• COPD Pharmacology
• Asthma Pharmacology
• Asthma
• Asthma Guidelines

The BTS/SIGN/NICE have recently published updated guidance for the treatment of asthma.1 The guideline advises that inhaler choice should be based on the lowest environmental impact among suitable devices and provides a hyperlink to a decision aid informing patients about the relative carbon footprint of dry powder versus metered dose aerosol inhalers. The advice is consistent with much of the published literature recommending measures whereby healthcare systems might decrease their impact on climate change and help deliver the UK National Health Service’s net zero carbon strategy.2–4

The purported environmental benefits of switching to dry powdered inhalers conveniently support the need to address the UK’s overuse of aerosolised short-acting beta agonists and the benefits of switching to maintenance and reliever treatment (MART) using a single inhaler that is typically in a dry powder formulation.5–7

While this new guidance is laudable and consistent with current best evidence for asthma treatment, the science of climate change suggests that unfortunately the purported environmental benefits are negligible. A similar situation has arisen within anaesthesia. National guidance altering practice towards the decreased use of anaesthetic gases has been based on an oversimplification of the science of climate change, resulting in perverse responses that risk increasing CO₂ emissions and compromising patient care.8

These problems have occurred because the medical sector, with the best of intentions, is using an idealised greenhouse gas emission metric, known as the Global Warming Potential (GWP) and its related CO₂ equivalence (CO₂e), to infer climate impacts and carbon footprints. CO₂e is calculated as the product of GWP and the emission of a particular gas. The computed CO₂e is then often used to express these emissions in terms of car miles. Consequently, the impact of hydrofluoroalkanes (HFAs) in the healthcare literature is typically represented as the distance travelled in a motor vehicle burning fossil fuels. Illustrating greenhouse gas effects in this way is easily understood by healthcare professionals and patients and has been widely adopted.

Unfortunately, there are major flaws with this approach that make the purported climate impacts, carbon footprints and car miles invalid. Despite its name, GWP says nothing about the potential global warming from a greenhouse gas emission. It was created in 1990 as a simple tool to aid international negotiations and inform multigas climate policies such as the Kyoto Protocol.9 As described, GWP does not take into account the complexity of the climate system and its response to specific gas emissions.8

Of relevance to medical practice, there is an extensive body of evidence in the climate literature, showing that GWP is invalid for short-lived gases with lifetimes <20 years such as the volatile anaesthetic agents and metered dose inhaler (MDI) propellants.10 11 The response of the climate system to these gases is fundamentally different to that for long-lived gases, such as CO₂ and nitrous oxide, that typically survive in the atmosphere for >100 years. GWP was never designed to micromanage the minute emissions of the short-lived gases and grossly overstates their potential climate impact.9 Likewise, the conversion to CO₂e emissions, based on GWP, overinflates their carbon footprints and purported car miles.

Recognising the limitations of assessing climate impacts based on emission metrics, climate scientists advocate radiative forcing (the difference between the energy entering the planet and the energy leaving it) as the fundamental measure of the temperature effects of human activity.12 This metric closely represents actual temperature effects (figure 1). Radiative forcing thus provides a physically robust comparison of the actual contribution of individual gases to global warming and, unlike GWP, inherently takes account of the differing behaviour of short-lived vs long-lived gases.

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Figure 1

Change in effective radiative forcing (ERF) and surface temperature change from 1750 to 2019 by contributing forcing agents. Note the scale of the uncertainty in the radiative forcing and surface temperature change. CH₄, Methane; N₂0, Nitrous Oxide. Reproduced from figures 7.6 and 7.7 in chapter 7: The physical science basis. Contribution of working group I to the sixth assessment report of the intergovernmental panel on climate change, doi: 10.1017/9781009157896.008.

Climate science is clear that the radiative forcing and warming from CO₂ will continuously increase as ongoing emissions accumulate decade by decade. However, for short-lived HFAs, the radiative forcing does not increase inexorably because emissions cannot accumulate in the atmosphere beyond the lifetime of the gas.11 Expressing HFA emissions in terms of CO₂e misrepresents their short-lived impact on global temperature and grossly overstates their potential long-term climate impact.10 Every kilogramme of CO₂ we emit accumulates in the atmosphere through our lifetime and irreversibly changes the climate for generations to come, whereas the effect of every kilogramme of HFA is lost within a decade or so. It is important to note that it is the emission rate of HFAs that matters. If MDI usage continued at its current rate, for example, no further warming, which is already minute, would take place. In contrast, there is no safe level of CO₂ emissions.

The two main gases used as propellants in MDIs are HFC-134a and HFC-227ea.13 Neither is an ozone-depleting substance, but both are active greenhouse gases. HFC-134a is the most abundant HFA and the most used in MDIs. It has a high GWP of around 1470, but its lifetime in the atmosphere is approximately 14 years. Like the anaesthetic gases, this makes it a short-lived gas. HFC-227ea is also used in MDIs. It has a longer lifetime of 36 years and a GWP of 3580.

HFC-134a emissions and atmospheric abundance are increasing. Its global annual mean mole fraction reached 113 ppt in 2020, up from 89 ppt in 2016.14 The abundance of HFC-227ea is much lower at 1.7 ppt. Thus, in 2020, HFC-134a from all emission sources contributed a global radiative forcing of 19.5 mW/m² and HFC-227ea contributed 0.46 mW/m². To put these numbers in context, the radiative forcing by CO₂ is estimated at 2160 mW/m² (figure 1).

The HFAs used in MDIs are only a small proportion of all the sources of emissions. HFC-134a is widely used as a refrigerant in mobile and domestic refrigerators and freezers, a blend component for stationary air-conditioning and commercial refrigeration, a foam-blowing agent and a propellant for industrial aerosols. HFC-227ea is widely used as a firefighting agent.

Estimates of the global annual emissions from MDIs are difficult to find. The European Fluorocarbons Technical Committee reported that in 2018 the global manufacture of 800 million MDIs used around 11 500 tonnes (11.5 Gg) of HFAs of which HFC-134a constituted 92%.15 Global emission estimates of HFC-134a from all sources are around 160 Gg/year. If all these MDIs were used, they would contribute no more than 6% of annual HFC-134a total emissions. Similarly, HFC-227ea containing inhalers would contribute no more than 15% of the total annual emissions of 6.3 Gg/year.

Assuming that the atmospheric composition is proportional to the fractional emission rate from inhalers, and that all the manufactured inhalers were used in the same year, then a ballpark, worst-case estimate of the contribution of inhalers to the total radiative forcing by HFC-134a of 19.5 mW/m² is ~1.2 mW/m². Using the same logic for HFC-227ea gives a worst-case estimate of 0.07 mW/m². This means that the worst-case estimate of radiative forcing from worldwide MDIs is ~1.27 mW/m². When compared with the estimated CO₂ radiative forcing of 2160±250 mW/m², any contribution from MDI propellants is way down in the noise. Like anaesthetic gases, the climate effects of medical MDIs are extremely small and inconsequential8. Removing MDIs from medical practice will have no impact on the climate system and the trajectory of global warming.

Simple emission metrics have become popular because of their ease of use but clearly have limitations. All emission metrics are gross oversimplifications of the climate response to greenhouse gases, but the Global Temperature Potential (GTP) metric has several advantages over GWP. GTP attempts to account for fundamental aspects of the climate response to radiative forcing including the climate sensitivity and the thermal inertia of the oceans10. However, it is an endpoint metric based on temperature change at the end of a specified time period (ie, 100 years), and so assumptions are made about future emission trajectories. Like GWP, GTP can be converted to CO₂e. Both metrics are reported in the WMO/UNEP Scientific Assessment of Ozone Depletion.14

A comparison of GWP and GTP (table 1) highlights the extent to which the use of GWP to derive CO₂e substantially overestimates the climate impact and carbon footprints of short-lived gases such as methane and MDI propellants. If GTP were used instead of GWP, CO₂e estimates as quoted by NICE would be reduced by as much as 80%. Using this more appropriate metric significantly impacts the extent to which carbon footprints might be reduced by removing MDIs in clinical practice but is still a substantial oversimplification of the complexity of the climate system.

Contrary to the suggestion that we might achieve ‘quick wins’ by switching to ‘low carbon’ inhalers, the minuscule radiative forcing and short lifetimes of the HFAs in medical aerosols make them irrelevant to the trajectory of climate change.16 Similarly, the switch to more ‘climate-friendly’ propellants is unlikely to result in significant climate benefits. Instead, the cost to the environment of direct CO₂ emissions from stopping existing MDI manufacturing, essential new drug development research activity and the introduction of new manufacturing processes will take decades to offset. Lengthy safety assessments, delayed availability in the marketplace and new patents will more than likely result in increased costs to the healthcare economy and disproportionately affect children.17

Although it has been suggested that switching to dry powder equivalents from aerosols might achieve financial savings to the UK National Health Service, this analysis did not consider the shift to MART regimes as currently recommended.18 Following NICE/BTS/SIGN guidance is likely to result in cost savings but this will be due to decreases in the need for hospital treatment rather than the use of greener inhalers.19

If there are no detrimental effects on patient care and no unnecessary costs, there are good clinical arguments to switch to dry powder inhalers when possible, although this is not an option for preschool children. Arguments related to carbon footprints and climate impacts of MDIs should be corrected to ensure that they accurately represent the underpinning climate science. Over-emphasising the green credentials of best practice risks being a distraction from the many other more significant ways in which healthcare professionals can reduce their damaging effects on the environment. As previously highlighted in this journal, making patients feel compelled to change their inhalers might also stigmatise or unsettle individuals who are already highly vulnerable.20

A better understanding of the science underpinning the relative importance of different greenhouse gases to global warming has important implications for how healthcare practitioners might approach choice of inhaler device, the advice from policymakers about best practice, and the development of new drug delivery devices by the pharmaceutical industry.21 Healthcare professionals who want to meaningfully reduce their greenhouse gas emissions, especially CO₂, might usefully offer telephone consultations when safe to do so, rigorously separate medical waste from household-type waste to avoid unnecessary incineration, minimise the use of nebulisers, avoid unnecessary follow-up, unnecessary duplicate investigations and administrative inefficiencies, switch off air conditioning, avoid air travel, walk or cycle whenever possible and share necessary car trips with colleagues. Changing practice to decrease the burning of fossil fuels is the most effective strategy that healthcare professionals can use to minimise their contribution to global warming.