Assessing the carbon footprint of clinical trials: a systematic review

BackgroundThe healthcare sector substantially contributes to global greenhouse gas emissions. While being pivotal for improving care, clinical trials involve various activities beyond routine care that contribute to their carbon footprint. We aimed to synthesize current evidence on the carbon footprint of clinical trials and the methodologies used to estimate these emissions. MethodsIn this systematic review, we searched PubMed, Embase, and Cochrane databases for studies published in English until April 16, 2024. Studies focusing on the carbon footprint of clinical trials were included. Abstracts without full-text availability were excluded. Four reviewers independently extracted data, focusing on trial characteristics, carbon emission quantification methodologies, and reported emissions per trial and patient. The risk of bias was assessed using a transparency checklist for carbon footprint calculations. FindingsThe review included 12 studies (6 analytical studies and 6 expert opinions). Total emissions per trial varied widely, ranging from 18 to 2,498 tons CO2eq, with emissions per patient ranging from 25 to 2,452 kg CO2eq. Methodologically, the three most recent studies included nearly all emissions domains with high levels of data completeness, whereas the other three studies considered fewer than half of the emission domains, with medium to low data completeness. Only two studies fully disclosed their conversion factors. Four expert groups agreed on the need to develop standardized estimation tools for prospective use. Experts unanimously called for the involvement of all research stakeholders in raising global awareness of the carbon footprint of clinical trials. InterpretationThe carbon footprint of clinical trials shows substantial variability, primarily due to differences in methodology and the domains of emissions assessed. Addressing these methodological inconsistencies with standardized and openly accessible tools is essential for developing strategies to reduce the environmental impact of clinical research, aligning with broader global efforts to mitigate climate change. FundingNo funding Panel: Research in contextO_ST_ABSEvidence before this studyC_ST_ABSPrior to this review, evidence on the carbon footprint of clinical research was sparse and inconsistent. A few opinion papers had briefly summarized the existing literature, but no formal review had been conducted. With a growing number of analytical publications in recent years - utilizing diverse assessment methods and reporting varying emissions - it became necessary to conduct a systematic review to compare and evaluated these methods and findings. Additionally, key recommendations made by experts, which seemed to align on several points, needed to be formally summarized. Added value of this studyThis is the first systematic review to critically evaluate and compare methodologies for estimating the carbon footprint of clinical trials. Our findings reveal wide variability in reported emissions, influenced by differences in study design, the emission domains assessed, the type of conversion factors used, and the reporting scale (by trial, by patient, or by year of execution). We emphasize the need for standardized, validated tools for consistent prospective carbon footprint assessments and advocate for the engagement of the research community to raise global awareness about this topic. This study lays the groundwork for advancing sustainable clinical research practices. Implications of all the available evidenceThe review highlights the importance of developing and adopting standardized tools for estimating the carbon footprint in clinical trials. These tools should be comprehensive, covering all relevant emission domains, and applied prospectively to support effective mitigation strategies from the start of the trial.