Scientists Find Toilets to Be Covid-Spreading Culprits

Left: Vortex caused by common annular flushing. Right: Large-scale spread of virus particles after flushing. (Photo courtesy of J.-X. Wang)

(CN) — Flushing a toilet is a particularly turbulent affair. The resulting high-speed winds can fling virus particles and bacteria onto unsuspecting bathroom surfaces, quickly spreading viruses such as the novel coronavirus that causes Covid-19.

This is especially true in high-traffic areas such as public restrooms, but remains a potential threat even at home. According to a new study published Tuesday in the journal Physics of Fluids, 40-60% of particles expelled when a toilet is flushed will fly above the seat, and can linger there long enough to be inhaled.

“Fecal-oral transmission is a common transmission route for many viruses, including this SARS-CoV-2,” the study’s authors wrote. “Blocking the path of fecal-oral transmission, which occurs commonly in toilet usage, is of fundamental importance in suppressing the spread of viruses.”

This particularly unpleasant transmission pathway can cause cross-infection of the virus to unsuspecting individuals if preventative measures are not taken.

The authors employed computational fluid dynamics using a volume of fluid model to simulate the flushing process of common toilets, along with a discrete phase model to determine the particle trajectories of expelled matter wafting through the air zone above the bowl.

When a toilet is flushed, water enters the bowl under pressure and mixes with the water seal. The resulting vortices reach an upward velocity up to 5 meters per second — that’s over a third that of gale-force winds. This creates quite a stir and sends particles flying around places you wouldn’t normally expect to find them.

In the future, you may end up sitting on a smart toilet — one with included sensors that can scan for harmful viruses and bacteria by analyzing users’ excreta, or track an emerging pandemic. Another promising toilet design is a water-free version that restricts the turbulent airflow caused by flushing and suppresses the resulting spread of pathogens.

For now, your best bet is to close the lid before flushing and wash your hands regularly.

While bathrooms are certainly risky, it remains more likely that one would be exposed to SARS-CoV-2 going about your other daily business. That’s why social distancing and face masks have become so important.

The authors of another study coming out Tuesday in the journal Physics of Fluids modeled a coughing incident by simulating a subject wearing a mask and comparing the transmission of droplets at various intervals across different areas of the mask to check for leaks and to gauge the effects of coughing.

“Wearing a standard surgical mask blocks the forward jet of droplets but allows leakage around the top, bottom and sides,” co-authors Talib Dbouk and Dimitris Drikakis, from the University of Nicosia, said.

“An N95 mask reduces the droplet leakage around the mask edges during the cough. However, the pressure inside the mask increases during coughs and the turbulent jet is directed through the front. Although both surgical and N95 masks decelerate the turbulent jet, none of them will prevent the droplets entirely from penetrating or escaping the mask.”

N95 masks are the standard protective equipment employed by health care professionals across the world. When worn correctly, these can block transmission of airborne particles by up to 95% — hence N95. Surgical masks, on the other hand, are loose fitting, prone to leaks and often worn in conjunction with an N95 mask underneath in high-risk settings.

Still, both N95 and surgical mask ratings ignore the effects of coughing and neglect droplet leakage around the edges.

“The manufacturers and regulatory authorities should consider new criteria for assessing mask performance, to account for the flow physics and cough dynamics,” Drikakis said. “We provided a simple criterion for assessing the efficiency of the mask that takes into account the efficiency reduction during a cyclic coughing incident.”

The efficacy of any mask depends largely on the shape of the wearers face and how closely the curvature of the mask can conform around it. The average minimum distance between an N95 mask and the face is four millimeters, while the maximum can average 1.4 centimeters around the nose piece. The closer the fit, the more effective the mask will be as fewer airborne droplets are allowed to escape through gaps around the perimeter.

Droplets that sneak around the edges can still be transmitted several meters away from the subject depending on wind conditions. By contrast, an unmasked individual can spread droplets up to 18 feet away while coughing.

“Since the use of masks does not provide complete protection, we should respect social distancing and also implement guidelines for different distances depending on different environmental conditions,” Drikakis added.

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