There are issues surrounding the processing of N95 and other masks in vaporized hydrogen peroxide (VH2O2) sterilization, which is what is being done in most hospitals. Hydrogen peroxide is referred to as H2O2. Subjects to be covered are:

  • Why are these (normally) single-use devices?
  • What are the differences between doing this in STERRAD and STERIS V-PRO?
  • How do the processes work?
  • What is required for user safety?

Let’s first see what the manufacturers involved in this have to say. 3M, the world’s leading supplier of N95 and FFP2 masks states that the method must:

  • inactivate the target organism, such as the virus that causes COVID-19
  • not damage the respirator’s filtration
  • not affect the respirator’s fit
  • and be safe for the person wearing the respirator [1]

The sterilizer manufacturers who have received authorization from FDA have the following positions. ASP’s announcement states:

  • Due to incompatibility, the STERRAD Sterilization System is not authorized for use with respirators containing cellulose or paper materials.
  • All compatible N95 respirators used in the STERRAD Sterilizers must be free of visible damage and soil/contamination (e.g., blood, dried sputum, makeup, soil, bodily fluids).
  • Compatible N95 respirators should be discarded after two decontamination cycles.
  • Any compatible N95 respirator whose traceability was lost or number of decontamination cycles not able to be identified should be discarded.
  • Decontaminated compatible N95 respirators are not sterile.[2]

STERIS’ announcement [3] is identical to ASP’s but you are allowed to process a mask 10 times in a V-PRO Non-Lumen cycle instead of only two times in the STERRAD machines. 3M has also evaluated Sterilucent’s process and found similar results to the STERIS V-PRO and Battelle’s process and found that it can be used for up to three processings.

Why are these (normally) single-use devices?

Surgical and protective masks are normally used once and discarded. Why is this true under normal circumstances?

  • You can’t clean them
  • You can’t decontaminate them
  • They are too inexpensive to make it worth the effort
  • They have limited compatibility with disinfection/sterilization processes
  • Even if you do decontaminate them, you can’t be sure that they are still doing the job of keeping infectious agents away from the person breathing through them.

But we are decontaminating them in a highly abnormal time when production can’t keep up with the need, under special permission from the U.S. Food and Drug Administration (FDA). As the curse goes, “May you live in interesting times.” We are living in very interesting times.

Let’s have a look at each of these points.

You can’t clean them

If the mask has been sprayed with liquid or otherwise stained, the mask will be penetrated and the filters that make up the mask are really good at retaining anything they are exposed to. Right to the garbage can, preferably a biohazard bin!

You can’t disinfect them

You can’t use liquid and surface disinfectants on these devices because they can change the filtration performance of the masks, making them less effective (or too effective, blocking airflow), providing a false sense of security.

But we are decontaminating them using a sterilizer, not validating sterilization. They come out pathogen-free, which is what you want.

ASP and STERIS recommend packaging the masks in standard Tyvek peel pouches and using a chemical indicator to avoid cross-contamination and to show that H2O2 reached the mask in the expected concentration.

They are too inexpensive to make it worth the effort

True. Until you don’t have any.

They have limited compatibility with disinfection/sterilization processes

If the mask contains cellulose-based or paper materials, it will absorb peroxide and limit the effectiveness of the process for the rest of the load or cause the cycle to abort for lack of sufficient pressure rise when the peroxide is admitted. You should consult with the manufacturer of the mask to ensure that their product can be processed in VH2O2.

Even if you do disinfect them, you can’t be sure that they are still doing the job of keeping infectious agents away from the person breathing through them

The definition of an N95 mask’s performance is, “The ‘N95’ designation means that when subjected to careful testing, the respirator blocks at least 95 percent of very small (0.3 micron) test particles.” [4] If the mask no longer blocks 95% of 0.3-micron particles, it isn’t an N95 mask anymore and the user of the mask may not be protected.

What are the differences between doing this in STERRAD and STERIS V-PRO?

Not a lot. The key difference is that you can process masks up to 10 times (if there is no visible damage) in a STERIS V-PRO Non-Lumen cycle and only twice in STERRAD 100S, NX (standard cycle), or 100NX Express cycle.

Why only two cycles in STERRAD and 10 in STERIS V-PRO? All three sterilizers begin with 59% H2O2. The STERRAD NX and 100NX then concentrate it to increase effectiveness. STERIS does not concentrate the H2O2. All I can guess is that the higher VH2O2 concentration of the STERRAD NX and 100NX cycles damages the masks more than the lower concentration in the STERIS V-PRO, limiting the number of processings to two. This explanation doesn’t account for the STERRAD 100S, which does not concentrate the H2O2, only being allowed to be used for two cycles per mask, but those are the FDA-approved instructions and they must be followed.

How do the processes work?

VH2O2 sterilization processes have the same general cycle sequence.

  1. Pull a very deep vacuum on the load
  2. Steam sterilizers, at best, pull a vacuum removing 93–95% of the air in the load. This is done three to four times to get good steam penetration.
  3. These sterilizers pull a vacuum removing 99.95% of the air in the load. This is done to ensure that the H2O2 vapor penetrates the load.
  4. Admit H2O2 vapor to the chamber and load.
  5. Wait for the required exposure time.
  6. Add air in to drive the H2O2 vapor into the load to reach portions of the load that aren’t easily reached by diffusion.
  7. Evacuate to remove most of the H2O2 from the load.
  8. Repeat at least once to meet the requirement of running a half cycle twice to ensure a 10-6 sterility assurance level (V-PRO does it three more times).
  9. Do something to decrease residual H2O2.
  10. In STERRAD machines, evacuation to a low pressure helps H2O2 diffuse out of the load, and a gas plasma is used to destroy the H2O2 that diffuses out. The plasma has nothing to do with the sterilization.
  11. In V-PRO machines, multiple pulses of air and evacuation are used to remove residuals.

Is the final step 100% efficient? Probably not for filtered items like masks. Does this potentially make the masks unsafe for use? It might. How do you know?

To find out, you need to place a processed mask into the smallest box that the mask will fit in with an H2O2 monitor. This monitor can be electronic or a dosimeter. The box cannot absorb H2O2 nor be ventilated. Letting the box sit for eight hours simulates a test per OSHA and NIOSH exposure limits of 1 ppm over an eight-hour period. If the result is greater than 1 ppm, you need to aerate the masks before use.

Aeration? I thought we were done with that when ethylene oxide faded!

Yes, we were. But these are different. you can aerate in a drying cabinet or on a well-ventilated shelf. Test after the aeration for residual hydrogen peroxide again to make sure it is gone. You can do the testing once on three masks (in three boxes) to verify the result. If you don’t, you risk causing respiratory distress and facial burns on the user.

So you can process masks in VH2O2:

  • If they are clean
  • If you don’t exceed the allowed number of processings
  • If you make sure that your process doesn’t leave H2O2 on the masks.


    1. “Decontamination Methods for 3M N95 Respirators,” 3M Company, Technical Bulletin April 2020, Revision 4.

    1. “Instructions for Healthcare facilities: Decontamination of Compatible N95 Respirators in STERRAD Sterilization Systems,” Advanced Sterilization Products (ASP) Inc., Accessed April 11, 2020.

    1. “Instructions for Healthcare Facilities: Decontamination of Compatible N95 Respirators Using the STERIS Sterilization Systems,” STERIS Corporation, Accessed April 9, 2020.


Bio note: Dr. Wilder has brought his background in physical chemistry and surface physics to bear upon all healthcare thermal and chemical sterilization methods. He has four patents in H2O2 and peracetic acid vapor sterilization.