The Future of Ethylene Oxide Sterilization
By Stephen Conviser
- Reviews EO sterilization basics.
- Discusses how EO sterilization has improved.
- Introduces a new EO/HFC sterilant gas blend.
Ten years ago, Central Service Managers had two choices for sterilizing steam-sensitive medical devices: 12-88, a non-flammable mix of EO in CFC-12, and pure, flammable EO. As EO/CFC-12 was phased out by the EPA (CFC-12 is implicated in depleting stratospheric ozone), alternative low-temperature sterilizers were introduced to the CS Manager. Today, the CS Manager must decide how best to use three different types of low-temperature gas sterilizers: 1) pure EO (provided by STERIS and 3M Health Care); 2) non-flammable EO/HCFC blends that replaced 12-88 (sold under the brand names of Penn Gas 2 and Oxyfume, used in sterilizers provided by Getinge/Castle and STERIS); and 3) vapor phase hydrogen peroxide plasma (sold under the brand name STERRAD®).
Many CS Managers plan to use EO into the indefinite future and several plan to use this workhorse sterilant more. This article will discuss why so many managers feel they need EO and highlights two EO sterilizer changes that will help CS Managers.
EO is a basic chemical. People use 17 billion pounds per year worldwide. Some of the products made from EO include textiles, detergents, and anti-freeze. Nearly half of all medical devices sterilized by device manufacturers are sterilized by EO. The other half is sterilized by radiation. The uses of EO continue to grow with the economy.
The average hospital has always used less than 500 pounds per year of EO sterilant (5,000 pounds per year of gas blends). But, in spite of small volume use, hospital reprocessors have always considered EO to be vital. Answering some key questions about EO explains its singular role and predicts its direction for future use.
How Does EO Sterilize?
EO sterilizes by alkylation. EO substitutes for hydrogen atoms on molecules needed to sustain life, and, by attaching to these molecules, EO stops these molecules' normal life-supporting functions. Some of the key molecules that EO disrupts are proteins and DNA. Under low-temperature sterilizing conditions, so much EO is used that this disruption proves lethal to microbial life.
The other low-temperature gas sterilizing process, plasma, uses a super-oxidizer, hydrogen peroxide. Super-oxidizers destroy life-supporting molecules. Not much is needed to be lethal. But, super-oxidizers react with many other molecules in the medical devices and packaging; hence, to avoid damage to the device, they are not used with certain sensitive materials and are used only in small amounts with other materials.
Factors That Make an EO Sterilizer Different
Efficacy--The ability to kill microbes in difficult to reach places.
The Sterility Assurance Level [SAL] for EO sterilizers is 10-6. This means that after sterilization, there is a one in a million chance that a live microbe is in the sterilized load. EO sterilizers are validated using an "overkill" procedure. Sterilizer manufacturers measure how long it takes to reach an SAL of 10-6, then we expose the load for twice that time.1
To prove efficacy, it is not enough to know how long it takes EO to kill microbes but also how long it takes to penetrate the types of barriers that a typical sterilizer load may contain. For this reason, all EO efficacy testing is done in barrier packs. An example is the AAMI challenge pack, in which two biological indicators (BIs) each contained in a syringe are placed in the middle of 12 layers of cotton towels, wrapped in two wrappers. Also, to absorb and divert EO from the syringes, the pack contains a plastic airway and 10" of latex tubing.2 Often for day-to-day operations, the hospital uses simpler challenges such as BIs in syringes.
Residues--The Need to Aerate
After exposure to EO, the medical devices and packaging contain trace amounts of EO. For patient and worker safety, operators need to aerate the devices until the EO residues are reduced to safe levels.
Long Cycle Time
Because operators need time to ensure good efficacy and to aerate the devices, the EO cycle can be as long as 15 hours, but improved cycles reduce this time.
Best Possible Material Compatibility and Penetrability|
EO at sterilizing temperatures has shown that it kills microbes in hard-to-reach and hard-to-clean spots, and it does so with no damage to devices.
Health and Safety
The hospital must manage the toxic and reactive hazards of sterilizing chemicals. Chemicals that kill microbes can harm people.
- OSHA 29 CFR 1910.134 Respiratory Protection
- OSHA 29 CFR 1910.1200 Hazardous Communications
- NIOSH Pocket Guide to Chemical Hazards (guideline)
- ACGIH Threshold Limit Values for Chemical Substances (guideline)
Ethylene oxide users comply with a special EO Rule (29 CFR 1910.1047). Administered by OSHA since 1984, it references rules for other chemicals but provides more comprehensive requirements for risk management.3
Manage EO Flammability
EO burns easily. The hospital may use pure EO (3M Health Care and STERIS 3017) or a non-flammable blend of EO, usually in fluorocarbons (Oxyfume 2002 and Penn Gas 2). Pure EO must be used in small chambers 4 to 8 cubic feet, must be operated under vacuum, and can only be supplied in small cartridges (about 100 grams).
Non-flammable blends may be used in large chambers and supplied in large cylinders. About 70% of all EO used by hospitals is used as non-flammable blends. Significant changes are underway in blend sterilizers.
Why Hospitals Choose EO for Low-Temperature Sterilization
First, they can get the best possible SAL with EO. Side-by-side studies (EO blends vs. pure EO vs. hydrogen peroxide plasma) show higher safety margins offered by EO with the best SALs achieved by the EO blend sterilizers.4-5 EO sterilizers use an overkill cycle to achieve an SAL of 10-6, which operators confirm by a BI challenge in a barrier pack in every sterilizer load.
Secondly, those who use the blend sterilizers rely on a "workhorse" process where there are almost no material limits, almost no device configuration limits, no size limits, high load limits (for chamber sizes up to 70 cubic feet), and for large chambers, the highest per cycle through-put with the greatest variety of devices.
- Non-critical devices that need only cleaning and disinfection. SAL not as low as 10-6.
- Devices that can be steam sterilized.
- If an alternate low-temperature sterilizer is onsite, those devices that can be safely and consistently sterilized to a SAL of 10-6.
- Reprocessable devices that can be replaced often by single-use devices if such replacement is economic and consistent with the hospital's policy on waste disposal.
- Improvements in EO Blend Sterilization
EO blend sterilizers are being made better in two ways: improved cycles and a new sterilant blend that meets the next century's regulatory needs. Improved cycles are being introduced now in the US, and the new sterilant blend is now being introduced in Europe and Canada.
The hospital community has long felt that EO blend cycles could be improved. But, with a heavy focus on complying with the OSHA EO Rule over the last 15 years, few resources have been available to improve the sterilizer as a sterilizer. Indeed, two obvious improvements can readily be made: the sterilizer can use less gas and operate with shorter cycles. Equipment change would be minimal, and the new cycles can be validated in a very straightforward manner.
Gas use can be reduced. In fact, medical device makers use less gas per cycle than in the past. Lower gas use has been validated for EO/CO2 sterilizers. Controlled laboratory studies show how to use less gas. Finally, less gas use has been validated in hospital sterilizers using the current EO/HCFC blends.
- Exposure temperatures were set at ~ 135ºF (versus current use of 120ºF to 130ºF).
- Exposure times were increased from two to three hours.
- Validation, as needed, usually by independent service provider, using AAMI ST24 and ST41.
Following these steps, gas use per cycle was reduced 25%. If we are to reduce cycle time, we must reduce aeration time. It accounts for over 80% of the total time to operate a blend sterilizer. From what medical device manufacturers have learned about improved aeration and from what is known about gas processes, we know of three ways to improve aeration.
Raise aeration temperature. For every 18º F increase in temperature, aeration time is reduced about 50%. To avoid device damage, aeration temperatures should be less than 140º F.
Pulse continuously from vacuum to atmospheric (approximately) pressure and back. This helps sweep EO from the chamber. The rate of pulsing will be limited by equipment capability. No pulses have been reported at times less than 15 minutes, time from atmospheric pressure to vacuum back to atmospheric pressure.
Aerate longer at vacuum than at atmospheric or higher pressures. At vacuum there is little to no air or water molecules to block and slow the rate of EO degassing from the medical device.
Using less gas to sterilize reduces the amount of EO to be degassed. But it doesn't reduce total cycle time. Using 25% less EO to sterilize reduces aeration time about 5 to 10%--a savings of one hour in aeration. But, to use 25% less EO, we must increase exposure time by one hour. Using less EO results in a net trade-off of zero in reducing total cycle time, one hour more exposure offset by one hour less aeration.
We validate shorter aeration times, so we know we can reduce aeration time safely without leaving higher EO residues on the medical devices. The validation occurs in the following manner. First, measure the residue left after aeration, using the current long cycle. We measure residue on a hard-to-aerate material, PVC tubing. Although PVC is not generally in re-usable devices, medical device manufacturers have found it to be one of the toughest of all device materials to aerate. PVC tubing is placed in the chamber at the same locations we use barrier packs to measure efficacy. Second, measure residues on the PVC tubing using shorter aeration times. Third, if the residue left after short cycle aeration is no more than the residue left after the current aeration, we have demonstrated that the short cycle did not leave a higher residue and worked as well as the current cycle. Note: Residues are measured by an outside laboratory that specializes in such procedures.
Some service companies now offer short cycle upgrades. Upgrade costs range from $5,000 to $20,000. The hospital must validate these service company cycles. One firm is developing new control systems and seeking an FDA 510K so that they may offer them to any US hospital. The hospital would not have to validate a 510K cycle. These systems are expected to cost $15,000 to $20,000 installed. For either type of upgrade, the hospital can save 25% on gas cost, can double the number of cycles run each day, or can start the sterilizer late in the afternoon, knowing that sterilized devices will be ready for early morning surgery.
21st Century Blend--Steriflo® Sterilant Gas
Oxyfume 2002 and Penn Gas 2 blends use HCFCs which have 0.02+ Ozone Depletion Potential (ODP), a measure of a chemical's potential to deplete the stratospheric ozone that protects life on earth. The ODP is 2% of the ODP of 12-88 sterilant that was phased out in 1995. For this reason, under the Clean Air Act, hospitals may continue to use these materials in existing sterilizers until the year 2030 and may buy new sterilizers with these materials until the year 2015.
A new blend, with the brand name, Steriflo, has been developed. It has zero ODP and, under the Clean Air Act, may be used into the indefinite future with no expectation of a phase-out date. The new blend uses HFCs to control EO flammability.
The HFC blend offers other benefits over the current blends. It contains about 5% more ethylene oxide, so no matter what cycle the hospital uses, it can use about 5% less gas. In cycle studies with barrier packs, Steriflo sterilizes much faster than the current blends, meaning the operator can sterilize with less exposure time or in the same time with less gas, and Steriflo can be substituted in the same sterilizer with few equipment changes.
- Highest available low-temperature SAL.
- Compatibility with the most device and packaging materials.
- Ability to sterilize most types of and largest size devices.
- Usability in improved cycles.
- Extended use of a sterilizing technology proven over the last 50 years.
Steriflo is now being validated in Europe and the US by medical device manufacturers and sterilizer makers. First conversions will begin in Canada and Europe. US use of the new blend will not start until it has been validated and is registered as a pesticide by the EPA. Expected date release for this product is 2001 to 2002.
Hospitals will continue to use EO sterilization because it sterilizes in a penetrating fashion. Hospitals will continue to use blend sterilizers because EO blend sterilizers have shown the good sterility assurance and high throughput. Improved cycles make the blend sterilizer both economic and flexible. The 21st Century Blend EO/HFC further enhances sterilizer performance while its zero ozone depleting properties make it a sterilant for the future. Most importantly, it is not subject to the 2030 phase-out date that affects the current blend sterilants.
Steve Conviser is the sterilant manager at Honeywell (Morristown, NJ).
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