Evaporative residues from aerosol droplets key to understanding COVID-19 survivability on different surfaces

A recognized avenue for COVID-19 transmission occurs when airborne droplets from infected individuals fall to a surface that then can be touched by others. Several emerging studies to date show that these virus-laden aerosol droplets, formed by human breathing, speaking, sneezing, and coughing, have the ability to persist on different surfaces for sustained periods of time (hours to even days). What has not been well studied or understood to date is the physical mechanism for how the virus can persist for such long periods of time on various surfaces.

Images of droplets and residuesA recently submitted study by researchers under the direction of Jiarong Hong, faculty at the St. Anthony Falls Laboratory (SAFL) and associate professor in the Department of Mechanical Engineering at the University of Minnesota, devised a series of experiments looking at what happens after aerosol droplets fall on five different surface types and evaporate under different temperature and humidity scenarios. Using both a nebulizer and human exhaled gas to form droplets on glass (coated and uncoated), steel, plastic, and copper surfaces, the research team found that the droplets do not disappear upon evaporation, but leave behind residues that still are large and stable enough to ‘protect’ the much smaller viruses for extended periods of time.

These residues vary in number and shape depending on the physical characteristics of the surface as well as the temperature and humidity of the surrounding environment. The ability to remove residues or preventing their formation is also a function of the surface characteristics and environmental conditions.

Key findings from the study include:

  • Evaporative residues formed on stainless steel, glass, and plastic surfaces have the ability to remain stable for extended durations (up to or more than 24 hours). Copper surfaces do not form residues even under moderate humidity (40%)
  • Higher surface roughness, higher hydrophobicity (how the surface repels water), and low thermal conductivity result in longer residue survival times. Plastic and glass surfaces consistently had the largest residue survival time.
  • When in an environment of higher temperatures and lower humidity, residue formation is lower across all surfaces and if residues do form, they have a shorter survival time. Lowering indoor air humidity (below 15% relative humidity (RH) for metals and below 10% RH for glass/plastics) can completely prevent residues from even forming.
  • Wiping, even with water-absorbent tissues, is the most effective means for removing residues on all surfaces. Ventilation with hot air is effective at removing residues on metal surfaces, but not as effective on glass or plastic surfaces. Lowering humidity levels in indoor environments can also effectively prevent residue formation.

These findings can support development of practical guidelines and prevention measures that can reduce risks of infection, not only for COVID-19, but other viruses whose transmission relies on respiratory droplets as well.

“Our study adds one more piece to the puzzle in our growing understanding of how to help mitigate the risk of contracting COVID-19,” says lead author Santosh Kumar, a PhD student advised by Hong. “By understanding the mechanism of how viruses are able to persist on different surfaces, we can improve our efforts in removing these residues that can make people sick.”

Read the study.