Plastic Parts Off-Gas Hydrogen Peroxide Vapor After Sterilization

Ethylene oxide (EtO) as a sterilant gas has two main drawbacks; the first is health – EtO is an irritant and a carcinogen and the second is economic; a typical EtO sterilization cycle may take 12 to 15 hours. While the actual sterilization step may only last a couple hours, the rest of the time is needed for the sterilized product to aerate and give up the EtO trapped within it.

Since the mid 1990s many alternative gases and vapors have been explored as low temperature gas or vapor sterilants. Obviously none of them are harmless since sterilization is achieved by exposing the device to high concentrations of reactive gases. Typical gases used are hydrogen peroxide or ozone, both strong oxidants, and formaldehyde (steam formaldehyde), an alkylating agent similar to EtO (not currently used in the US).

In addition to avoiding EtO, the main selling features of hydrogen peroxide sterilization have been the short cycle times. The two leading manufacturers of hydrogen peroxide sterilizers, Advanced Sterilization Products and Steris Corporation, both now offer cycles times as short as 28 minutes. These features have been so attractive that hydrogen peroxide sterilization is now the dominant form of low temperature gas sterilization used in the US today.

The conventional wisdom is that EtO has a long cycle time because porous and some plastic products dissolve or otherwise retain the EtO and the long cycle time is required to allow the EtO to diffuse out of the product; whereas for hydrogen peroxide little or no aeration time is required.

A recent study by Japanese researchers from the Division of Infection Prevention and Control, Tokyo Healthcare University Postgraduate School challenged the conventional wisdom by showing that some plastic devices after hydrogen peroxide sterilization, off-gas the hydrogen peroxide vapor and take a much longer time for the vapor to clear than some of the current hydrogen peroxide sterilization cycles allow.

The researches took 11 kinds of plastic test panels, made of the polymers common used to manufacture medical devices and sterilized them using a hydrogen peroxide sterilizer and then measured the hydrogen peroxide vapor off-gassing from the surface of the plastics. They found that some plastics retained the hydrogen peroxide for much longer than others. A similar effect is seen with EtO, where some polymers such as PVC are notorious for slowly releasing dissolved EtO. The initial concentration of hydrogen peroxide emitted ranged from ~40 to over 300 ppm and many of the plastic panels with the initially higher hydrogen peroxide continued to emit concentrations over 50 ppm more than 25 minutes later.

In another test, the researches took a medical stapler made of polyetherimide, sterilized it, and found that the stapler initially emitted over 300 ppm hydrogen peroxide and took six days for the emitted hydrogen peroxide concentration to fall to 10 ppm, and 24 days before the concentration fell below 1 ppm (the OSHA PEL). In another test, flexible scopes continued to out gas hydrogen peroxide above 10 ppm for 18 to 40 hours.

The researchers started the work in response to complaints or eye and respiratory system irritation from healthcare workers who work around the sterilizers but the use, clean & sterilize and reuse times of many devices such as flexible scopes is often much less than 40 hours and so the researchers went to comment that healthcare workers should be on the look out for adverse effects in patients (as well as in their colleagues performing the sterilization).

The conventional wisdom is that hydrogen peroxide sterilization is much superior to EtO sterilization in large part because the latter does not require long aeration periods and so can perform its sterilization function with short cycles which saves time and resources as equipment can be sterilized and put back into service more quickly. This study by Rika Yoshida, Hiroyoshi Kobayashi directly challenges the belief that there is no significant out-gassing with hydrogen peroxide and shows that the concentrations being emitted are not only above the OSHA PEL (1 ppm 8 hr TWA), but in some cases over the NIOSH Immediately Dangerous to Life and Health level (75 ppm). These results parallel a study from 1997 by MacNeal and Glaser ["Comparison of healthcare based sterilization technologies: Safety, efficacy, and economics" in Journal of Healthcare Safety, Compliance & Infection Control (1997), 1,(2, December), p 91 to 107] where they found that

"Packages removed from the sterilizer after on hour continued to emit residual H2O2 gas at short-term or instantaneous concentrations of up to 2.5 ppm, for up to 1.3 hours following their removal from ther sterilizer."

Surprisingly, there do not appear to have been any other studies that looked at out-gassing of hydrogen peroxide from sterilized medical devices, though with the questions raised by these two research groups, we are sure that much more attention will be given to this important subject going forward.