Life cycle assessment of the human energy expenditure in mobility activities

Purpose All activities performed by a person demand an amount of energy expenditure (E.E.) by the human body, but few life cycle assessment (LCA) studies discuss this matter. The analysis of this variable can provide valuable information for consumers to achieve more sustainable lifestyles, including adopting mobility habits with lower environmental impacts. This paper aims to verify the sensitivity of environmental impacts of mobility lifestyles to the inclusion of human E.E. in the LCA. Methods A Consumer LCA was performed considering as the functional unit the mobility of one person to four destinations using one transport alternative for each location for a period of 1 year. The mobility alternatives considered were journeys on foot, by bicycle, car (private and shared), bus, and airplane. A baseline scenario assessed seven midpoint and three endpoint indicators, while the human E.E. was simulated through a double-tiered sensitivity variable evaluated upon a sampled set of mobility lifestyles. Results and discussion Some mobility lifestyles were more sensitive to human E.E., especially when the modes of transport presented low input and output inventory parameters. The inclusion of E.E. increased the impacts by approximately 4-33% on climate change, 6-62% on particulate matter formation, and 1-42% on human toxicity, depending on the mobility lifestyle. Ecosystems were revealed as the most sensitive damage category. Agricultural life cycle stages and processes were the main contributors to the lesser performance of the environmental indicators. In addition, a vegetarian meal as a proxy for energy income presented lower sensitivity and environmental impacts compared to a regular meal. Although cycling and walking were the most sensitive transportation habits, both alternatives presented the lowest environmental impacts in absolute values. Conclusions The proxy variables had a significant influence mainly on the environmental profile of mobility activities with higher bodily energy demand, although lifestyles that prioritize cycling or walking habits remain less impactful than mobility habits based on motor vehicles. The impact categories most influenced by the additional E.E. had higher contribution from agricultural lifecycle stages. This research highlighted the importance of the definition of sensitivity parameters under multiple factors and tiers to observe the possible outcomes for the same product system.