Effects on the environment

Anaesthetic gasses and vapours are known to have an effect on both ozone depletion and climate change.

The stratosphere is the region of the atmosphere from about 10 to 50 km above the Earth’s surface, where ozone plays a vital role absorbing harmful short wavelength ultraviolet radiation from the sun and protecting the earth (Fig. 18.1). Stratospheric ozone is depleted by human-made chemicals including hydrochlorofluorocarbons (e.g. halothane, enflurane, isoflurane) and nitrous oxide.

Figure 18.1 The Earth’s blanket of air reaching approximately 10 000 km above its surface. Approximate temperatures are also shown.

The contribution of these agents to ozone depletion is a function of their lifetimes in the atmosphere, and these lifetimes depend on the reaction of the drugs with hydroxyl radicals in the troposphere¹ (Table 18.1). The relatively short lifetimes of these agents along with their minimal production means they have been seen as relatively ‘ozone friendly’. However, with the reduction of chlorofluorocarbons globally, the influence on ozone depletion by volatile anaesthetics is potentially of increasing importance.⁴

Nitrous oxide, however, has a much longer lifetime (similar to the traditional chlorofluorocarbons, well known for their ozone depleting effects, and which have been successfully reduced since the 1987 Montreal Treaty) and has now been shown to be the single most important ozone-depleting emission. This is expected to remain throughout the 21st century.⁵

The term ‘greenhouse effect’ was first used in the 1800s to describe the naturally occurring function of gasses in the atmosphere to absorb heat. These gasses (mostly water vapour, carbon dioxide, methane, nitrous oxide and fluorocarbons) absorb and then re-emit longer infrared wavelength energy, warming the Earth to 30°C warmer than it would otherwise be and, therefore, enabling life. It is now a term which has negative connotations and is associated with global warming. This is due to the dramatic increase in fossil fuel combustion in the last century and, with this, carbon dioxide production.

Nitrous oxide is an exceptionally potent greenhouse gas with approximately 290 times greater global warming potential than carbon dioxide. Although the amount of nitrous oxide generated from medical sources compared with the total global production is small, its contribution in the light of environmental issues and pressures is still significant and difficult to ignore. It would, therefore, seem prudent to reduce the use of nitrous oxide when there are alternative agents and techniques available, including the more potent volatile agents, and regional and intravenous anaesthetic techniques.

In anaesthesia, the increasing use of single-use devices in the past decade has also contributed to the global picture of increased carbon dioxide emissions. Within the wider context of atmospheric pollution, recycling in healthcare needs to be addressed. Healthcare institutions lag behind other industries on this due to understandable concerns regarding the potential for contamination from the reuse or recycling of biohazardous material. However, the vast quantities of waste generated in healthcare environments that are not currently recycled will become an issue in the future.

Effects on individuals

Chronic exposure to low concentrations of anaesthetic gasses has been associated with adverse health effects. There have been studies and case reports of these effects since the 1960s, although the evidence is sometimes conflicting. Some animal and human studies^6–11 have suggested that as a result of chronic exposure to inhalational agents amongst theatre personnel there is a demonstrable increase in:

In other studies, long-term exposure to nitrous oxide has been shown to result in:

The Health and Safety Commission’s Advisory Committee on Toxic Substances reviewed the literature on the toxic effects of anaesthetic agents in the workplace in 1996.¹² They made the following conclusions based on the data available:

They therefore set maximum exposure limits for volatiles at levels at which no adverse effects were seen in animal studies and thus represent levels at which there is no evidence to suggest the development of adverse effects in humans.

Legislation

Various organizations in different parts of the developed world have introduced recommendations for maximum acceptable levels of pollution to protect staff working in these areas. Due to the rather inconclusive evidence on adverse effects of volatile agents, these limits vary in different countries.

In the USA, for example, several organizations such as the federal Occupational Safety and Health Administration (OSHA), the National Institute of Occupational Safety (NIOSH) and the American Conference of Industrial Hygienists (ACIGH), have this responsibility. In the UK, it is now the responsibility of the Health and Safety Executive (HSE) following its merger in 2008 with the Health and Safety Commission. In Europe it is the Scientific Committee on Occupational Exposure Limits (SCOEL) that sets these limits under the Chemical Agents Directive.

In the UK, it is a legal requirement that employers control industrial and medical pollution. The legislation takes the form of a government approved code of practice entitled ‘Control of Substances Hazardous to Health’ (COSHH).¹³ This was first introduced in 1988, updated in 1994 and amended annually until 2002. There was a further new edition in 2005 (reprinted in 2008) and further amendments made in accordance with the European Commission’s new limits. The HSE’s Advisory Committee on Toxic Substances has drawn up this code of practice under Section 16 of the Health and Safety at Work Act (1974), for the purpose of providing practical guidance on the control of substances hazardous to health in the workplace.

It was in 1996 that COSHH defined the safe maximum exposure limits for a wide variety of substances, including anaesthetic gasses and vapours (EH40/96).¹⁴ Since 2005 ‘workplace exposure limits’ (WELs) have been the defined limits used to protect workers, replacing the previously used ‘maximum exposure limits’ (MELs) and ‘occupational exposure standards’ (OES). WELs are defined at concentrations of hazardous substance in the air, averaged over a specified period of time referred to as a time-weighted average (TWA). An 8 h time period is used.

COSHH recommends that: ‘Exposure should be controlled to a level to which nearly all the population can be exposed day after day without adverse effect on health’. Recommended exposure limits for anaesthetic gasses and vapours in some countries are set out in Table 18.2. In the UK in 2010 no WELs are available yet for sevoflurane and desflurane. Both OSHA and NIOSH recommend a global ceiling limit (concentrations that must never be exceeded during any part of the day) of 2 ppm for all volatile agents, though they have no regulatory authority. As a rough guide, substances with exposure limits below 100 ppm are considered highly toxic by inhalation, those substances with exposure limits of 100–500 ppm are considered moderately toxic by inhalation and those substances with exposure limits greater than 500 ppm are slightly toxic by inhalation.

The WELs for anaesthetic gasses (enflurane, isoflurane, halothane and nitrous oxide) have remained unchanged since 1996.

There are eight principles of good practice for the control of exposure to substances hazardous to health, published by the Health and Safety Executive in 2005.¹⁵ They are as follows:

To properly protect employees, employers have to follow these principles, and by doing so exposure should be below any WEL.

Control of pollution

When no steps are taken to avoid pollution, the exposure limits may be exceeded. One study from a 20 hospital survey reported that the levels of halothane varied between 0.1 and 60 ppm (mean of 2.8 ppm) and for nitrous oxide between 10 and 3000 ppm (mean of 388.5 ppm) when scavenging systems were not used.¹⁶ In the same study the installation of an active scavenging system in one particular hospital reduced the anaesthetist’s exposure to nitrous oxide (and halothane) from a mean value of 411 ppm (and 1.9 ppm) to a mean value of 24.5 ppm (and <0.1 ppm).