Considerations for vaporized hydrogen peroxide process development

In our recent webinar, “Achieving Effective H₂O₂ Bio-decontamination in Facilities & Containment Systems” we did not have time to answer all questions, so we answered by email afterward. In this blog, we share the questions and answers in three categories: bio-decontamination process development, low ppm measurement, and condensation.

Bio-decontamination Processes

QUESTION: Are representative locations sufficient to prove bio-decontamination?

ANSWER: I assume that you are asking if vaporized H₂O₂  is measured in representative locations, that this would be sufficient to prove bio-decontamination. The answer is that once a process has gone through a validated cycle development, then proper monitoring will suffice until the next cycle requalification is needed. However, inline monitoring of vaporized H₂O₂ monitoring is not a substitute for validation of the process using chemical, biological, or enzymatic indicators..

QUESTION: How is ppm (in terms of hydrogen peroxide concentration level) connected with effective bio-decontamination?

ANSWER: The combination of temperature, vaporized H₂O₂ ppm levels, and exposure duration are used to estimate the effectiveness of a bio-decontamination process. Other variables, like materials, temperature, relative humidity, relative saturation; all are involved. But it’s most important to understand that they are all connected

QUESTION: Is ppm concentration correlated to effective bio-decontamination? I ask because sometimes, a process at low concentration H₂O₂ ppm cycle will pass, and sometimes a high concentration H₂O₂ ppm cycle will fail…

ANSWER: The vaporized H₂O₂ ppm concentration required for a bio-decontamination process depends on a number of variables, including the type of micro-organisms, the space being bio-decontaminated and challenges associated with the space, such as the objects within it. It is possible that certain areas or processes require a longer duration at lower ppm levels, where others require higher ppm levels for shorter durations. There is no single correct method. The process you are using should be developed under conditions that are representative of a normal cycle.

QUESTION: Does altitude affect humidity and saturation during bio-decontamination?

ANSWER: Yes, altitude does affect the relative humidity and therefore relative saturation. The relative humidity and relative saturation can be expressed in terms of the partial pressures of water and vaporized H₂O₂ relative to the total saturation vapor pressure (the amount of the gas the air can hold at a specific temperature) of each gas. This relationship is linear with pressure changes, so the drop in pressure at altitude needs to be corrected for during measurement.

QUESTION: Why does H₂O₂ concentration vary at the end of a vaporized cycle, during aeration and afterwards, when a 1 ppm concentration is not reached?

ANSWER: Aeration time is going to vary from area to area, with a number of possible variables. The examples provided in the webinar were basic representations. The 1 ppm limit is a safety guideline for exposure over an 8-hour workday and is sometimes a goal in the aeration process. Different factors can affect this target level for aeration, namely material desorption, condensation effects, and chemical compatibility. Sometimes the aeration target is in the ppb levels for vaporized H₂O₂.

QUESTION: Why is vacuum vaporized H₂O₂ used for sterilization applications?

ANSWER: Vacuum conditions are typically related to the vaporized H₂O₂ delivery and aeration of the process. One method uses a deep vacuum to pull liquid hydrogen peroxide from a disposable cartridge through a heated vaporizer and then, following vaporization, into the sterilization chamber. 

In another approach, vaporized H₂O₂ is brought into the sterilization chamber by a carrier gas such as air using a slight negative pressure (vacuum). There may be other benefits and concerns for performing the cycle under vacuum as well. Aeration is typically performed using fairly powerful suction as well.

QUESTION: How long do you have to hold the conditions at 100% saturation to get effective decontamination?

ANSWER: The decontamination phase of the cycle is going to vary in length, depending on a number of variables determined by process development. Factors to consider are; the type and volume of the space being bio-decontaminated, the types of micro-organisms being neutralized, and the contents and the materials present in the space. The typical process times vary widely. For example, room bio-decontamination can range from 4 – 24 hours, whereas isolator bio-decontamination processes may only last 1-4 hours.

QUESTION: Is there a rule of thumb for how much liquid volume of peroxide is consumed per bio-decontamination process? I am thinking of a number in terms of milliliters per cubic foot of meter for 35% or 50% H₂O₂.  Obviously, there are many factors: the shape of the room or item being decontaminated, level of log reduction desired, etc., but if you have a value in mind, I'd be quite interested.

ANSWER: Unfortunately, I do not have a very good answer to your question, as the type of generator we use is customized, and I don’t know if we have tracked the consumption of liquid H₂O₂ use. Also, vapor generators vary in output and flow rate, so it’s difficult to give an answer on this, but I would recommend contacting the vaporizer manufacturer.  They produce and sell vaporized H₂O₂ products and services and will likely have better information for you.

Low ppm Measurement

QUESTION:  Is the 1 ppm for H₂O₂ a European standard?

ANSWER: The European Chemicals Agency ECHA has an evaluation document "Regulation (EU) No 528/2012 concerning the making available on the market and use of biocidal products"

In the US, the ACGIH, OSHA, and the National Institute of Occupational Safety and Health (NIOSH) all have set an average daily occupational exposure limit of 1 ppm.

QUESTION:  How does the Vaisala hydrogen peroxide probe measure low concentration? 

ANSWER: Currently, HPP270 products measure down to 0 ppm with accuracy ± 10 ppm the minimum. Therefore, Vaisala does not offer a measurement solution designed specifically for low level vaporized H₂O₂ measurement. There are a number of solutions on the market, and I would refer you to the reference materials we provided for more insight into low-level measurements.

QUESTION:  If the lowest concentration level measured by the HPP270 is 10 PPM, how do we know when it is safe for operators to re-enter a decontaminated area?

ANSWER: The HPP270 Series probes are designed for in-line, process-level measurements. Those measurements can be used to manage vaporized H₂O₂concentrations during the decontamination phase. The HPP270 probes were not designed for safety level measurements. The duration of aeration (after cycle development and validation) can account for the evacuation time required, and the rate of change from the HPP270 series probes’ real-time measurement can aid in that estimation. There are other low-level measurement options currently available.

Condensation

QUESTION: Which method is more effective in bio-decontamination: Dry or wet (with visible concentration)?

ANSWER: The effectiveness of wet vs dry vaporized H₂O₂ bio-decontamination is a hotly debated topic. This is another item that should be determined by process development. In general, Vaisala sensors are good choice for both cases and provide accurate and stable measurements even during high-condensing processes. Condensation is why the HPP270 probes provide the relative saturation value; this is the only parameter that allows us to estimate when condensation will occur.

QUESTION: Can we achieve micro-condensation with spraying H₂O₂ through atomizer nozzle?

ANSWER: Micro-condensation is a condition created with a careful balancing of the relative humidity, relative saturation and temperature of both the air space and the surfaces in the space. However, Vaisala does not manufacture vaporizer or atomizer products so I cannot comment on the delivery capabilities of an atomizer, as most of our experience is related to hydrogen peroxide vaporization.

QUESTION: Is condensation recommended during a process?

ANSWER: This is debated in the scientific community and there is no clear consensus. In micro-condensing processes, there is actually an invisible condensate formed during the process, but being at sub-micron level, it is invisible to naked eye. This is achieved by maintaining the relative saturation extremely close to 100%. There are scientific papers that suggest that micro-condensation is necessary in hydrogen peroxide bio-decontamination and that micro-condensation can happen in dry processes as well. We recommend a review of the research that focus on processes similar to your own.