Prior to the eruption of Taal Volcano or the standstill brought about by the COVID-19 pandemic, most Filipinos had probably never heard of N95 respirators. Even now, six months and counting under one form of community quarantine or another, there persists the idea that N95 respirators act like a net or a colander—that they easily filter out large particles but let smaller ones slip through. This idea, while born of everyday experience, fails to capture the astounding ingenuity and materials science behind N95 respirators.
Unknown to most people, N95 is an air filtration standard for respirators set by the National Institute of Occupational Safety and Health, a United States (US) federal agency. Respirators are classified based on their oil resistance and their particle filtration efficiency; the “N” in N95 means that the respirator is not resistant to oil, while “95” means the respirator can filter out 95 percent of all airborne particles.
There also exist N99 respirators, which filter 99 percent of all airborne particles, and respirators designated “R” or “P” that are somewhat resistant or fully resistant to oil, respectively.
Charged up
N95 respirators in their current form owe their existence to Dr. Peter Tsai, a Taiwanese American and former University of Tennessee (UT) professor who first patented the respirator’s underlying technology in 1995. His patent was for a process of electrostatically charging a web or a film—a process that was suitable for use in electrocharged filtration in respirators.
“Microfibers have a capability to capture the small particles, but if we can put the charges into the fibers, in addition to the mechanical mechanisms, we [can] use electrostatic force to attract the particles,” he elucidates.
This electrostatic charging is achieved through a process called corona charging—entirely unrelated to the SARS-CoV-2 virus that causes COVID-19—which uses high voltage “to generate [a] highly-intensified electric field…[that] ionizes the air, so the air becomes charged,” Tsai explains, “then we use the same electrical field to introduce the charges into the fibers.”
Interestingly enough, Tsai notes that corona charging also creates “opposite charges at the same time during the charging process, so the fabric has both positive and negative charges.” In the process, the microfibers become quasi-permanent electrets, which can be described as the electric version of magnets. In the same way that magnets create a magnetic field due to being a magnetic dipole having a north and south pole, electrets create an electric field due to having both positive and negative charges—an electric dipole.
When asked how long these electrets last, Tsai replies that the charges on a respirator can stay for at least five years “if it is properly made”. He assures that he has “tested the material after 15 years, and the [filtration] efficiency is still there,” noting that proper storage can help prolong the charge on the mask. “You are not supposed to store this kind of respirator in a warehouse that has a very high temperature,” he adds.
Layers all the way down
Composed of several layers of non-woven polymer fabrics, Tsai says that N95 respirators typically consist of three layers of nonwoven fabrics. These three are a protective external layer; an electrostatically charged middle layer; and a thick, form-fitting inner layer. The process that produces these nonwoven fabrics is called melt blowing and is not too dissimilar from the method used to make cotton candy.
Tsai elaborates that this generally begins by forcing a reel of melted polymer “through small holes to make fibers…then we use hot air to attenuate the fibers—to pour the fibers—from a thick size to a very thin size; in this case, about two microns.” As the attenuated fibers depart from their source, they cool and deposit onto a collection vessel such as a conveyor belt or drum. Enough of these fibers bonded onto one another eventually forms a fabric, sans weave.
Multi-faceted filtration
“If the requirement is to filter out 95 percent or more particles down to 300 microns in diameter or larger, there is no single filtration method that works,” explains Dr. Mahesh Bandi, an associate professor from the Okinawa Institute of Science Technology. He adds that N95 masks employ “a range of mechanisms” when filtering the air.
The first of these is inertial impaction, which works for particles larger than one micron in size, or one-thousandth of a millimeter. They are large enough that their inertia carries them in what is essentially a straight line into the vast web of microfibers where they get lodged. Once caught, these particles stay there “using what we know in Physics as van der Waals forces, which are weak but sufficient to keep the particles pinned down,” Bandi explains.
Conversely, small particles—those less than 0.1 microns in size—diffuse into the nonwoven fabric of the respirator. At microscopic scales, these particles are pushed around in random directions by air currents in a phenomenon known as Brownian motion. Due to these erratic movements, small particles are easily filtered out by the respirator due to an increased likelihood of contact.
Finally, particles between 0.1 and one micron in size are the most difficult to intercept. As Bandi elaborates, “We use the method known as electrocharged filtration,” which works “[by attracting] charged dust and aerosol droplets [toward] them [the fibers] due to long-range electrostatic Coulomb forces.”
Back to help
As the COVID-19 pandemic incapacitates healthcare systems around the world, shortages of personal protective equipment (PPE) threaten to worsen the already dire conditions experienced by healthcare workers. Despite having retired in 2018, Tsai was recently motivated to embark on new research specifically focused on the disinfection of N95 respirators. “After SARS, a lot of papers about [the] sterilization of masks [were published] in case there was a shortage,” he recalls. Researchers found that keeping respirators under sustained dry heat for specific amounts of time was able to kill the SARS-CoV virus, and the same methods have worked for SARS-CoV-2, with some minor tweaks.
Tsai relays, “Dry heat [of] 70 degrees Celsius for 60 minutes will kill [the virus that causes] COVID-19, so this can be used for the sterilization of N95 [respirators], but you need to handle it carefully: the heat needs to be uniform.” Using ultraviolet light to disinfect also works, he adds, although there is a concern that this only disinfects the surface of the respirator. The US Food and Drug Administration, additionally, has greenlighted the use of hydrogen peroxide to disinfect respirators.
While it seems like a good idea, Tsai cautions against using a 70 percent alcohol solution to disinfect N95 respirators for reuse. He reasons, “[Alcohol] will erase the charges [in the mask], so you cannot use ethanol or alcohol to do a sterilization…but 70 percent ethanol can be used to do the sterilization for disposing [of] respirators.” According to the former professor, without the electrostatic charging, “the [filtration] efficiency [for mid-sized particles] is reduced to one-tenth of the charged [filtration] efficiency.”
