Ethylene Oxide – the future of gas sterilisation
15 September 2009
So far, 99% of the global use of EtO has been in the manufacture of Ethylene Glycol, an intermediate in the production of anti-freeze and Polyester.
However, Ethylene Oxide (EtO) has been used as a sterilant for nearly eighty years now and has stood the test of time as a very effective sterilant, being good both at killing a wide range of pathogens and at sterilising the most complex shapes. It continues to be a major technology for the sterilisation of medical devices world wide. EtO is very good at reaching all parts of complicated shapes. For instance, a 1 metre stainless steel tube, with 2mm lumen open only at one end can be readily sterilised with EtO. EtO has no effect on the item being sterilised, so designers are not restricted in the plastics they can use, as would be the case when sterilising with gamma irradiation.
The successful adoption of EtO technology has been driven by its low toxic threat, low cost, high effectiveness and low environmental impact.
Toxicity:
The only statistically significant epidemiological study on the carcinogenicity of EtO was conducted by NIOSH (National Institute Of Safety & Health in the USA), Mortality Among Workers Exposed To Ethylene Oxide – Steenland 1991. This study was updated in 2003 and covered 18,235 men and women who had worked at 14 plants (belonging to 10 different companies) from the early 1940’s through to the 1980’s. In 1985 OSHA lowered its 8hr TWA from 50ppm to 1ppm, meaning that this study cannot be repeated or extended. Workers in the study were exposed to ten’s of ppm EtO, for the duration of every working day, for tens of years.
The study compared the death rates and causes of death in the general U.S. population with those in the study. The study found that overall the occurrence of cancer in the study population (that had been exposed to EtO) was LOWER than in the general population. The study also found that there was a trend of increased cancer incidence with increasing years of exposure. With less than ten years exposure the incidence of cancer in the study population was 77% of that in the general population. This rose to 91% for those exposed for 10 - 20 years, and reached 103% for those exposed for more than 20 years. It is this TREND that leads NIOSH to classify ETO as a “potential human carcinogen”.
There have been other studies using laboratory animals. However the levels and modes of exposure in these studies bear no resemblance to any human activity. Consequently OSHA state that EtO “has been shown to cause cancer in laboratory animals”.
When you understand the health risk presented by EtO, in the context of a sterilisation facility where the operator is not going to be exposed to any EtO, you do wonder whether the Europeans and Japanese are missing out on something.
Cost:
There are two aspects to this. One is the straightforward cycle cost, either in house or at a sub-contractor. The second is the cost of the time a device has to be out of commission before it can be used in the next procedure.
Though it is not possible to give meaningful global figures on costs, EtO is cheaper than alternative technologies and cycles are now as short as 3 hours.
Contract sterilisation cycles for small lots (63 litre chamber) in the UK will typically be around £100 - £150 and for traditional large chambers the prices will be even lower. The large chambers are only available to hospitals through contract sterilisers, whose business is predominantly in sterilising single-use devices. Hospitals will simply not do enough re-sterilisation to justify bringing this technology in house. Typically items will be loaded on pallets and many pallets at a time go into the one chamber. Whilst this is cost effective, cycles are slow, typically two days, and the transport time tends to mean that hospitals have to allow two weeks between uses of each device. That can mean purchases of multiple copies of expensive devices, to support the frequency with which the relevant procedures are performed.
The technology is now available to surgeons and HSDU managers that allows EtO to be brought in house. Cabinets conforming to HBN 13 Supplement 2 and delivering the 10-6 SAL (Sterility Assurance Level) required by BS EN 556-1:2001 are available for around £10k in the UK. The consumable cost is around £12 per cycle. These cabinets will allow the surgeon to re-use a device after only some four hours.
The challenge for EtO as a technology has also been the requirement for aeration before devices could be used on a patient, as much as the actual cycle time itself. It is a feature of EtO that it is absorbed by many materials, particularly soft plastic, and this may need to elute before a device can be re-used. However, the cabinets delivering a 3 hour cycle run at 50ºC. Items can be left in the cabinet for a further hour to drive off residual EtO. This highly efficient steriliser gives users the ability to use the most effective sterilant and reuse their delicate and expensive equipment again the same day.
Effectiveness:
There is also a debate that has developed over the best way to handle devices that have historically been cold soaked. There has long been an acceptance that this technology did not sterilise devices. The process was never believed to kill all the micro-organisms on a device, and the term disinfection, or decontamination, is widely used to make this clear. So, much work has gone into improving the performance of soaking technologies, both in terms of the level of disinfection achieved and in terms of the damage caused to the items being treated. The result has been Automated Endoscope Reprocessors (AER’s) that achieve consistently high levels of disinfection with little or no damage to the scope.
However two problems do remain. First, since immediately after disinfection devices are rinsed to remove the disinfectant, there are stringent requirements on the water used for rinsing, with the costs of testing and control that go with that. Second, the items being soaked are not in sterile packaging. This means that as soon as they come out of the disinfection solution they begin to pick up bioburden. The British Society for Gastroenterology (BSG) has recommended that “All endoscopes must have been exposed to a full decontamination cycle not more than 3 hours prior to use”. This poses significant planning and operational challenges, whilst not delivering a sterile device to the surgeon.
The BSG has also recommended that “Endoscopy should be avoided whenever possible in patients with suspected or confirmed vCJD”. Clearly, it can be very difficult to know if a patient has vCJD, since we do not have a practical way to test for the presence of the causative agent. In a similar way there are problems with re-sterilising devices if patients have aids or hepatitis. This would involve screening all patients prior to operation and then having an established route for handling devices should a patient test positive.
It should be made clear that the difficulties over a system for testing the presence of vCJD do make it difficult to know whether any technology with deactivate the infectious agent. However, the mode of action of EtO through chemically modifying the infectious agent does seem make this one of the more promising options.
In all cases where disinfection is currently accepted, EtO stands ready to deliver true sterilisation (10-6 SAL).
Environment:
EtO is certainly a toxic gas. It is a sterilant after all. So there are maximum allowable levels of EtO in the workplace. In the USA the 8 hour TWA (time weighted average) allowed is 1ppm. In the UK HSE has set the WEL (workplace exposure limit) at 5ppm. This compares with 1ppm for Hydrogen Peroxide and 0.05ppm for Glutaraldehyde. The 5ppm limit in the UK and the 1ppm limit in the USA are easily met with the current generation of small ETO cabinets, which use as little as 5ml EtO per cycle.
The waste ETO is pumped to the outside where there are virtually no restrictions. In the USA there are no federal restrictions provided annual emissions are less than 2,000 lbs. Small in house cabinets will emit nowhere near this level of EtO, though large traditional chambers used by contract sterilisation companies will have to employ abatment technology. There are local regulations in some districts in the USA, which require more than 98% of the EtO used each cycle to be absorbed rather than emitted to atmosphere. This applies regardless of the amount of EtO used per cycle. So the 2% emitted from large chambers will exceed the total usage per cycle in small chambers. However, it has driven the development of technology for small abators that has found a use outside these specific districts in the USA.
Customers may choose to adopt EtO abator technology either because they wish to be seen to be using the best available technology, or for more practical reasons. In some establishments the EtO steriliser is not situated close to a suitable outside wall. Here it can be convenient to release the exhaust gas into the common venting system to be ducted up for emission from the roof. In this case absorbing more than 98% of the EtO in the exhaust, before releasing it into the common ducting system, ensures that any escape on the way to the roof cannot generate levels of EtO in the workplace that pose any risk.
The freedom to emit EtO in the UK arises because it is not a greenhouse gas (Kyoto Protocol), it is not a volatile organic compound (VOC, Geneva Protocol), and it is not an Ozone depleter (Montreal Protocol). Also EtO is not covered by the Pollution Prevention and Control Act 1996, which is the UK implementation of the EU Council Directive 96/61/EC. This makes sense because EtO is rapidly absorbed by water in the environment and degrades either by conversion to ethylene glycol or by alkylating available organic material.
In short by virtue of the ability of very small amount s of EtO to achieve high levels of sterility and then to degrade in the environment, EtO is an environmentally friendly, green product.
Adoption:
The adoption of EtO for re-sterilisation is driven principally by cost. There are two scenarios that allow hospitals to save money using EtO.
First they may be able to reduce the number of expensive items, typically endoscopes that a hospital needs to buy. There are very limited choices available to hospitals over what technology to use to sterilise endoscopes. This is a pressure that applies particularly to hospitals in more affluent countries, such a Europe and North America. The difference in uptake between these two regions may be partially explained by the greater level of state funding in Europe and hence the greater cost pressure on hospitals and clinics in North America.
Second, in less regulated environments, hospitals may reuse devices that are accepted as being single use in developed countries. Much as surgeons may bemoan the cost of items that are thrown away after use, including items that are opened but not used, there is little they can do in a country such as the UK if it has been purchased with the single use symbol on it. Whilst the gains in this area are potentially huge, it will take some pressure from purchasers to motivate suppliers to supply multiple use versions of items that are currently single use. Clearly this will have to be done intelligently, as few would countenance being a patient in a procedure with a second hand cardiac catheter. However, the re-use of items such as a staple gun does seem to be a project whose time may be arriving, with the pressure that state funding is going to be under during the coming years.
So, Ethylene Oxide has the effectiveness and cost competitiveness to address a range of operational issues, as well as the green credentials to satisfy the modern marketplace. No wonder it’s global fan base seems set to continue growing.
