Background The global health sector is responsible for approximately 5% of global net carbon emissions and is therefore a major contributor to climate change. Reducing unnecessary tests and treatments that have a large carbon footprint may be low hanging fruit but there is a paucity of evidence about how best to reduce the carbon footprint of clinical activity.

Objective To synthesise the available evidence on the effects of interventions implemented by clinicians or healthcare services designed to reduce carbon emissions of health care.

Search methods We searched CENTRAL, MEDLINE, Embase, Clinicaltrials.gov and WHO International Clinical Trials Registry Platform from inception to 16th December 2021 with no language restriction. We also searched reference lists of included studies and relevant reviews, undertook citation tracking and contacted experts in the field.

Selection criteria We included randomised, pseudorandomised and non-randomised controlled trials, interrupted time series and controlled or uncontrolled before-after studies that evaluated interventions designed to reduce carbon emissions of clinical activity within any healthcare setting. We excluded studies that evaluated interventions designed primarily to reduce waste, water, energy use and/or pollutants. Our primary outcome was carbon or greenhouse gas emissions. Secondary outcomes included patient-relevant outcomes, effectiveness in terms of individual study aim (e.g. number of anaesthetic bottles consumed), adverse effects, costs, engagement, and acceptability.

Data collection and analysis Two reviewers independently selected studies for inclusion, extracted data, assessed risk of bias using a checklist for observational studies adapted by a Cochrane group and the certainty of evidence using GRADE. The primary comparison was any intervention compared to no intervention. Vote counting based on direction of effect was used to synthesise results because of inconsistency in the effect measures reported across studies.

Results 12 uncontrolled before-after studies met our inclusion criteria. Interventions targeted anaesthesia (n=5), unnecessary test requests (n=3), waste (n=3) and energy (n=1). Clinician education (n=10) and environmental restructuring (n=7) were the most frequent interventions. Most (n=11) studies were susceptible to bias: in particular, confounding (58%), generalisability (83%) and outcome reporting (75%) biases. Only one-quarter of studies (n=3) used the gold standard of life cycle assessment to measure carbon emissions. Although most studies reported effect estimates favouring the intervention (carbon emissions 9/10; costs 7/9; adverse effects 1/1; and effectiveness 10/12 studies), we are uncertain whether interventions designed to decarbonise clinical activity have any effect because the certainty of the evidence is very low. We did not identify any studies that measured intervention engagement or acceptability.

Conclusions We are uncertain whether interventions of any type designed to reduce carbon emissions of clinical activity compared to no intervention are effective. Rigorous studies using life cycle assessment are needed to determine their true effects. Targeting low value care that has a large carbon footprint should be prioritised with the dual aim of both reducing iatrogenic harms and addressing the environmental sustainability of health care.