Case Study: Ductwork Decontamination

Traditional sanitation procedures typically do not include air handling units or their accompanying ductwork. Chlorine dioxide gas is the only residue free fumigation method that can successfully decontaminate ductwork and HVAC systems, including HEPA housings. Being a dry process and a true gas at room temperatures, CD is able to navigate the bends and turns of the ductwork system without condensing and getting "stuck." Typically, ductwork is decontaminated along with the rooms that it handles.  On one occasion, there was only a need to use chlorine dioxide gas to decontaminate the ductwork itself.

The exhaust ductwork in the BSL-2 and BSL-3 research laboratory of a major pharmaceutical company needed to be replaced. Since the ductwork was used to exhaust biological safety cabinets (BSCs) for testing on HIV and Hepatitis C, special precautions would be required prior to its renovation. The company decided that a fumigation style decontamination should be performed, and chlorine dioxide gas was chosen due to its ability to reach all surfaces and distribute throughout the entire length of ductwork without condensing or leaving a residue. The laboratory was located on the third floor of the building and consisted of a four-room BSL-2 area and a smaller, two-room BSL-3 area. There was a total of fourteen BSCs with exhaust ductwork that required decontamination and two ceiling exhausts. One chlorine dioxide gas generator was set up, and gas injection tubing was run to one BSC in the BSL-2 area and to one BSC in the BLS-3 area. The gas was then pulled through the exhaust system on the fifth floor and down to the distribution system and then into each BSC. A total of fifteen Bacillus atrophaeus biological indicators (BIs) were placed in each BSC and the two room exhaust vents to validate the decontamination. The results of the cycle yielded a greater than 720 ppm-hr decontamination time, which is more than adequate to provide a 6-log sporicidal reduction. All biological indicators were negative after the seven-day incubation period allowing the renovation crew to work in a safe environment without having to wear personal protective equipment (PPE).

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Follow Your Nose: CD's Best Safety Feature

While all decontaminating agents are by nature dangerous, chlorine dioxide (CD) gas has many traits which make it the safest method available. The best safety feature with CD is that it is self-alerting.  Chlorine dioxide gas has a discernible odor at safe levels, allowing you time to shut down the system and address the situation safely if it is smelled. Other agents, such as Ethylene Oxide (EtO) and Vapor Phase Hydrogen Peroxide (VPHP), cannot be sensed until you are exposed to extremely high concentrations.  This dangerous trait is why natural gas is given a sulfur-like odor additive, to act as an alert. VPHP users (and surrounding colleagues) become aware of a harmful exposure only when coughing and choking occurs, therefore a reliance on external sensors to prevent adverse health effects is more necessary.  With CD, this need for external equipment is not as strong because of its odor.  CD has an odor threshold at or below the 8-hour Time Weighted Average (TWA), so the user is self-alerted to exposure at a low level and the reliance on external sensors is not as imperative as it is with VPHP.  This makes CD safer for both the user, and any surrounding personnel who may be working nearby.

Visit our Safety page to learn more important differences between chlorine dioxide and hydrogen peroxide.

Is Chlorine Dioxide Carcinogenic?

A major factor in choosing a decontamination method is safety. All decontamination agents are dangerous as this is their function. However, gaseous chlorine dioxide can be used more safely than other fumigation methods due to its chemical properties and safety profile. One example of this is that chlorine dioxide gas is not a carcinogen. Formaldehyde is “known to be a human carcinogen” as described by the US National Toxicology Program. Formaldehyde was once a widely used method for decontamination, but its classification as a carcinogen has limited its use and caused it to be banned by some health agencies. The ACGIH designates vapor phase hydrogen peroxide (VPHP) as an A3, Confirmed Animal Carcinogen with Unknown Relevance to Humans. Chlorine dioxide gas is not considered to be carcinogenic, with no health organization listing CD as a carcinogen of any kind. In fact, it is used to treat fruits, vegetables, poultry, and other food products. Chlorine dioxide has also been used in the treatment of drinking water since the 1920’s both domestically and internationally.

Do you have safety concerns about the use of chlorine dioxide for decontamination? Attend our CD Gas 101 webinar on September 18th to ask questions and learn more.

Biological Safety Cabinet (BSC) Decontamination

The Class III Biological Safety Cabinet (BSC) is a gas-tight enclosure designed for work with highly infectious microbiological agents and for the conduct of hazardous operations and provides maximum protection for the environment and the worker.  A Class III BSC is typically decontaminated on a periodic basis and always before filter change out and repairs. Formaldehyde and chlorine dioxide gas are the only approved decontamination methods by NSF International. However, chlorine dioxide gas provides a much quicker cycle time than formaldehyde, is not a carcinogen, and does not leave a residue.

The Tufts New England Regional Biosafety Laboratory (RBL) is dedicated to the study of existing and emerging infectious diseases, toxin mediated diseases, and medical countermeasures important to biodefense. The facility’s two main decontamination choices are vapor-phase hydrogen peroxide (VPHP) and chlorine dioxide (CD) gas. Both agents are known to be efficacious, and both are sterilants. VPHP has been used longer, and many papers have been published on the process. Some issues of concern were that VPHP condenses and, when it does, the droplets become more aggressive or concentrated. Because of the increased concentrations, it has been documented to damage painted surfaces, epoxy surfaces, and electronics. Additionally, VPHP vapors have been shown to have limited distribution and penetration abilities. CD easily penetrates and distributes into all spaces. It covers an entire room, penetrates deeply into equipment, and gets into the hard-to-reach places. Setup is simple and requires very few extras (only 1 or 2 fans and a portable humidifier). Based on the needs to decontaminate this RBL, CD gas was the best choice as it provided complete decontamination of all surfaces within the spaces and inside the Class III BSC.


The Class III BSC can be decontaminated as part of the room (by opening the gull wing door), or it can be decontaminated on its own through use of the built-in connectors. The components needed are RH probe, mix box (which contains a humidity generator), blower motor, DC/AC controller, pressure relief scrubber, and the Minidox generator. The CD gas concentration is monitored via a gas sample port. This hose is connected to the Minidox, which then, on the basis of real-time readings, activated the gas injection system as needed. The scrubber removes any CD gas during this process. A standard cycle of 5 mg/L for 30 minutes of exposure is often used for Class III BSCs. However, due to the nature of this particular facility’s work, the cycle time was extended to 45 to 60 minutes. All biological indicators (BIs) were repeatedly killed, and no issues of corrosion were evident. All components continue to remain free of any imperfections. Due to that, chlorine dioxide gas is now the method of choice for the decontamination of Class III BSCs.

To read more about Tufts New England Regional Biosafety Laboratory’s utilization of chlorine dioxide gas for both BSC and room decontamination, click here.