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Cradle-to-Grave Life-Cycle Assessment of Wooden Pallet Production in the United States 9 Table 4—Selected impact indicators, characterization models, and impact categories Impact indicator Characterization model Impact category Greenhouse gas emissions Calculate total emissions in the reference unit of CO 2 equivalents for CO 2 , methane, and nitrous oxide. Global warming Releases to air decreasing or thinning of ozone layer Calculate the total ozone forming chemicals in the stratosphere including CFCs, a HCFCs, b chlorine, and bromine. Ozone depletion values are measured in the reference unit of CFC equivalents. Ozone depletion Releases to air potentially resulting in acid rain (acidification) Calculate total hydrogen ion (SO 2 ) equivalent for released sulfur oxides, nitrogen oxides, hydrochloric acid, and ammonia. Acidification value of SO 2 mole equivalents is used as a reference unit. Acidification Releases to air potentially resulting in smog Calculate total substances that can be photo- chemically oxidized. Smog forming potential of O 3 is used as a reference unit. Photochemical smog Releases to air potentially resulting in eutrophication of water bodies Calculate total substances that contain available nitrogen or phosphorus. Eutrophication potential of N-eq is used as a reference unit. Eutrophication Extraction of nonrenewable primary resources (fossil) at a rate higher than the replacement rate Calculate the quantities of natural gas, oil, and coal resource consumed at rates greater than nature replenishes them (MJ surplus). Fossil fuel depletion a CFC, chlorofluorocarbons. b HCFC, hydrochlorofluorocarbons. (ISO 2017). ISO 21930 Clause 7.2.2 suggests biogenic carbon entering the product system is considered as removal from the natural environment and is characterized as a negative emission. Biogenic carbon that leaves a system as product, and coproducts, and is emitted directly to the atmosphere when combusted was considered and accounted for as emitted CO 2 . Note 2 of ISO 21930, Section 7.2.11, states that forests are considered as sustainably managed forests if the forest carbon stocks are identified as stable or increasing. Because U.S. forest stock volumes are increasing, U.S. forests are considered to be sustainability managed forests (FAO 2015, Oswalt and others 2019, Hoover and Riddle 2020). 2 Life-Cycle Inventory Analysis The data were collected in line with the data quality requirements addressed by ISO 14044 to ensure quality and reliability. Also to ensure the reliability and accuracy of the data used in the analyses, mass and energy balances were developed from survey data. In addition, sensitivity and scenario analyses were conducted to address completeness, consistency, and uncertainty issues relevant to the data used. The methodology used, the process input and output data, and the LCI generated in this study are provided in detail to allow other LCA practitioners to reproduce the results presented in this study. 2.1 Industry-Wide Data Calculation Rules The facilities reported the number of pallets produced in 2018. Data collected were weight-averaged using the following equation (missing data were not averaged as zeros): 1 weighted 1 2 2 1 2 1 weighted 2 weighted n i i 2 i i 2 i 2 i 2 n i i P x 1 P x 1 i i P x i i P x 1 x 1 2 2 2 2 (2) Where P is the weighted average of the values reported by the facilities, P i is the reported mill value, and x i is the fraction of the mill's value to total production for that specific value. Because the surveyed mill data varied between facilities, a statistical analysis was conducted. In this study, the weighted coefficient of variation (CoV w ) was calculated (NIST ITL 1996). The weighted coefficient of variation (CoV w ) is the weighted standard deviation (sd w ) divided by the weighted mean (P weighted ): 2 1 w 1 sd 3 sd 3 sd 3 sd 3 sd 3 1 sd 3 1 w sd 3 w 1 N i w sd 3 i w sd 3 i sd 3 i sd 3 N sd 3 N sd 3 i i x x i w x x i w N w 1 N w 1 N w N w 1 N w 1 N N w N i N w i i N w i N sd 3 sd 3 N w N w sd 3 sd 3 N w N w x x x x i w x x i w i w x x i w sd 3 sd 3 N w N w (3) w w weighted sd CoV 4 CoV 4 CoV 4 w CoV 4 w w CoV 4 w sd CoV 4 sd P weighted P weighted CoV 4 CoV 4 (4) Where w i is the weight of the ith observation, N is the number of nonzero weights, and x w is the weighted mean of the observations. SimaPro version 9 (PRé Consultants 2019) was used as the accounting program to track all the environmental inputs and outputs crossing the system boundary. 2.2 Primary Data Sources Primary (foreground) data were collected using survey questionnaires sent to wooden pallet manufacturing and recycling–remanufacturing facilities in the United States. All measurable data on raw material and energy inputs, product and coproduct outputs, emissions to water, air, and land, and waste generated were collected for the unit processes included in the current wooden pallet production facilities in the United States. Material and energy flows