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When It Comes to Droplets, Size Matters

In the past year, the words aerosol, particle, and droplet have all been brought front and center as we learn more about SARS-CoV-2, the virus that causes COVID-19 and its transmission. In the world of surface disinfection, these words are equally important particularly when it comes to evaluating new disinfection technologies. However, in my experience both inside and outside the public health community, these words tend to be used interchangeably. It leads me to question how the cleaning industry is supposed to differentiate and identify the information most relevant to them and the safety of their workers. Are all droplets really aerosols? Are particles the same as droplets? How does the particle or droplet size impact the safety of the various surface disinfection applications?

To help clarify, let’s break it down:

  1. Aerosol: A suspension of tiny particles or droplets in the air.1 Aerosol is often used both to define respiratory droplets and particles that are small in size, as well as to explain the collection or cloud of these droplets in the air.2
  2. Particle (also known as particulate matter): Tiny pieces of solids or liquids that are in the air. Examples of these particles include things like dust, smoke, dirt, and drops of liquid. Particles can range in sizes. Some are big enough (or appear dark enough) to see with our naked eye, e.g., you can often see smoke in the air. While others are so small that you cannot see them in the air. As illustrated in the below image, the range of particle size can vary considerably.3
  3. Droplet: Simply defined as a tiny drop of liquid.4 A droplet is a type of particulate matter.

In April 2021, the Centers for Disease Control and Prevention (CDC) released their updated guidance for prevention of COVID-19 when using electrostatic sprayers, foggers, misters, or vaporizers. In this guidance, they caution operators that “these devices aerosolize chemicals, or suspend them in the air, and they can stay in the air for long periods of time, especially if the area is not well ventilated.” In terms of safety, the size of the droplets or particulate matter in the air is what matters when determining if something is truly inhalable or breathable and sizes can vary greatly.6 Large droplets or particles generally fall to the ground quite quickly because of gravity. To provide scale, inhalable particles or droplets that enter the airways are usually anything below 30 µm (30 microns) and respirable particles or droplets are typically defined as anything less than 10 microns in diameter.7 5  The CDC guidance correctly points out that users should review the requirements and safety protocols for these technologies, but it is important to knowthat the technologies provide different levels of performance and require different safety precautions. 

So where do the common surface disinfection technologies fall in terms of particle or droplet size?

  • Misters or Foggers have the smallest particle size (typically average size about 10-30 microns).8,9 These droplets are so small that they often hang in the air for a while before being pulled down by gravity. These small particles are also well within the breathable range. This means they have the potential to reach the deep lung after inhalation. Because of this, you will see higher Personal Protective Equipment (or PPE) requirements, instructions for sealing vents and leaving the area during fogging, and longer re-entry times.
  • Trigger and Pump Sprayers deposit large droplets on a surface (average > 100 microns). These large droplets are dependent on gravity to fall to surfaces, have patchy distribution patterns, and are clearly visible to the applicator.
  • Electrostatic Sprayers have a more medium-sized particle size (average > 40 microns).8 If you were goldilocks, you might say they are “just right” because they have high coverage rates and emit droplets that are on average outside the inhalable and respirable range. This means that vents do not need to be sealed and there are generally no or very low re-entry times for bystanders when using this technology. Additionally, unlike the trigger sprayer or foggers and misters, the electrostatic particles have an electrical charge making them attracted to the surfaces being sprayed. This results in the particles dropping out of the air quickly and not just waiting for gravity to pull them down.
infographic showing droplet size of solution through electrostatic sprayers versus misters

As you can see, foggers, trigger sprays and electrostatic sprayers offer a range of disinfecting benefits. While electrostatic sprayers and foggers are often considered similar, their performance demands different levels of personal protective equipment (PPE), room preparation, and room re-entry wait times. Regardless of the technology you decide is right for your cleaning and disinfection protocols, the droplet size of the delivery system does make a difference.

References

1. CDC. The National Institute for Occupational Safety and Health (NIOSH): Aerosols [Internet]. Centers for Disease Control and Prevention. 2010 [cited 2021 Apr 28]. Available from: https://www.cdc.gov/niosh/topics/aerosols/default.html
2. CDC. Science Brief: SARS-CoV-2 and Potential Airborne Transmission – Updated Oct. 5, 2020 [Internet]. Center for Disease Control and Prevention. 2020 [cited 2021 Apr 28]. p. 4. Available from: https://www.cdc.gov/coronavirus/2019-ncov/science/science-briefs/scientific-brief-sars-cov-2.html
3. CDC. Air Quality: Particle Pollution [Internet]. Centers for Disease Control and Prevention. 2019 [cited 2021 Apr 28]. Available from: https://www.cdc.gov/air/particulate_matter.html
4. Merriam-Webster. Droplet [Internet]. 2021 [cited 2021 Apr 28]. Available from: https://www.merriam-webster.com/dictionary/droplet#:~:text=Medical Definition of droplet,drop (as of a liquid)
5. CDC. Safety Precautions When Using Electrostatic Sprayers, Foggers, Misters, or Vaporizers for Surface Disinfection During the COVID-19 Pandemic [Internet]. Centers for Disease Control and Prevention. 2021 [cited 2021 Apr 28]. Available from: https://www.cdc.gov/coronavirus/2019-ncov/php/eh-practitioners/sprayers.html
6. Baron P. Generation and Behavior of Airborne Particles (Aerosols) [Internet]. National Institute for Occupational Safety and Health. 2020 [cited 2021 Apr 28]. Available from: https://www.sdpt.net/rd/Aerosol_101.pdf
7. European Aerosol Federation. Guide on Inhalation Safety Assessment for Spray Products. 2013.
8. EPA. Instructions for Adding Electrostatic Spray Application Directions for Use to Antimicrobial Product Registrations [Internet]. United States Environmental Protection Agency. 2021 [cited 2021 Apr 28]. Available from: https://www.epa.gov/pesticide-registration/instructions-adding-electrostatic-spray-application-directions-use
9. WHO. Space spray application of insecticides for vector and public health pest control [Internet]. World Health Organization – WHO. 2003 [cited 2021 May 1]. p. 45. Available from: https://apps.who.int/iris/bitstream/handle/10665/68057/WHO_CDS_WHOPES_GCDPP_2003.5.pdf?sequence=1