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Explained: How to measure aerosols with a particle counter, after accounting for dust

The concentration of aerosols in public spaces can give a measure of infection risk.

Written by Kabir Firaque | New Delhi | Updated: December 29, 2020 8:09:25 am
coronavirus spread, aerosols measurement, particle counter, Covid virus concentration, Indian express newsMany particle counters are available in the market. While this study used a device marketed as Fluke 985, the researchers said similar results were obtained with other particle counters.

The novel coronavirus spreads primarily via aerosols, or tiny droplets generated by a cough or a sneeze that may be carrying the virus. The concentration of aerosols in public spaces, therefore, can give a measure of infection risk, but making that measurement usually requires specialists and specialised equipment.

Now, scientists have demonstrated a simpler alternative: use a commercially available particle counter. The study is published in Physics of Fluids, a journal from the American Institute of Physics.

Many particle counters are available in the market. While this study used a device marketed as Fluke 985, the researchers said similar results were obtained with other particle counters.

The reading you get from such handheld particle counters, however, will include background dust besides the aerosols. How can you distinguish these dust particles from aerosols that arise from people breathing, speaking, sneezing, and coughing? The researchers overcame this with a simple subtraction.

“We can measure the amount of dust particles when there are no aerosols and then take the difference with when people do generate aerosols by speaking or coughing. It’s just a simple subtracting of the background,” Daniel Bonn, one of the authors of the study, said by email. Dr Bonn is a physicist at the University of Amsterdam.

While the device used comes with six different size channels — 0.3, 0.5, 1.0, 2.0, 5.0, and 10.0 microns ( 1 micron is a millionth part of a metre) — a lot of the dust is so fine that aerosols in that range cannot really be measured. More than 98% of the dust, in fact, is contained in the first two channels (smallest particles) of size 0.3 and 0.5 microns. The study did not consider particles in these channels for the aerosol assessment. “But there’s a reasonable sized range where you can detect the aerosols,” Bonn said.

For validation, the researchers compared their measurements with those from specialised laboratory techniques. Aerosol concentration is often measured using a technique called laser diffraction, in which a laser beam passing through a sample lights up different-sized particles differently. Results from this highly specialised technique and the method used in the study, the researchers found, matched up perfectly.

Bonn noted that the results are not unique to the device they used, and can be extended to other particle counters as well. “I bought a cheap one (50$) that I keep on my desk; the air quality index turns red if there are too many aerosols or small dust particles. In both cases you want to open a window,” Bonn told The Indian Express. “Anyone can do it.”

The findings suggest that well-ventilated areas can have aerosol concentrations more than 100 times lower than poorly ventilated areas, such as public elevators or restrooms. Ventilation, therefore, plays a large role in indoor spaces.

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