Currently used metrics overestimate the environmental effects of aerosol metered dose inhalers

<bold>Introduction</bold> It is commonly stated that metered dose inhalers (MDIs) contribute 3-4% to the global health care carbon footprint. We compared radiative forcing, a metric estimating actual temperature effects, with global warming potential (GWP), a metric commonly used to convey the environmental impact of the hydrofluoroalkanes (HFAs) emitted from MDIs. <bold>Methods and Results</bold> HFC-134a comprises 92% of the aerosols in MDIs and HFC-227ea the other 8%. Both are short lived gases for which GWP is invalid. The estimated annual global manufacture of 800 million MDIs uses ~ 11.5 Gg of HFAs. If all manufactured MDIs were used, they would contribute a maximum of 6% and 15% of annual HFC-134a and HFC-227ea total annual emissions respectively. Assuming that atmospheric composition is proportional to fractional annual emission rates, worst-case estimate of the contribution of inhalers to the total radiative forcing is ~1.2 mWm-2 for HFC-134a and 0.07 mWm-2 for HFC-227ea. These can be compared with the radiative forcing of 2160 mWm-2 from carbon dioxide (CO2) emissions and are thus inconsequential for climate change. Using Global Temperature Potential, a metric more suited to short lived gases, the estimated climate contribution of MDIs in terms of equivalent CO2 emissions is 70-80% lower than current estimates based on GWP. <bold>Conclusions</bold> GWP substantially overestimates the climate effects of MDI propellants. Removing MDIs from medical practice or reformulating with new propellants will have no impact on the climate system. Decreasing the burning of fossil fuels is the most effective strategy to address global warming.