Why ETO?

ETO sterilization is a well-known technology with excellent history and high reliability

Environmental considerations

  • Strict emissions requirements
  • No depleted radiological waste to contend with

Effect of radiation on product or packaging materials is sometimes prohibitive
Effect of radiation on drugs may be problematic



NASP provides medical products sterilization by ethylene oxide gas (commonly called ETO or EO). This process inactivates living microorganisms which might contaminate a device. Exposure to ethylene oxide gas causes alkylation to the microorganisms at the nuclear level. This means that the ETO replaces a hydrogen group atom of the organism with an alkyl group. It causes damage to the cellular DNA and therefore causes the microorganism to die. Ethylene oxide (EO) sterilization is also used as the sterilization process for many other industries such as food and agriculture, spices, and cosmetics. EO for use as a sterilant makes up less than 1% of its use in the United States. 99% of its use is as a chemical intermediate and the remaining percent is as a fumigant or insecticide.

EO sterilization process consists of several steps of evacuating air from the product and introducing humidity, ETO gas and nitrogen gas. For this reason, medical products sterilized with ETO require porous packaging that allows the rapid transfer of these gasses through the package without damaging the package.

ETO Process Overview


Typically the ETO sterilization process consists of three basic steps:

  • Preconditioning
  • Sterilization
  • Aeration



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Preconditioning may be performed in the sterilizer chamber or a separate room. The purpose of preconditioning is to heat and humidify the product load. Temperature ranges from 110°F-130°F and relative humidity ranges from 35%-80%. This aids in the kinetics of the gas transfer process and also aids in the death of the microorganisms. Essentially the product humidifies allowing transfer of heat to the product which warms up the microorganisms allowing them to be killed easier. Preconditioning is done with fully packaged finished product on pallets. Depending on the density of the product, and the relative ease of humidity and heat transfer, this process may be lengthy. It is important that the entire load be equally conditioned and it is easy to see that the product on the inside of the pallet cube will take longer to reach equilibrium than will the product on the outside of the load.



There are four very important factors in the sterilization phase which can influence the lethality of the ETO sterilization process. These are:

  • Humidity
  • Gas concentration
  • Gas dwell time
  • Temperature

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The humidity level must be at a certain percentage to allow the proper gas transfer and ensure that the microorganisms are able to be killed. Too much humidity will cause saturation which in turn may give the reverse effect. Common humidity ranges are from 35%-80%.

Gas concentration is very important. We must ensure that the amount of EO in the chamber is enough to kill the microorganisms, but is not too much where EO residual problems after the fact are created. Typical EO sterilization cycles concentrations range from 450mg/L to 700mg/L.

Dwell time is also extremely important to ensure that the product load is being exposed for the proper amount of time in order to ensure the product will have the required sterility assurance, taking into account the time it takes to inject the gas and the time it take the gas to penetrate into the products within the chamber.

Perhaps the most important of the four factors is temperature, because the higher the temperature of the product the higher the lethality. EO sterilization follows the Q10 effect which in essence means that for every 10°C increase in temperature, the lethality of the cycle is doubled. Common temperature ranges from 110°F-130°F

General Steps in the ETO Sterilization Process
  • Evacuation
    This phase removes the air that is in the sterilization chamber.
  • Nitrogen dilution
    This introduces nitrogen into the chamber. It is injected and then immediately removed. This also aids in the removal of any air that is in the chamber.
  • Humidity injection and humidity dwell
    Humidity is injected into the chamber and it is held at a certain pressure for a certain amount of time. This replenishes the moisture in the load which was lost during its removal from the preconditioning room and transfer to the chamber, and during the evacuation and nitrogen dilution phases. The moisture in the chamber also allows easier absorption and desorption of the EO gas in and out of the products.
  • Sterilant injection and EO dwell
    The EO sterilant is injected into the chamber and is held in a dwell phase for a period of time in order to achieve the validated sterility assurance level.
  • Sterilant removal
    This is the process by which a vacuum is pulled deeper to remove all of the ethylene oxide from the chamber and the product.
  • Nitrogen wash
    There are usually a series of nitrogen washes. Their purpose is to essentially wash off any ethylene oxide that is remaining in the chamber.
  • Air break
    This phase releases the vacuum and brings the sterilization chamber back to atmospheric pressure before the doors can be opened.

Heated Aeration


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Following the sterilization phase, the product load is transferred to heated aeration cells where it remains for a period of time in circulated heated air. This process aids in the removal of any remaining ethylene oxide residues from the load. There is no added humidity in this phase. After heated aeration, the product load is removed and usually transferred to a warehouse where it awaits formal release.