Anaesthetic Gases & Inhalers
To understand how anaesthetic gases and inhalers contribute to the climate crisis lets look again at greenhouse gases.
Greenhouse gases (GHGs) make the earth warmer by trapping energy inside the atmosphere (Gadani et al, 2011). GHGs include water vapour, carbon dioxide, methane, nitrous oxide, chlorofluorocarbons (CFCs), halogenated fluorocarbons (HCFCs), ozone (O3), hydrofluorocarbons (HCFs) and perfluorinated carbons (PFCs).
Table 1: The main GHGs and their main global sources (Adapted from US EPA, 2020).
The effect of a greenhouse gas on climate change depends on three main factors:
How much is in the atmosphere
Concentration measured in parts per million or billion.
How long it stays in the atmosphere
Each gas can stay in the atmosphere for different amounts of time ranging from a few years to thousands of years.All the gases remain in the atmosphere long enough to become mixed so that the amount measured in the atmosphere is roughly equal throughout the world regardless of the emission source.
How strongly it impacts the atmosphere
Some gases are more effective than others at warming the planet. A Global Warming Potential (GWP) has been calculated for each gas to reflect how long on average it lasts in the atmosphere and how strongly it absorbs energy.Stronger absorption leads to a higher GWP and therefore contributes to more warming.
Radiative Efficiency, Atmospheric Lifetime and Global Warming Potentials (GWP)
GHGs trap heat in the atmosphere by absorbing energy and slowing the rate it can escape to space. GHGs differ from each other in their ability to absorb energy (radiative efficiency) and how long they stay in the atmosphere (atmospheric lifetime).
Radiative efficiency is the change in solar energy irradiance on the earth’s atmosphere that occurs with a change in the concentration of a particular compound and is usually given in parts per billion (McGain et al, 2020).
GWP provides a common unit of measurement, allowing comparison of the global warming impact of gases. It is a measure of how much energy the emissions of one ton of gas will absorb over a given period of time, relative to the emissions of one ton of carbon dioxide. The time period is normally taken as 100 years (GWP100), which is the standard used by the Intergovernmental Panel on Climate Change (IPCC).
Table Two: The GWP 100 for the main GHGs.
Anaesthetic gases include the hydrofluorocarbons sevoflurane and desflurane, the chlorofluorocarbon isoflurane, and nitrous oxide. The NHS estimates that 5% of the total carbon footprint of an acute NHS trust is attributable to anaesthetic gases, with anaesthetic nitrous oxide contributing to 1-3% of all global nitrous emissions (McGain et al, 2020).
Atmospheric Lifetime of Anaesthetics Gases
The atmospheric lifetime of a molecule depends upon how quickly it is metabolised by the OH- radical (McGain et al, 2020). The bond strength of carbon-fluorine (C-F) is greater than that of C-H, C-Cl and C-Br. Therefore, atmospheric OH- radicals less easily displace F atoms and consequently sevoflurane, isoflurane, and desflurane have differing atmospheric lifetimes of 1, 3, and 14 years respectively. Sevoflurane has a shorter atmospheric lifetime than desflurane as OH- radicals react more readily with its carbon atom attached directly to two carbon atoms and a third carbon atom via the ether group (McGain et al, 2020).
GWP100 of Anaesthetic Gases
Volatile anaesthetic GWP variability is predominantly caused by differences in atmospheric lifetime rather than radiative efficiency (McGain et al, 2020). Table Three summarises the GWP100 of modern anaesthetic gases. As shown, per mass, desflurane has a GWP100 approximately 20 times that of sevoflurane, 5 times that of isoflurane, and 10 times that of nitrous oxide (McGain et al, 2020).
Table Three: GWP100 of Anaesthetic Gases:
Ozone depleting substances are those that produce ozone depleting radicals. Atmospheric chlorine and bromine atoms form ozone depleting radicals, and therefore halothane has an ozone-depleting potential (ODP), however this anaesthetic gas is becoming obsolete (McGain et al, 2020). Nitrous oxide also has an ODP. In contrast, fluorine does no destroy ozone and therefore sevoflurane and desflurane have no ODP (McGain et al, 2020). As the Montreal Protocol 1987 limited the release of most ODPs, at present anthropogenic nitrous oxide generation is responsible for most ongoing ozone depletion. As a result, the global anaesthetic contribution of nitrous oxide should not be ignored (McGain et al, 2020).
Carbon Dioxide Equivalents (CO2e) and Anaesthesia
This is a quantity that describes, for a given mixture and amount of GHG, the amount of CO2 that would have the same GWP when measured over a specified timescale (normally 100 years) (Campbell et al, 2014).
To help put the CO2e emission from the use of anaesthetic gases we can now convert this to distance driven in a car per hour (km).
So in summary, if its blue its bad for the environment. Isoflurane and Sevoflurane are not much better, and it essential that we practice low flow anaesthesia and look to alternative techniques such as total intravenous anaesthesia (TIVA) and regional anaesthesia if we are going to reduce our carbon footprint.
Click here to learn more about reducing your carbon footprint
Metered Dose Inhalers
Now this isn't just anaesthetists trying to shift the blame here (I promise), but metered dose inhalers have a significant impact contributing to 3.9% of the carbon footprint of the NHS (SDU 2018). This is as the drug delivery (eg Salbutamol/Budesonide) occurs thanks to propellants hydrofluoroalkane (HFA) tetrafluoroethane (HFA134a) or heptafluoropropane (HFA227ea), which are incredibly potent greenhouse gases. Lets have a look at them a bit closer.
Table 4: GWP 100 of common metered dose inhaler propellants. Modified from (Wilkinson er al 2019).
So at best current MDIs are 1300 times more warming than CO2 over a period of 100 years. Some inhalers contain more propellant than others. All salbutamol MDIs use HFA134a, but one (Salamol) contains less due to the presence of an alcohol co-solvent. The difference in carbon footprint between the small volume (Salamol) and the large volume inhaler (ventolin) is 18kgCO2e (Wilkinson er al 2019).
"Surely there must be another way!"
Well yes their is (and please stop calling me Shirley.) Dry Powder Inhalers (DPIs) have a carbon footprint ranging from 1.5-6kg CO2e / 200 doses, compared to MDIs (8-36kg CO2e), while aqueous mist inhalers like the Respimat have a carbon footprint of 780gCO2e (Wilkinson er al 2019).
8-36 kg CO2e
1.5-6 kg CO2e
Aqueous Mist Inhaler
MDIs have other benefits too as they have a dose counter, and systematic review data shows their to be less user error when compared to MDIs (even with spacers) (Sanchis et al, 2016). If MDIs are used (when clinical reasons takes precedent), there impact can be less if they are recycled or incinerated thus recapturing or destroying excess HFA.
Table 5: Strategies to reduce the carbon footprint from inhalers. Modified from Wilkinson er al 2019.