Chlorine dioxide (ClO₂) is a chemical that can sterilize medical devices, food and even public drinking water, making it a potential alternative for ethylene oxide (EtO).
Chlorine dioxide can make certain medical devices safe for use by killing microbes that could infect patients. It does this through oxidation, using negatively charged molecules to perforate cell walls and kill viruses, bacteria or fungii. Like EtO, ClO₂ gas can permeate medical device packaging and any part of the device that’s not air-tight.
Chlorine dioxide has a key advantage as medical devices such as implants are increasingly powered by embedded batteries. Those batteries can be challenging or prohibitive entirely for heat or EtO sterilization, but chlorine dioxide does not damage batteries and isn’t explosive enough to be ignited by them (more on that later).
Chlorine dioxide gas has been EPA-registered as a sterilant since 1988. Johnson & Johnson developed chlorine dioxide sterilization in the 1990s and patented its “method and apparatus for the generation, use, and disposal of chlorine dioxide.”
More recently, the FDA approved chlorine dioxide for contract sterilization of medical devices in 2021. Finished and packaged medical devices — such as implantable contact lenses, artificial joints, suture products, surgical kits, vial stoppers, endoscopes, electronic devices and prefilled syringes — are sterilized with chlorine dioxide in airtight chambers.
Chlorine dioxide is a toxic gas with a greenish yellow or reddish yellow color and smells like chlorine.
Sterilization usually requires about a 4% concentration of ClO₂, and is not flammable or explosive at sterilization concentrations. Chlorine dioxide at concentrations over 10% is explosive and prohibited from transport by the U.S. Department of Transportation unless it’s shipped frozen.
On-site generators can produce the gas by passing chlorine gas through different chemicals. For example, one ClO₂ gas generator evaluated by the EPA uses bleach, hydrochloric acid, sodium chlorite and distilled water.
The FDA has warned consumers not to drink liquid chlorine dioxide products dangerously promoted for the prevention or treatment of COVID-19, other viruses such as influenza and HIV, cancer, hepatitis and autism. But chlorine dioxide sterilization does not pose a risk to patients because it does not leave residue on medical devices.
Chlorine dioxide sterilization versus EtO
EtO is the most commonly used method for sterilizing medical devices, used on about 20 billion medical devices per year. That’s approximately half of all medical devices that require sterilization, thanks to its ability to permeate packaging and sterilize truckloads of product at once.
However, a heightened focus on EtO’s safety risks by the EPA, FDA and medtech industry means commercial sterilizers will likely face new rules and regulations to protect workers and communities. That — along with the maxed-out capacity for EtO sterilization — has spurred interest in EtO alternatives.
Electron beam (e-beam) sterilization is one of the fastest-growing EtO alternatives, but e-beam sterilization and other forms of irradiation can’t match EtO’s materials compatibility or ability for bulk sterilization.
Chlorine dioxide gas is more comparable to EtO for materials compatibility and offers shorter sterilization cycle times than EtO. Gassing a pallet of medical devices with EtO might take 12 to 24 hours, plus another 12 hours to several days for aeration (removing the gas and any residue). ClO₂ gas can sterilize the same pallet in six to eight hours, including 90 minutes to two hours for aeration.
One factor adding to EtO’s aeration time versus chlorine dioxide is that most materials used in medical devices do not absorb chlorine dioxide. Many materials absorb EtO, requiring mechanical aeration to remove residual EtO.
Unlike EtO, chlorine dioxide gas has not been linked to cancer or birth defects. Because there are no cancer studies on human exposure to chlorine dioxide, the EPA does not have enough information for carcinogenicity classification.
Another advantage is that chlorine dioxide is not explosive under normal sterilizing conditions. Following explosions at EtO sterilization facilities in the 1990s, former National Institute for Occupational Safety and Health Director Linda Rosenstock warned of the danger in an industry alert.
“If ignited from overfeeding in industrial sterilization processes, EtO can explode with enough force to lift a 50,000-lb sterilization chamber three feet off its foundation and blow out steel ductwork,” she said.
Because EtO is explosive, the gas can’t safely sterilize devices containing batteries without a thorough safety review. That might make chlorine dioxide sterilization a better option for devices with embedded batteries, such as pacemakers
Chlorine dioxide sterilization drawbacks
Chlorine dioxide is not as readily available as EtO or irradiation sterilization methods such as e-beam, gamma ray and X-ray. And chlorine dioxide chambers at commercial sterilizers are not yet as large as the biggest EtO chambers, which reduces how much product can be sterilized simultaneously.
ClO₂ sterilization is also more expensive than steam or heat sterilization, though medical devices that can be sterilized through those methods are less likely in need of alternatives than medical devices sterilized with EtO.
Corrugated cardboard poses a particular challenge for chlorine dioxide sterilization. Corrugated cardboard used in medical device packaging soaks up ClO₂ more than EtO, which means sterilization will require more gas and more time for aeration.
For that reason, a manufacturer might want to use chlorine dioxide sterilization for packaged products in-house before boxing them up for shipment, while a contract manufacturer might remove packaged devices from cardboard shipping boxes before sterilization. One way to avoid time lost due to unpacking for sterilization is to send finished devices to a contract sterilizer in plastic-wrapped baskets instead of cardboard boxes.
Medical device engineers should consider their options for sterilization when designing their devices and product packaging, keeping materials compatibility and size limitations of sterilization chambers in mind.