Stay Safe In Virus-Town: Put The Toilet Lid Down

Stay Safe In Virus-Town: Put The Toilet Lid Down


Key Takeaways:

  • COVID-19 is also a GI disease and live virus can be found in fecal matter
  • Aerosolized particles of the virus can remain airborne for several hours
  • Flushing a toilet creates a “toilet plume” of aerosolized particles of varying sizes being shot into the air. These can contain live virus.
  • Some of these particles remain in the air, others settle onto exposed surfaces.
  • Putting the toilet seat down before flushing could reduce the number of these particles by up to a factor of 12.
  • Don’t leave your toothbrush or contact lens case on the counter.
  • Regularly wipe down surfaces in the bathroom.
  • Ensure that your home environment has proper ventilation.

1. How The Virus Is Transmitted

COVID-19 is spreading, and it is up to us to do what we can to slow it down. Every measure we take lowers the R0 by a little bit more, and can lead to many lives saved over the course of this pandemic.

Thus far COVID-19 has been primarily categorized as a respiratory disease. Most of the deaths occur when it develops into pneumonia and acute respiratory distress syndrome (ARDS), and it is thought to spread primarily from person-to-person contact via droplets produced when coughing or sneezing, which are then inhaled by anyone standing within 6 feet1

It is also becoming evident that COVID-19 has a gastrointestinal component. Although a large study in China found diarrhea in only 3.8% of subjects 2, a smaller study of 204 patients across 3 hospitals found that 50% of them experienced digestive symptoms, with diarrhea in 34% of cases.3

This makes sense when we look a bit closer at the physiology. The virus enters into human cells via docking with an enzyme called ACE2. A recent bioinformatics analysis of a normal human lung and gastrointestinal system was carried out to identify where ACE2 was most strongly expressed, and revealed that ACE2 was not only highly expressed in the lung AT2 cells, but also all throughout the intestinal tract.4 Once infected with the virus, gut permeability goes up and absorption goes down, potentially leading to symptoms like diarrhea.

Just as saliva and mucous leave our nose and throat carrying the virus, so to do feces exiting our large intestine. SARS-CoV-2 (the virus that causes the disease known as COVID-19) has been detected in stool samples up to 30 days after illness onset 5 SARS-CoV-2 RNA has been detected in blood and stool specimens and live SARS-CoV-2 virus has been isolated in cell culture from the stool of a patient with pneumonia 15 days after symptom onset.7

Now back to transmission.

It was recently shown that (in a laboratory setting) it is possible for the virus to survive on various surfaces:9

  • up to 4 hours on copper
  • up to 24 hours on cardboard
  • up to 2-3 days on plastic and stainless steel (13 hr median half-life on steel; 16 hr median half-life on plastic)
Viability of SARS-CoV-1 and SARS-CoV-2 in Aerosols and on Various Surfaces. NOTE: The vertical axis is plotted on a logarithmic scale. SOURCE: DOI: 10.1056/NEJMc2004973

In light of this study, it was widely publicized that it is extremely important to routinely disinfect surfaces that are commonly touched or may have been exposed to droplets from sneezes or coughs.

What most of the media coverage glossed over was the first portion of the study: the aerosols. To understand why this was so, we must first understand the term.

2. The Difference Between Droplet and Airborne Transmission

Disease transmission is typically classified as either droplet or airborne/aerosol.

  • Droplet transmission: transmission of diseases by expelled particles that, due to their size, tend to settle quickly to the ground within 1m of the site of generation.
  • Airborne/aerosol transmission: the transmission of disease caused by dissemination of droplet nuclei that remain infectious when suspended in air over long distance and time.

The World Health Organization (WHO) classifies the cutoff size between these two categories as 5 µm in diameter.10 In the same document, they state regarding influenza that “Transmission of infection occurs at close range, mainly through droplets and occasionally through contact.” This is because coughing and sneezing tend to produce droplets of >5 µm in diameter.

Bioaerosols (short for biological aerosols) are a subcategory of particles released from terrestrial and marine ecosystems into the atmosphere. They consist of both living and non-living components including organisms, dispersal methods of organisms, and excretions.11

As with influenza, COVID-19 has thus far been classified as only having droplet transmission.12 In light of this, even though the above study found that SARS-CoV-2 particles could remain suspended in the air, that shouldn’t matter because they would have no way of getting there. In the aforementioned study, the researchers had to use a nebulizer to artificially produce the aerosols.

Further support for droplet-based transmission comes from a study by virologist Ke Lan of Wuhan University. He and his colleagues took air samples from the rooms of 35 COVID-19 patients across 2 hospitals and found no aerosolized particles. They did, however, find coronavirus aerosols near patients’ toilets in Wuchang Fangcang Field Hospital. 13

3. How A Virus Can Become Aerosolized

During the 2003 SARS outbreak in Amoy Garden in Hong Kong, over 300 residents in a single-apartment block with thought to have been infected by contaminated sewage, originating from a single SARS patient suffering from diarrhea. 14 Further examination found that exposure and disease propagation was likely due to virus-laden aerosols originating in the sewer drain.15 These studies suggest that SARS aerosols can remain contagious over significant time and distance.

It is thought that this may have been one mode of transmission within the Diamond Princess cruise ship, with 3,700 people, among whom at least 742 have been confirmed to be infected with SARS-CoV-2.

Outside of cruise ships and countries with poor sanitation, this type of aerosolization is thankfully of little concern.

There is, however, another way that those virus-containing fecal matter particles can get into the air. Through a simple device found in every home, school, business, and hospital: the toilet.

Photo by Gabor Monori on Unsplash

4. The Dangers of the Toilet

Every time you flush a toilet, the movement of toilet water (i.e., bubbling, swirling, splashing) causes thousands of tiny droplets to be shot into the air in a phenomena often referred to as a “toilet plume.” Some of these are larger droplets that end up settling on exposed surfaces in the bathroom, containing significant samples of bacteria and virus particles from the toilet bowl. 16 One study measured the particles in the air above the toilet bowl 15 seconds after flushing and found that up to 145,000 such particles were produced per flush, with 95% of droplets <2 μm diameter and >99% <5 μm (the precise number varied based on toilet type, with a greater flush volume and energy being associated with increased particle counts).17 Even after flushing, a significant number of virus particles remain adsorbed to the porcelain surface of the toilet bowl, and airborne particles were found after second and third flushes 18

One study seeded toilets with indicator organisms at levels required to mimic pathogen shedding during infectious diarrhea. They found that the quantity of Serratia marcesens bacteria and MS2 bacteriophage virus found in the air after flushing was ~1370 CFU per cubic meter for Serratia and ~2420 PFU for MS2 page. Even after a first flush, large numbers of micro‐organisms remained on the toilet bowl surface and in the bowl water which were launched into the air by further flushes.19

5. The Importance of Lids

In a 2012 study, researchers20 decided to measure the role of toilet lids in lowering the infection risk from aerosolized particles. They flushed a toilet seeded with fecal suspensions of Clostridium difficile. They measured fallen droplets near the toilet as well as particles in the air above the toilet at multiple heights. The results showed a widespread dissemination of large fallen droplets with the lid up but not with the lid downC difficile was recovered from air sampled at heights up to 25 cm above the toilet seat and up to 90 minutes after flushing, at concentrations 12-fold greater with the lid up than with the lid down. They concluded that lidless conventional toilets increase the risk of C difficile environmental contamination and thus discouraged their use.21

Another 2018 study examined lidless toilets in hospitals and found that Toilets produce aerosol particles when flushed, with the majority of the particles being 0.3 μm in diameter. The particles aerosolized include microorganisms remaining from previous use or from fecal wastes. Bioaerosols generated from toilets containing fecal waste resulted in a bacterial concentration of 278 CFU/m3 (SD ± 149). No signifiant change was found over the 1 meter distance from the toilet or a 30 minute study period, indicating that the particles spread out and remain in the air for significant periods of time. They also found a significant amount of aerosolized bacteria even when there was no fecal matter in the toilet bowls, indicating once again that bacteria can set up shop on the walls of the bowl itself.22

6. Why Does It Matter?

One of the scariest qualities of COVID-19 is its long incubation period. The median incubation period was estimated to be 5.1 days, with a full range of 1-14.23 There is also a significant amount of evidence pointing towards the disease being spread by people who are still asymptomatic.24

If the only ways for it to spread were through sneezing and coughing, this wouldn’t seem to make much sense. People rarely sneeze and cough when they aren’t sick (unless they have allergies, the examination of which would make for an interesting epidemiological study).

But everyone poops.

If you share a bathroom with someone, you may be sharing their disease.

7. What Can We Do About it?

While we can never fully eliminate the risk (short of a complete sterilization and air filtration after each bathroom use) there are many things that can be done to lower the odds of infectious transmission.

  1. Close your toilet lid before flushing.
  2. Don’t leave contact lens cases or toothbrushes out on an exposed bathroom surface.
  3. Wipe down everything in your bathroom as often as possible. This is especially important if you are in the unfortunate situation of sharing a living space with someone with the disease.
  4. Get a continuous toilet cleaner system to minimize residual particles. This will help prevent more disease bioaerosals from being release each time you urinate and flush.
  5. Make sure that you have proper ventilation in your living environment. Increasing the airflow minimizes the amount of time that aerosolized particles can stick around. 26
    1. One study found that in simulations using empirical transmission levels observed in households, bringing ventilation to recommended levels had the same mitigating effect as having 50% to 60% of the household vaccinated. 27

The first two are extremely easy and can have a very large impact. The cleaning may seem like a pain, but building that habit could do wonders for reducing your overall risk of infection.

BONUS: How To Disinfect a Toilet

NOTE: Coronaviruses are surrounded by a lipid envelope that makes them more vulnerable to soap than most bacteria or influenza viruses. For this particular epidemic, even a basic cleaning should get the job done.

Share The Word

These are very easy steps that everyone can take but few are thinking about. Sharing this article–or just this message–could have a meaningful impact on someone’s getting or spreading the disease.


Quick note: I’m spending most of my time these days keeping up with as much of the newly published research around COVID-19 as possible, as well as diving through the past literature to figure out ways that we can help our situation. For the constantly updated result of all this research, visit

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