Sterilization with Nitrogen Dioxide Gas: A New Sterilization Tool (If the clean room manufacturers are working on the option is not clear)

Nitrogen Dioxide (NO2) gas is a rapid and effective sterilant for use against a wide range of microorganisms. The unique physical properties of NO2 gas allow for sterilant dispersion in an enclosed environment at room temperature and ambient pressure. These same properties also allow for quicker removal of the sterilant and residuals through aeration of the enclosed environment. The combination of rapid lethality and easy removal of the gas allows for shorter overall cycle times during the sterilization (or decontamination) process and a lower level of sterilant residuals than are found with other sterilization methods. NO2 has a boiling point of 21oC at sea level, which results in a relatively high saturated vapor pressure at ambient temperature. Because of this, liquid NO2 may be used as a convenient source for the sterilant gas. Liquid NO2 is often referred to by the name of its dimer, dinitrogen tetroxide (N2O4).

In the gaseous state, NO2 and N2O4 are in equilibrium, with the former being heavily favored at the concentrations used during the sterilization process. In order to deliver sterilant to an enclosed environment that is at either ambient pressure or under a vacuum, a pre-chamber and vacuum pump system is used to meter the prescribed dose of NO2. The pre-chamber, which is connected by solenoid valves to a cylinder of liquid NO2, is first evacuated. The valve between the pre-chamber and cylinder is then opened, which causes the liquid NO2 to evaporate into the evacuated pre-chamber until a pre-set pressure is reached in the pre-chamber. This pre-set pressure corresponds to the prescribed dose of NO2 gas in the enclosure to be sterilized, while accounting for the NO2 and N2O4 equilibrium. Once the pre-chamber has been filled to the appropriate pressure of NO2 and N2O4 gas, it is opened to the evacuated sterilization chamber, or to the air stream that is circulating within the isolator generator system. This introduces the sterilant gas to the volume to be exposed to the NO2. At this point, humid air may be added to the sterilization chamber.

However, the isolator environment will have already been humidified to a recommended level by using some means of water vaporization. Since the saturated vapor pressure of NO2 gas at 21dc is 1 atm and the concentration used are typically around 1% of the saturated vapor pressure, there is no need to heat the chamber in order to support the desired effective dose. Sterilant removal from an exposed volume is readily accomplished by air exchanges with fresh air that has been passed through a HEPA filter.

The sterilant in the exhausted air can be scrubbed from the air stream by chemical means. The chemical removal of sterilant from the air stream uses a solid chemical scrubber material. The scrubber material neutralizes and captures the NO2 in a non-hazardous, solid material. The spent scrubber material may be disposed of as non-hazardous solid waste with no special handling measures required. The scrubber also allows the scrubbed air to be vented directly to the room.

The most-resistant organism (MRO) to sterilization with NO2 gas has been proven to be the spore of Geobacillus stearothermophilus, which happens to be the same MRO for both steam and hydrogen 2 peroxide sterilization processes. The spore form of G. stearothermophilus has been well characterized over the years as a biological indicator in sterilization applications. Microbial inactivation of G. stearothermophilus with NO2 gas proceeds rapidly in a log-linear fashion, as is typical of other sterilization processes.

The mechanism for lethality is degradation of DNA in the spore core by absorbed NO2 kills the exposed organism.

NO2 can be effectively used as a sterilizing agent. NO2 exposure provides a rapid inactivation process and can be quickly aerated from exposed materials. The mechanism of action is found to be single strand breaks of the organism DNA.

Several literatures are available based on some studies done so far. If the clean room manufacturers are working on the option is not clear.