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Understanding the sporicidal action of VPHP

This article aims to describe the fast-action vaporised hydrogen peroxide (VPHP) sporification process clearly and review the validity of the ‘conventional’  VPHP cycle
Tim Coles BSc(Hons) MPhil 
Managing Director, 
Pharminox Isolation Ltd,
Cambridge, UK
A paper on the action of vaporised hydrogen peroxide (VPHP) by Agalloco and Akers(1) concluded with the statement “The healthcare industry has experienced considerable difficulty in the implementation of the process”. Indeed, the subject of VPHP sporicidal gassing has been the subject of much debate  since it was first introduced by Amsco (now Steris) some 25 years ago. The process has long been regarded as something of a ‘black art’, with validation seen as a lengthy and complex undertaking.
This impression is not improved by the relatively high cost of the equipment. Perhaps some of the reasons for the uncertainty stem from commercial interests, but basically, it would appear there has been a fundamental lack of understanding of the fast-action VPHP sporicidal process. This article aims to describe the process clearly and then review the validity of the ‘conventional’ VPHP cycle.
The misconceived VPHP process
The basic layout of the VPHP process is relatively simple. A solution of hydrogen peroxide is evaporated by some means, and the resulting vapour is introduced into a steam of air, the mixture being directed into an enclosure, such as a pharmaceutical isolator. Here it acts to kill viable micro-organisms on the surfaces of the enclosure, rendering it suitable for aseptic processes.
It was noted early on that the VPHP process was extraordinarily effective and, under the right conditions, quickly inactivated large populations of resistant test micro-organisms, such as Geobacillus stearothermophilus spores.
It was assumed, not unreasonably, that high concentrations of hydrogen peroxide vapour, in the region of 2mg/l or 1200 ppm, were responsible for the quick ‘kill’.  Thus every effort was made to raise the vapour concentration quickly and maintain this high level, for a period of time until kill was achieved.
While widely used and, until now, little challenged, it seems that this concept is in fact quite wrong. Pure hydrogen peroxide vapour does not produce a fast kill, even at high concentrations. The fast kill observed is actually produced by a different effect.
The true VPHP process
If insufficient hydrogen peroxide vapour is introduced to the air stream passing into an enclosure, a ‘weak’ mixture results. This delivers vapour above its dew point to the enclosure and, at best, only a slow kill will result.
On the other hand, if too much vapour is delivered to the air stream, a ‘rich’ mixture results. This delivers vapour close to its dew point and frank, visible condensation develops on the walls of the enclosure. Once again, only a slow kill will result. 
If, however, the correct level of vapour is delivered to the air stream, the ‘Goldilocks’ level, then something different happens. This ‘something’ is the phenomenon that has been termed ‘micro-condensation’, and it is this micro-condensation that is responsible for the fast kill effect of the VPHP process.
The concept of micro-condensation was first put forward by workers at Bioquell Ltd in the UK.(2) They showed that, under the correct conditions, an invisible form of condensation takes place on the surfaces of an enclosure. The droplets are a few microns in diameter and thus cannot be seen. Importantly, the workers showed that these droplets are of high concentration, perhaps 60% or 70% hydrogen peroxide. Furthermore, the droplets tend to form preferentially on the surface of organic material, thus giving a ‘double whammy’ blow to micro-organisms, quickly inactivating them.
Part of the evidence for this micro-condensation comes from sophisticated calculation, but a simple piece of evidence comes from plotting the concentration of hydrogen peroxide vapour against time, during any correctly-run VPHP gassing cycle. The hydrogen peroxide concentration rises quickly at the start of the cycle and is then generally held steady for a time, after which the supply of hydrogen peroxide vapour is cut off and pure air is used to purge down the atmosphere of the enclosure to a safe level. Somewhat counter to intuition, the concentration of peroxide actually rises for a time, after the supply of peroxide is cut off. Until the work of Bioquell, this effect was poorly explained, or not explained at all.
The reason for the brief rise is that the micro-condensation on the surfaces of the gassed enclosure forms an equilibrium with the incoming vapour. There is a continuous input and output of vapour to and from the micro-condensation. When the supply is cut off, there is only output, as the micro-condensation evaporates. The micro-condensation has high concentration, and so the vapour concentration rises accordingly.(2) 
Given that the VPHP process is in fact a condensation process, a much clearer picture of the way in which it may be applied then emerges. Among other factors, it brings into question the need for dehumidification of the air in the enclosure before introduction of the gas. 
Is dehumidification required?
The ‘conventional’ gassing cycle as originally developed by Amsco, some 25 years ago, has four phases:
(a) Dehumidification of the air inside the enclosure
(b) Conditioning. Raising the concentration of VPHP in the enclosure quickly to a high level, around 1200 ppm.
(c) Sterilisation. Maintaining a steady high concentration for a time
(d) Aeration. Removal of the VPHP from the enclosure, down to a safe level, normally defined as 1 ppm.
The logic of dehumidification is to allow more ‘space’ into which the peroxide solution can evaporate and thus produce the highest VPHP concentration. This, until now, has logically been assumed to be the best route to a fast kill. We might term this the ‘low relative humidity (RH) type of cycle’.
However, more recently developed VPHP devices do not use the dehumidification phase. Instead, they simply use the ambient air in the enclosure as it is. We might term this the ‘high RH type of cycle’.
The measured VPHP concentration in these cycles is much less. It is of the order of a few hundred ppm, but nonetheless, these cycles can still demonstrate a log 6 reduction. (Log 6 reduction refers to the decease of a population of viable test organisms by 10.[6] This is conventionally taken as the performance indicator for a gassing cycle to be used in an aseptic isolator or enclosure).
The reason for this is that the all-important micro-condensation can be produced both from the low RH approach and from the high RH approach. From this information, the measured ppm level of VPHP is not an indicator of probable kill, because it is the all-important micro-condensation deposited on the surfaces of the enclosure that is responsible for the rapid kill. Log 6 reduction can be demonstrated with hydrogen peroxide vapour readings of only two or three hundred ppm.
There are some very significant potential advantages in using the ‘high RH type of cycle’:
  • The lower concentrations of hydrogen peroxide vapour in the enclosure may lead to less adsorption by the materials of the isolator and its load. This then may lead to reduced aeration times, this being the time taken for the peroxide concentration to drop to a safe level. Conventionally, this safe level is taken as 1 ppm, the eight-hour occupational exposure level (OEL) for hydrogen peroxide vapour. Arguably the one-hour OEL of 8 ppm might be an acceptable safe level to open an isolator. In most VPHP cycles, it is the aeration time that is by far the longest phase of the process, and so any reduction would be most welcome 
  • Removal of the dehumidification equipment from the VPHP gas generator makes it much simpler, more reliable and less expensive. And of course, the time for dehumidification is taken out of the complete cycle, again reducing the overall cycle time
  • A minor benefit is the use of less peroxide solution, cutting cost and perhaps improving safety.
Several VPHP devices have become available recently, operating without dehumidification of the enclosure air. Experience suggests that these devices work well and can give the required log 6 reduction within the same time frames as the ‘conventional’ low RH type of cycles. However, perhaps not surprisingly, some account has to be taken of the RH, and indeed of the temperature of the air within the enclosure.  With the low RH approach, the starting point RH for the cycle is established. If active dehumidification does not take place, then the RH of the enclosure air may be changeable from time to time and from day to day. This then means that a significant variable is introduced into a process that needs to be fully controlled.
In practice, the VPHP process is likely to be operated in an environment with fairly well controlled conditions of temperature and humidity, normally around 50% RH and 20oC. Provided that the RH and temperature limits of the high RH cycle have been established, this apparent lack of control may not in fact present a real issue.  
The critical issues
What, then, are the really significant factors for a successful VPHP process?  There are two main points to take on board:
  1. The VPHP process is only the final ‘polishing’ stage of the complete bio-decontamination process.  The bio-decontamination process starts with sweeping up any broken glass, moves on to methodical and validated mechanical cleaning, and only ends with VPHP treatment. If the surfaces of the enclosure, the process equipment, the process materials, products and tools it contains are not clean, then the enclosure will not be properly sanitised by the VPHP process. (Note the use of the word sanitise and not sterilise. Only dry heat, wet heat and irradiation can produce true sterility. The VPHP process can be very effective, but application of the word sterilise is not appropriate.)
  2. The VPHP process is a condensation process. The micro-condensation may be invisible, but it is there. Without the micro-condensation, the process will not operate correctly and cycle times will be long, or even totally ineffective.
Once these two points are established, the development of a VPHP cycle becomes less confounding. Indeed, with some input from an experienced operator, cycles can be developed by relatively unskilled technicians. Furthermore, this can be done fairly quickly, perhaps over a period of days for a simple sterility testing isolator, and weeks for a complex vial filling line isolator. In the past, many months have been taken to validate some VPHP applications.
The subject of VPHP cycle development is covered in the next article in this section.(1)
  1. Agalloco J, Akers J. Overcoming limitations of vaporized hydrogen peroxide. Pharm Tech 2013;37(9):46–56.… (accessed 16 December 2013).
  2. Watling D, Parks M. The relationship between saturated hydrogen peroxide, water vapour and temperature. Pharm Tech Eur;March 2004.