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The electrostatic sprayer method kills nearly 100% of pathogens. It also kills the COVID-19 virus. But is that overkill?
Bacteria love surfaces. They need them to survive and thrive. Through adhesion to surfaces, they can form biofilm, a slimy extracellular matrix, that makes them hundreds of times more effective in resisting antibiotics.1
Viruses also love to cling to things. That’s why one of the first responses to SARS-CoV-2 was to find out just where and how long it survived outside the body.
And early research duly alarmed the world. Laboratory studies suggested that the virus could last anywhere from hours on clothing to 3 days on paper money to 6 days on hard surfaces such as plastic and stainless steel. In early 2020, scientists identified RNA of SARS-CoV-2 on a variety of surfaces in the cabins of both symptomatic and asymptomatic infected passengers aboard the Diamond Princess up to 17 days after the cabins were vacated but before disinfection procedures were conducted.2 They could not at the time determine whether there was any transmission from surfaces.
More recently, investigators have found that surfaces can be positive for the COVID-19 virus but that the viral material isn’t actually able to infect cells. Transmission of COVID-19 via surfaces is rare. Emanuel Goldman, PhD, a professor of microbiology at Rutgers New Jersey Medical School, became irritated by the way COVID-19’s viability on fomites (inanimate surfaces or objects) was being tested. In a comment letter to The Lancet Infectious Diseases in August 2020, Goldman wrote that the risk was being exaggerated.3 Although investigators were claiming virus survival of up to 6 days, he said, they were placing very large initial virus titer samples on the surface being tested.
“None of these studies present scenarios akin to real-life situations,” he said. In an interview with Nature published in January 2021, he said, “The viral RNA is the equivalent of the corpse of the virus. It’s not infectious.”4
So how to deep-clean contaminated surfaces? One relatively new way is with electrostatic sprayers (ESSs). The method has been used for more than 50 years in agriculture to reduce spray drift when applying pesticides to crops, but it has been used only in the past 10 years or so in health care settings. Thanks to COVID-19, the past 2 years have been profitable for disinfectant manufacturers. By the end of 2020, global sales of surface disinfectant totaled $4.5 billion—a jump of more than 30% over the previous year.4 To give that context: In 2019, the market size was expected to cross just $1.9 billion by 2025.5
The COVID-19 virus does not last long on surfaces that have a lot of holes or microscopic grooves, nooks, or crannies, said Frank Esper, MD, in an article for Cleveland Clinic’s website—but those areas are where ESS shines.6 Most pathogens love complex surfaces and, given the chance, will settle right in. Norovirus, for instance, which can infect with minute amounts, can stay transmissible on objects and surfaces for days or weeks. Moreover, it can survive some disinfectants.7
ESS offers a fast way to clear out pathogens from wide areas. EvaClean, which is certified by the Occupational Safety and Health Administration, has an ESS that uses disinfecting tablets registered with the Environmental Protection Agency (EPA). EvaClean states it takes approximately 30 to 45 minutes to clean and disinfect a hospital room using traditional methods but its ESS can reduce those times by 40%.8 Moreover, depending on the disinfectants used, the ESS can kill 99.99% of bacteria found in biofilm, Clostridioides difficile, Mycobacterium tuberculosis, and other viral and fungal infections, EvaClean states.
An ESS is based on the universal interaction between positive and negative forces. (Remember rubbing a balloon to create static and then sticking the balloon to a wall or your head? Same general principle.) Most surfaces have a (grounded) negative surface charge. The sprayer positively charges the disinfectant as it passes through the sprayer nozzle, and the charged disinfectant droplets seek out and stick to negatively charged surfaces—that is, they produce static.
Like magnets that shoot away from each other when held rear to rear, the positively charged particles in the droplets repel each other and spread out—at many times the force of gravity—looking for exposed surface. The spray droplets can also reverse direction, moving against gravity and coating all sides of the target object. Thus, one of the advantages of electrostatic spraying is that it offers “wraparound” coverage, meaning it can quickly coat high-touch surfaces that may be time consuming to clean such as pens, light switches, faucets, and keyboards.
Because the solution comes out in a thin mist, it covers the targeted area evenly. Sprayers with smaller droplet sizes may be safer for sensitive equipment. Surfaces that are already covered will repel the charged droplets, which redistributes them, making the task that much more efficient and reducing the chance of overapplication.
In addition to time savings, the ESS offers cost savings. Used properly, the ESS can save on disinfectant and cut cleaning time by 50% or more, compared with spray-and-wipe methods. Moreover, according to an EPA decontamination study, because an ESS provides more uniform distribution, using a minimal amount of solution, it significantly reduces waste streams and costs associated with liquid hazardous waste disposal. Compared with a backpack sprayer, the ESS generated almost 75 times less liquid waste.9
On the Other Hand…
A downside is that although the droplets give the disinfectant sustained contact with the contaminated surface, the protection doesn’t last forever. The sprayer’s ability to deposit smaller amounts of disinfectant over a wide area may be both an advantage and disadvantage, according to the EPA. With less disinfectant applied, disinfection efficacy may diminish if the surface does not remain wet for the required contact time.10 (See chart below.)
The required time often is specific to the pathogen. One study, for instance, found a 5-minute contact time was required for C difficile spores.11 On real-world surfaces, C difficile spores were reduced but not eliminated completely after a single spray application of the disinfectant. The investigators say that failure to eliminate the spores is partly because curved or vertical surfaces typically had drying times of approximately 2 minutes. Thus, in settings where C difficile is a concern, they say, repeated application may be required to maintain 5 minutes of wet contact time. Moreover, left intact, the solution will last long enough for the pathogen to be deactivated, but only until the surface is touched again.
Some of the advantages of ESSs depend on the components.
A team of EPA investigators compared several spray parameters for 6 electrostatic sprayers, 2 foggers, and 1 hand-pumped garden sprayer.12 “The electrostatic spray process is complicated and involves multiple physical phenomena,” they wrote in PLoS One on November 30, 2021. Parameters that might affect a disinfectant’s ability to inactivate the virus on surfaces include spray droplet size distribution, electrostatic charge, ability of the spray to wrap around objects, and loss of disinfectant chemical active ingredient due to the spray process. Also, not all electrostatic sprays carry the same charges. In this study, 2 ESSs that used alternating current had a negative charge, whereas the battery-powered ESS carried a positive charge.
The investigators tested the sprays on a variety of surfaces, including a metal trashcan and other surfaces such as a stepladder, a clip-on lamp, and a fold-out chair.
For all the devices evaluated, “[w]e were surprised to find there was not much of a ‘wrap-around’ effect of the spray from ESS, a claim that some ESS suppliers use for marketing—albeit this is just based on the limited tests we did,” Joseph Wood, senior research engineer with the EPA’s Center for Environmental Solutions and Emergency Response, and primary author on the study, told Infection Control Today®. For instance, the deposition was greatest at the front of the trashcan, with some minor amounts of spray deposited on the sides and only minimal amounts deposited on the back of the can.
Due to the range in recommended ESS surface coverage, types of surfaces and materials, varying disinfectant chemistries and parameters such as droplet charge, and site-specific environmental conditions, surfaces may not remain wet for the required contact time of the disinfectant. Wood’s top recommendation for using ESS: “Ensure that the surface you are disinfecting remains wet for the required contact time of the disinfectant you are using, and follow all label directions, including use of PPE [personal protective equipment].”
On the plus side, the investigators found that little of the active ingredient in dichlor- and hydrogen peroxide–based disinfectants was lost to the air (ie, below occupational health levels of concern) and the active ingredient concentrations collected 3 feet away from the nozzle did not decrease.
Those are serious considerations. For one, although ESSs allow for contactless cleaning, aerosolized disinfectant can hang in the air for long periods of time, especially if the area is not well ventilated. Aerosolized disinfectant can irritate skin, eyes, and airways.
The Centers for Disease Control and Prevention and EPA advise extreme caution when using ESSs, stating that they should be used only:
An ESS does indeed kill nearly 100% of pathogens. It also kills the COVID-19 virus. But is that last function overkill? Fomite transmission of COVID-19 is a “ghost problem,” according to Joseph Allen, associate professor and director of the Healthy Buildings program at the T.H. Chan School of Public Health at Harvard University; Charles Haas, professor of environmental engineering at Drexel University; and Linsey Marr, professor of civil and environmental engineering at Virginia Tech.13
In an opinion article for The Washington Post in December 2020, they advised that it’s much better, when dealing with an airborne virus, to “shift our effort toward cleaning shared air, not shared surfaces.”
They wrote, “We don’t have a single documented case of COVID-19 transmission from surfaces. Not one….When we look at [the] entire causal chain, it’s easy to see that if fomite transmission is happening, it’s minor and certainly not driving the pandemic.”
Investigators, though, are still not saying definitively that COVID-19 is never spread by surface contact. Fomite transmission is difficult to prove, in part because respiratory transmission from asymptomatic individuals cannot be ruled out. Furthermore, in hospital settings, COVID-19 is still not the only infectious threat. Bacteria and viruses and more still linger on bed frames, bedside tables, monitors, and wheelchairs.
One of the lessons learned from this pandemic has been that handwashing is a primary preventive of infection. One investigative team says that even with low adherence to handwashing—with just 1 in 4 individuals disinfecting their hands after surface contact—median infection risks from fomite contact were reduced by 0.16 to 2.2 log. With high adherence—3 of 4 individuals disinfecting—median risks decreased by 3.4 to 4.0 log.10,14
Enhanced cleaning is “hygiene theater,” the authors wrote in The Washington Post article. But because surface transmission is still—although remotely—possible, Marr told Nature, handwashing is still crucial. Despite the efficiency of ESSs, the oldest disinfecting tool—handwashing—may be the best.
Jan Dyer is a writer and editor, specializing in clinical topics. She lives in Suffern, New York.