Clouds of Viruses: Why You Should Wear a Mask in Public Restrooms

Researchers use computational fluid dynamics to model the particle movement that occurs with the act of flushing.

Michael Hunter, MD
2 min readSep 10, 2020


Photo by Thomas Layland on Unsplash

Researchers recently simulated the movement of a virus-laden particle from urinal flushing. They discovered that the particles spread externally. Alarmingly, the particle can reach 0.84 meters (a man’s thigh) in only 5.5 seconds, while a toilet flush hit 0.93 meters in 35 seconds.

Do I have your attention? These alarming observations are from scientists in China. They used computational fluid dynamics to model the movement of particles associated with toilet or urinal flushing.

Computational fluid dynamics (CFD) has been around since the early 20th century. CFD can solve complex fluid flow and heat transfer problems. This fluid mechanics branch uses applied mathematics, physics, and computational software to visualize how to analyze situations involving fluid flow. CFD can simulate the free-stream flow of a fluid and the interaction of liquids and gases with surfaces.

The Covid-19 pandemic continues. There have been over 27 million cases worldwide, with nearly 900,000 deaths from the disease. In my home country of the USA, the total case number is now over 6 million. Today we look at whether you should worry about the risk of Covid-19 in public restrooms.

Let’s go back to the public restroom. Flushing involves an interaction between liquid and gas surfaces. When we flush, we cause a massive release of aerosol particles to be released from the toilet or urinal. The scientists simulated and tracked this particle motion.

The minute particles released spread primarily externally; what I mean is that nearly 60 percent of the particles traveled away from the urinal. It took 5.5 seconds to reach a man’s thigh. In contradistinction, toilet flushing led to particle spray, but it took 35 seconds to get slightly higher.

Photo by CDC on Unsplash

We have known that feces and urine can facilitate the transmission of viruses. Public restrooms can serve as vectors of the spread of the small particles, including the novel coronavirus, it appears. This computational fluid dynamic modeling reminds us of the importance of wearing a mask within public spaces, including restrooms. Now we need the scientists can get on means for reducing the diffusion of small particles in indoor spaces.


  2. American Institute of Physics. “Using a public restroom? Mask up! Flushing public restroom toilets or urinals can spew clouds of particles carrying viruses, including COVID-19.” ScienceDaily. ScienceDaily, 18 August 2020. <>.



Michael Hunter, MD

I have degrees from Harvard, Yale, and Penn. I am a radiation oncologist in the Seattle area. You may find me regularly posting at