
uncorrelated
use, same PWB
(%)
HSHI dryer HSHU dryer 1.3 4.5 14 7.3 37 50
HSHI dryer HU dryer 0.2 0.2 1.2 1.0 19 27
HSHI dryer Cotton roll towels 0.0 0.0 0.0 1.3 1.3 1.3
HSHI dryer Paper towels 0.4 0.5 0.4 6.5 6.2 6.4
HSHU dryer HU dryer 1.0 1.6 0.5 4.1 26 26
HSHU dryer Cotton roll towels 1.1 1.2 0.8 2.7 2.8 1.9
HSHU dryer Paper towels 7.8 7.6 6.4 9.4 9.0 7.8
Cotton roll towels HU dryer 65 65 67 85 85 87
Cotton roll towels Paper towels 59 59 60 59 59 60
Paper towels HU dryer 57 58 59 77 78 79
Int J Life Cycle Assess (2013) 18:1605–1617 1615
product Y.”) should only be made if the claim can be supported
by uncertainty analyses that demonstrate that the claim is robust
across all of these analyses. Second, if the uncertainty analyses
reveal significant overlap in the distribution of impacts associ-
ated with the alternatives, then assertions of relative product
impact need to be stated alongside a clear definition of key
framing assumptions (i.e., those assumptions that change the
relative impact of the products). Finally, given the uncertainty in
calculated environmental impact values and the variation across
equally plausible scenarios, we discourage the use of quantify-
ing relative impact (e.g., “Product Xhas a Z% lower environ-
mental impact than product Y.”) unless it is accompanied by a
specific confidence level (such as those listed in Tables4and 5).
While these broad recommendations in the standards are
useful, there is almost no specific guidance on conducting
uncertainty analyses for comparative LCAs. This case study
has illuminated several issues that should be included in such
guidance. First, although it is useful to aggregate the uncer-
tainty of multiple parameters in the parameter uncertainty
analysis, it will almost always be meaningful to conduct
further uncertainty analyses with specific parameters held
constant as a means of gaining insight on the impact of key
parameters on outcomes. This is analogous to conducting
parameter uncertainty analyses using multiple scenarios (as
was done here and in other work), but there should be
analytical justification for the selection of parameters that
should be analyzed further. An example of this can be seen in
Mattila et al. (2011). Second, the literature discusses a sep-
aration between parameter and scenario uncertainty, but in
reality, there is overlap between the two in the implementa-
tion of uncertainty analyses because many choices (related to
scenario uncertainty) manifest themselves as changes in
parameters. Thus, parameter and scenario uncertainty should
be analyzed together in an aggregate manner where possible
and then analytical methods should be used to determine
which parameters and/or choices should be analyzed further.
Of course, there are some choices that cannot be aggregated,
such as the use of different life cycle impact assessment
methods, and these will still need to be analyzed separately.
As with any LCA, our analysis has several limitations. One
limitation centers on data collection. We received data on the
HSHI dryer directly from a manufacturer (which meant higher
data quality and lower uncertainty), whereas we relied heavily
on data from previously published studies for other hand drying
systems, and in some cases, we did not have strong sources for
key pieces of information (such as observed use intensity of the
hand dryer systems). These challenges are common in compar-
ative LCAs and motivate the need for expanded uncertainty
analyses such as the ones we have presented here. In-depth
uncertainty analyses are the strongest way to understand the
implications of data limitations and other important assumptions
(that are part of all LCAs) on comparative assertions of envi-
ronmental impact. Additionally, due to resource limitations,
uncertainty analyses were only conducted for GWP impacts.
The results presented here should not be generalized to other
impacts without further study. Finally, this paper focuses on
uncertainty analysis among the various systems. A complete
assessment of uncertainty should also include sensitivity analy-
sis to isolate the main drivers of impact (and possibilities for
improvement).
Acknowledgments This work was commissioned and funded by Dyson.
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