VHP, HPV or V-PHP all stand for Vapor (or Vapour) Hydrogen Peroxide: but what is vapor exactly?

Hydrogen Peroxide is a very well-known molecule for the biodecontamination in the pharmaceutical industry. Its definition is as simple as writing its formula H₂O₂.

As water, hydrogen peroxide has different state at ambient temperature: liquid and vapor. While liquid definition is pretty straight forward, vapor understanding may be trickier when it comes to a practical explanation.

Let’s start from its scientific definition.

In physics, a vapor (American English) or vapour (English and Canadian English) is a substance in the gas phase at a temperature lower than its critical temperature, which means that the vapor can be condensed to a liquid from its gas phase by increasing the pressure on it without reducing the temperature.

This helps to understand that a vapor acts as a true gas as far as it does not condense back to liquid. A true gas (like oxygen) is different only for the fact that can be condensed into liquid only by reducing its temperature while increasing its pressure.

Vapor is also different than an aerosol as an aerosol is a suspension of tiny particles of liquid, solid, or both within a gas. Aerosol is made up with droplets, liquid state of H₂O₂ (not gas state), suspended within the ambient air.

From this definition we can few clarifications on H₂O₂ decontamination technologies:

1. Vapor H₂O₂ is an invisible gas which is distributed inside any enclosure following gas rules
2. H₂O₂ gas can condense back to liquid state whenever its pressure increases over a critical point (pressure of H₂O₂ gas is relative saturation)
3. Any suspension of liquid H₂O₂ droplets in the air is an aerosol and not a vapor, in fact aerosol is clearly visible to the naked eye.

It is important to identify the appropriate decontamination method for each application as specified also in the latest release of EU GMP Annex 1.