What is the aerosol and droplet risk in COVID-19 transmission?

In a recent study posted to the medRxiv* preprint server, researchers assessed the superposition of aerosol and droplets in coronavirus disease 2019 (COVID-19) transmission.

Study: Superposition of Droplet and Aerosol risk in the transmission of SARS-CoV-2. Image Credit: Evgenia.B/Shutterstock
Study: Superposition of Droplet and Aerosol risk in the transmission of SARS-CoV-2. Image Credit: Evgenia.B/Shutterstock

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is spread through three different methods: direct contact with a fomite (an infected person or object), aerial transmission by droplets, and aerosols. The last of these, the distinction between "droplets" and "aerosols" and their respective propensity to spread infection, has been controversial but is undoubtedly significant.

Extensive research is required on the relative contributions of droplets and aerosols to infection risks or the implications for mitigation strategies.

About the study

In the present study, researchers estimated the amount of virus in particles exhaled by a SARS-CoV-2-infected person and inhaled by an uninfected person to assess the risks of aerial transmission.

When an infected individual exhales, they release virus-laden droplets into the atmosphere. The medium-sized droplets hit a person, touch something, or fall to the ground. The tiny droplets or aerosols can drift through the air for hours, cover great distances, and perhaps enter a distant person's respiratory system. By assuming that this number is proportional to the initial droplet size as volume, the team estimated the number of virions disseminated across the respiratory tract. The team compared the amounts of virions transmitted at various distances to obtain a more accurate exposure estimate. In the literature, "droplet" and "aerosol" describe whether an object poses a concern for short- or long-range aerial transmission.

The following example was employed to define terms, list presumptions, and describe the analytical process. A non-infected person, Bob, is at risk of contracting an infection from airborne contact with an infectious person, Alice.

  1. Bob and Alice spend a lot of time together in a small space (an hour or more). Alice exhales droplets of saliva and lung fluid that vary in size, but all have the same initial anticipated concentration of virions per unit volume. Alice's droplets were divided into two groups based on size. 
  2. The team referred to droplets having a diameter of 8 to 75 m as medium droplets. Gravity pulls these downward, and if they get in Bob's mouth, nose, or eyes, they could potentially infect him. This is known as Route 1.
  3. Aerosols or small droplets are defined as droplets between 1 and 8 micrometers in diameter. These float across the air. They could be dangerous for Bob if he inhales them. This was considered Route 2.
  4. Two distinct models were applied—a rapid decay model and an inverse distance square decrease—to extrapolate what the Route 1 droplet inhalation would be at various distances.

Based on a mathematical model, Chen et al. analyzed short-range droplet transmission. According to them, there are two primary ways for short-range non-fomite transmission: large droplets that are projected at the same height directly into a person's mouth, nose, and eyes who is facing the source, and small droplets that enter the air stream and are inhaled.

Chen et al. concluded that droplets of medium size fall to the earth within a meter. Smaller droplets move with the air; larger ones move farther but will land on the ground because they descend horizontally more slowly. Furthermore, they conclude that most exposure occurs through inhaled droplets rather than deposited droplets at distances over 0.3m in the case of talking and over 0.8m in the case of coughing.

Results

The team discovered that facing an individual at a distance of 1 m produces 17 pL of inhaled droplets for every 1 L of exhaled droplets. By first calculating what portion of that 1 L gets aerosolized using the numbers, the long-range risk can be estimated. Although environmental factors alter the diameter at which particles remain airborne, the team assumed that an inhaled particle was aerosolized when it had a diameter of less than 8 m.

The measurement with the highest measured ventilation, 3.2 air change rates per hour (ACH), was employed. The decline of aerosol danger with distance is probably considerably less pronounced in the absence of ventilation; in fact, in small enclosed spaces, the steady state may be very uniform.

Conclusion

The study findings quantified the relative contributions of different routes via a literature review. Medium droplets elicited the largest infection risk (measured as exposure to droplet volume), when near the infectious person out to a distance of about 1 m, according to three published studies that assessed droplets larger than 8 m, and smaller than 8 m (small droplets or aerosols), and smaller than 75 m (medium droplets).

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Bhavana Kunkalikar

Written by

Bhavana Kunkalikar

Bhavana Kunkalikar is a medical writer based in Goa, India. Her academic background is in Pharmaceutical sciences and she holds a Bachelor's degree in Pharmacy. Her educational background allowed her to foster an interest in anatomical and physiological sciences. Her college project work based on ‘The manifestations and causes of sickle cell anemia’ formed the stepping stone to a life-long fascination with human pathophysiology.

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