The global SARS-CoV-2 pandemic has exposed serious and legitimate shortfalls in world supply chains of important materials. For example, the skinny, elongated cue tip (called a swab) that is used to collect nasopharyngeal or oropharyngeal samples for Covid-19 testing was only mass manufactured in two factories around the world. Many of us likely remember the massive hand sanitizer shortages that occurred in March and April.
These were early signs that the world was not ready to combat a global pandemic, and the shortfall of personal protective equipment (PPE) has solidified that notion. Reports have been flooding in since March that nurses and other healthcare professionals have been reusing their PPE for weeks on end. The masks, gowns, and gloves are typically designed to be single-use.
That being said, the production of PPE is still behind demand, and that does not seem likely to change in the coming months. Even the Centers for Disease Control (CDC) has released guidelines on how to best extend the life of the limited supply of various pieces of PEE. One of the major barriers to reuse is the issue of sterility. For example, a nurse uses the same mask for a 14-hour shift treating Covid-19 patients, are they supposed to take that mask home with them? How can these masks be safely and effectively sanitized? Today, we’ll take a look at emerging research indicating that ozone may a cost-effective and reliable mechanism of N95 mask sterilization.
What are N95 Masks?
N95 Filtering Facepiece Respirators (FFRs), usually referred to simply as N95s, are a type of mask worn by doctors, nurses, and many different individuals in the healthcare field. N95s are very different from cloth masks worn by the general public. Cloth masks have been shown to be extremely effective at blocking respiratory droplets as they leave an individual’s mouth, and they have been shown to reduce the chances of infection to a mask wearer by up to 65%.
These types of masks pale in comparison to N95s. According to the FDA, N95s are intended to act as “a respiratory protective device designed to achieve a very close facial fit and very efficient filtration of airborne particles.” The “95” in the name references the fact that these medical-grade masks block at least 95% of extremely small particles, those that measure roughly 0.3 microns. For scale, there are over 25,000 microns in a single inch. The width of a human hair is 70 to 250 times larger than the particles that N95s block.
What is Ozone?
Ozone exists at room temperature as a pale blue gas that exhibits an odor reminiscent of chlorine. First discovered in 1785 by Martinus van Marum, the utility of ozone has evolved into an area of intense academic research.
The ozone molecule, O3, is much less stable than other allotropes of oxygen, such as O2. This is part of the reason that O3 exists as a reactive molecule and acts as a powerful oxidizing agent. For this reason, ozone is commonly used to sanitize municipal water, recirculating air, and home medical devices.
Mechanism of Viral Inactivation
Ozone has long been used as a disinfectant because of its ability to kill or inactivate bacteria and viruses. But, how exactly does that work? This question has been studied by researchers for several decades. While some of the finer details are still unknown, there is an established body of evidence concerning this topic.
The structure of all coronaviruses, like the virus that causes Covid-19, are relatively similar. Each individual virion (the name for the individual infectious particle) is contained within a viral envelope, which is composed of a variety of proteins. Ozone, being a powerful oxidizing agent (meaning it does a good job of stealing electrons from other molecules), has the ability to disrupt these proteins. Presently, this is the working hypothesis for the mechanism of viral inactivation due to ozone.
Ozone to Sterilize N95
Now that we understand both N95 respirators and ozone, let’s combine them together: ozone has been shown to be 99.99% effective in reducing the viral load of enveloped viruses present on N95 masks.
Researchers at the Georgia Institute of Technology investigated the effectiveness of molecular ozone as a disinfectant for various forms of PPE. In order to understand their results, we must understand a few parameters of their study:
- The researchers did not test ozone efficacy on SARS-CoV-2 because of its contagious and virulent (dangerous) nature. Instead, they utilized other, similar enveloped viruses: Influenza A and human respiratory syncytial virus A2. The researchers state “these pathogens are similar in form and function to SARS-CoV-2, although not without some differences.”
- Three main types of PPE were selected for study: cloth face masks, PAPR (powered air-purifying respirator) hoods, and N95 respirators.
- The ozone was administered to the materials via three different methods: two ozone cabinets (according to the authors they are “used primarily to disinfect sports and playground equipment in schools, daycare centers, and the like) and a positive airway pressure (PAP) at-home cleaning device.
Researchers found that, under experimental conditions, ozone was able to reduce the viral load on N95 maks by at least 104-fold. The group measured this by infecting masks with a known quantity of virus, treating the masks with ozone, then harvesting the virus from the mask material. Afterward, this harvested virus was used to infect permissive (able to be infected) cells.
It should be noted that humidity played an extremely important role in the ability of ozone to inactivate virus. At high levels of relative humidity (RH), around 70%, an ozone concentration of 20 PPM for an average of 40 minutes was sufficient to produce a decrease in the viral load of 99.99%. At lower RH values, a much longer (up to four hours) treatment time was required to achieve the same level of inactivation.
Ozone Effect on Material Properties
Due to ozone’s ability to act as a strong oxidizing agent, it can degrade various classes of polymer-based plastics and rubbers via a process known as “ozone attack.” This presents a concern due to previous descriptions of stretched rubber being degraded by ozone (N95 masks have rubber straps for attachment).
The researchers confirmed that this phenomenon was problematic, stating “We found that even a short exposure of strained elastics to 20 ppm O3… often resulted in compromised headband integrity or complete failure.” While this effect is not desirable, it can be avoided by removing the rubber straps prior to O3 treatment. The rubber straps themselves can either be replaced or sterilized with other common methods such as ethanol.
A final concern specific to N95 masks is any possible reduction in filtration efficacy as a result of treatment with ozone. The group investigated this topic “in a manner closely analogous to the NIOSH N95 respirator penetration testing guidelines.” They determined that there was no significant reduction in the ability of the N95 mask to effectively filter particulates.
Written by: Patrick O'Hare