How many carbon emissions are saved by doing one less MRI?
Authors: Scott McAlister, Alexandra Barratt, Katy Bell, Forbes McGain
Journal: The Lancet Planetary Health (2024)

Different methods to quantify the carbon impact of health care produce different answers. The top-down method, environmentally extended input output (EEIO) analysis, uses financial data to track money flows of products and services between sectors within an economy.1Mattila TJ Use of input–output analysis in LCA.in: Hauschild MZ Rosenbaum RK Olsen SI Life Cycle Assessment; theory and practice. Springer International Publishing, New York City, NY2017: 349-372Google Scholar Knowing both the direct emissions of each sector, and the indirect emissions from a sector's purchases, a CO2 equivalent (CO2e) emissions per pound spent is calculated. EEIO only requires monetary input data, making this analysis quick and easy to perform. Although EEIO works well enough to calculate the total national impacts of the health-care sector, it can be misleading if used for individual health-care items. The method aggregates all products and services in a sector and relies on a linear relationship between financial cost and carbon emissions. As an example, pharmaceuticals in the UK are assumed to all have an emission factor of 570 kg CO2e per £1000.2Tennison I Roschnik S Ashby B et al.Health care's response to climate change: a carbon footprint assessment of the NHS in England.Lancet Planet Health. 2021; 5: e84-e92Summary Full Text Full Text PDF PubMed Scopus (322) Google Scholar This results in the unlikely finding of lower estimated emissions for generic drugs than their chemically identical originator drugs, or for the same drug being costed differently in different countries.3Godman B Hill A Simoens S et al.Pricing of oral generic cancer medicines in 25 European countries; findings and implications.GaBi J. 2019; 8: 49-70Crossref Google Scholar The other way of calculating emissions is the bottom-up process-based life cycle assessment. Granular data (eg, mass, composition, energy, transport distances, etc) on all processes that make up the whole life cycle of the product are collected and modelled to estimate emissions. The requirement for detailed data collection is more challenging but achieves valid comparisons between products within a sector. Consequential life cycle assessments estimate the consequences of changing demand, and as such reflect the actual impact of a change in clinical practice. A health-care example is the consequence of requesting an MRI scan. MRI machines use large amounts of electricity to cool their magnets, whether or not scans are being performed. The consequence of requesting a scan, however, is only the additional energy to perform the scan above the standby state keeping the machine cool.4McAlister S McGain F Petersen M et al.The carbon footprint of hospital diagnostic imaging in Australia.Lancet Reg Health West Pac. 2022; 24100459PubMed Google Scholar EEIO and consequential life cycle assessments provide very different estimates of the emissions for an MRI scan: an EEIO estimate of 77 kg CO2e versus a consequential life cycle assessment estimate of 1·1 kg CO2e, which equates to a 70-fold difference. Differences are also seen for other types of imaging—all many times higher when estimated by EEIO (table).TableEstimates of carbon emissions per scan for imaging in an Australian settingConsequential energy use (Wh)4McAlister S McGain F Petersen M et al.The carbon footprint of hospital diagnostic imaging in Australia.Lancet Reg Health West Pac. 2022; 24100459PubMed Google ScholarConsequential life cycle assessment CO2e (kg)4McAlister S McGain F Petersen M et al.The carbon footprint of hospital diagnostic imaging in Australia.Lancet Reg Health West Pac. 2022; 24100459PubMed Google ScholarFinancial cost (AU$)5Australian Government Department of Health and Aged CareMedical Benefit Schedule fee.https://www.mbsonline.gov.au/internet/mbsonline/publishing.nsf/Content/4C767FC45843CB02CA25898500825ACA/Φle/PDF%20version%20of%20the%201%20July%202023%20Complete%20MBS.pdfDate: 2023Date accessed: December 18, 2023Google ScholarEnvironmentally extended input output CO2e (kg)6Lenzen M Geschke A Wiedmann T et al.Compliling and using input–output frameworks through collaborative virtual laboratories.Sci Total Environ. 2014; 485–86: 241-251Crossref Scopus (154) Google ScholarX-ray100·58$517·13Ultrasound170·09$11816·75CT5121·09$31945·09MRI24571·14$55077·70CO2e=CO2equivalent. Open table in a new tab CO2e=CO2equivalent. Accurate CO2e estimates are crucial to achieve net zero health care. Users of evidence should review the methods used to ensure emission estimates are fit for purpose. AB is funded by a National Health and Medical Research Council of Australia Centre of Research Excellence Grant (grant number 2008937). FG has patents pending for a reusable N95 mask and the McConty Medihood and has received royalties for the McConty Medihood. SM and KB declare no competing interests.