Focusing on Droplets and Aerosols in Virus Transmission

We know that infected people generate and release droplets and aerosols by coughing, breathing, speaking, etc. Others who have contact with these droplets or aerosols may risk infection. Determining the exposure and infection risk for these individuals raises many questions. For example: 

• How do these particles spread in a room? 
• How does the air circulation in the room affect how far these particles can travel from the point of release? 
• How does the size distribution of the particles affect the dispersion and spread of the aerosol cloud? 
• What is the viral content and its viability within the aerosol droplets or particles? 

For years, our research customers have found TSI particle instruments to be valuable experimental tools in addressing questions like these.

The Evolution of Human-Generated Aerosols

Aerosols emitted by people when breathing, talking, sneezing, and coughing are liquid droplets. Because outside air is usually not as humid as the air inside a human respiratory system, the droplets begin to evaporate soon after emission. Evaporation shrinks them, which affects how far they can travel. 

The largest droplets settle onto the floor or other surfaces, due to gravity. The remainder stay airborne for some time, depending upon their final size after evaporation is complete. Droplets eventually become tiny solid particles that may contain viable microorganisms, including viruses. The smaller the particles are, the longer they can stay suspended in air, and the better they are able to spread over longer distances. We know that very small droplets and particles are able to deposit deep into the respiratory tract. Size, distribution, and concentration are particle characteristics that are relevant to the above research questions. Consequently, it’s vital to be able to measure these characteristics accurately. 

Aerosol Size Distribution Measurements

Sizing Solutions for 1 Micrometer and Larger

For measuring particles in the 0.5 to 20 µm range, the Aerodynamic Particle Sizer® (APS™) 3321 spectrometer is a field-proven instrument. It uses a robust ‘time of flight” technique that determines the aerodynamic particle diameter, a parameter that is commonly used to characterize the deposition of particles in the human respiratory system. The APS™ 3321 spectrometer is used by the US Army Lab to measure particles emitted by human sneezing. 

The Optical Particle Sizer (OPS) 3330 is another instrument that can measure airborne particles in the 0.3 to 10 µm size range, but with a lower size resolution compared to the APS™ 3321. This spectrometer uses a light-scattering technique to provide a response that is dependent on the refractive index of the measured particles. For example, the illustration below shows an experimental setup using TSI instruments. In this experiment, researchers sought to 3321 measure the size distribution and concentration of aerosol particles emitted by a person during coughing events. 

Sizing Solutions for Sub-Micron Particles

For measuring particles in the 1 to 1000 nm size range, TSI offers instruments based on the electrical mobility classification, followed with counting by a condensation particle counter. The Scanning Mobility Particle Sizer™ (SMPS™) spectrometer is configurable to measure different size ranges depending on the specific application; they have excellent sizing resolution and particle concentration range. The SMPS™ 3938 spectrometers made by TSI are serving worldwide in academia and industry for a variety of applications, including virus physical characterization studies. 

Leakage Around Respiratory Protection Devices

On a day-to-day basis, the most important question for a respiratory protection device is whether it is protecting the person wearing it. Correct fit of a device to the person’s face is a key component of that protection. The PortaCount® Respirator Fit Tester objectively measures how completely a mask seals to a wearer’s face. It is able to quantitatively fit test many types of respirators—gas masks, SCBAs, respirators, and disposables like N95s and FFP3s. 

A PortaCount® Respirator Fit Test system can identify a leaking facemask.

Size Selective Sampling for Biological Assays

When droplets and aerosols containing viruses travel across a room or other space, are they still viable? What about droplets or aerosols that have passed through a mask or other filter? Researchers are hoping to answer these and related questions. 

Particle size plays an important role in the infectivity and survivability of airborne viruses.¹ Because of this, virus viability in aerosol particles is often a concern in two applications: characterizing ambient air, and biological filtration efficiency or BFE testing. 

Characterizing Ambient Air

Airborne infectious viruses are typically difficult to recover due to their extremely low concentration in air and because of limitations of commonly used air samplers. SARS-CoV-2 RNA has been found in air samples inside patient rooms.² Reliable sampling methods for air sampling, including measurements of culturable virus, are crucial for understanding the role of ventilation systems in infectious aerosol exposure.

Measuring the Biological Filtration Efficiency (BFE) of Respiratory Protection Devices

Air that has passed through a respiratory protection device, such as a surgical mask can be tested to verify that the mask material retains viable microorganisms. In this case, testing the filtration efficiency of the material (media) used to make respiratory protection devices is key. 

BFE takes that notion a step further by quantifying how effective mask media is at capturing viable microorganisms. A United States standard test method (ASTM F2101) measures the BFE of medical facemask materials, calculated as the ratio of the upstream bacterial challenge to downstream residual concentration, and uses Staphylococcus aureus as the challenge organism. The equivalent version of this standard (EN 14683) applies in Europe for the same purpose. This same standard procedure is applicable to viral filtration efficiency measurements using bacteriophase phiX174 as the challenge organism. Read about a similar experiment in Italy: Masks are tested at Sant'Orsola: Laboratory set up in record time.

When performing BFE tests, standards require a cascade impactor for sampling. This allows a BFE calculation by particle size, since cascade impactors collect samples in a size-segregated fashion. The TSI 100NR MOUDI™³ is a cascade impactor that can segregate the sampled particles in eight size fractions between 0.18 and 10 µm. Similarly, the MOUDI™ 110NR has ten stages between 56nm and 10 µm. These impactors have sharp collection efficiency curves that allow users to assess the virus content and viability of the collected samples as a function of aerodynamic diameter.

Preserving Viability of Sampled Viruses 

Maintaining the viability of a collected virus throughout sampling and handling is essential to ensure accuracy. Impaction plates filled with Agar media or gelatin filters help to maintain virus viability during sampling. Conventional MOUDI™ models (100NR and 110NR) enable users to perform quantitative biological assays, such as polymerase chain reaction (PCR) analysis.

Every day, TSI helps researchers answer important questions and reach their goals. How can we help you advance knowledge in your field?

References

1. Zuo et al 2013, https://www.tandfonline.com/doi/full/10.1080/02786826.2012.754841 
2. Liu et al 2020, https://doi.org/10.1101/2020.03.08.982637
3. MOUDI™ stands for Micro-Orifice Uniform Deposit Impactor.