What gets dispersed with a single flush? Toilet particles may reach the breathing zone.

What gets dispersed with a single flush? Toilet particles may reach the breathing zone.

The moment you flush, an "invisible cloud" rises from the toilet

You use the toilet, press the lever or button, watch the water swirl and clean the bowl, and then walk away. It's an action repeated almost unconsciously every day.

However, during those few seconds, there is a possibility that tiny invisible particles are being pushed into the air. The phenomenon where the liquid in the toilet becomes fine droplets due to the force of the water flow and is carried upwards is called a "toilet plume." The particles themselves are not necessarily dangerous, but if bacteria or viruses are present in the toilet, they could become "bioaerosols" containing these microorganisms.

The German news site FOCUS Online introduced the latest systematic review by a research team from Flinders University in Australia, among others. The research team organized 22 studies, including those on household toilets, strong flushing systems used in public restrooms, and vacuum toilets in aircraft, comparing the amount and spread of aerosols generated during flushing.

This is not a single experiment that "recorded a new height of 163 centimeters in one toilet." It is a study that compiled existing research with different measurement methods and toilet designs to analyze which factors are related to the amount of particle generation. Without understanding this point, the numbers alone may be misleading.


Up to 163 centimeters, approximately 1.5 meters horizontally

In the studies included in the review, particles generated by flushing were detected at a maximum height of 163 centimeters, and examples were found where they were confirmed up to about 1.5 meters away from the toilet horizontally. 163 centimeters is close to the height of the nose or mouth of many adults, meaning particles from the toilet could reach a standing person's "breathing zone."

Many of the particles were less than 3 micrometers in diameter. One micrometer is one-thousandth of a millimeter, much smaller than the thickness of a hair, and some can ride air currents instead of quickly falling to the floor. In some of the studies, particles were observed for more than 20 seconds after flushing. In past experiments using high-power lasers to capture public restroom-type toilets, the invisible particle plumes were visualized rising vigorously and spreading upward.

The microorganisms targeted in the review included E. coli, Salmonella, Staphylococcus aureus, Clostridioides difficile, as well as adenoviruses and noroviruses. However, the conditions of the studies were not the same. Some experiments added substitute microorganisms to the toilet water to track them, while others measured particles in the air after normal use. Therefore, it is incorrect to interpret that "pathogens fly up to 163 centimeters in every flush."


Two factors influencing the amount generated

The analysis clearly indicated the impact of the concentration of microorganisms in the toilet and the volume of flushing water.

When the concentration of microorganisms in the toilet increased tenfold, the concentration of bioaerosols detected in the air tended to increase by about 2.3 to 2.6 times. Additionally, for every liter increase in the water used per flush, the concentration of aerosols or bioaerosols was shown to increase by approximately 1.3 to 1.4 times. The research team considers the "source load," or how many microorganisms are present in the toilet, and the energy with which the water moves as important factors.

From these results, in households with dual-flush buttons, the idea of using the small flush as needed emerges. This not only conserves water but may also reduce particle generation. However, the small flush is often intended for liquid waste. Forcing solid waste through with insufficient water could lead to clogs or require re-flushing, which could be counterproductive. It is essential to follow the manufacturer's instructions and use the appropriate flush for the type of waste.


"Particles exist" is not the same as "infection occurs"

The most important thing here is to separate the presence of aerosols from the actual risk of infection.

For an infection to occur, several conditions must be met: the particles must contain infectious pathogens, they must reach a person's nose, mouth, or eyes in sufficient quantities, and the person must be susceptible. Simply detecting particles in the air does not indicate whether they contain pathogens, are alive, or are present in amounts necessary for infection.

The U.S. Centers for Disease Control and Prevention (CDC) acknowledges that aerosols are generated during toilet flushing but states that there is no epidemiological evidence showing this poses a direct infection risk. The 2022 laser visualization study also demonstrated particle movement but did not directly investigate disease transmission.

In other words, the latest review is not a study that concludes "toilets we thought were safe are actually major sources of infection." It organized how much particulate matter is generated during routine flushing and what conditions affect that amount, creating a foundation for future risk assessments and toilet design.

The risks to be emphasized differ between situations such as a healthy person using a cleaned home toilet as usual, a shared toilet used immediately after a patient with infectious gastroenteritis, and a hospital room with someone with compromised immune function. Particularly in healthcare and nursing facilities or public restrooms with high foot traffic, more cautious infection control than in everyday households is necessary.


Does closing the lid solve the problem?

"Then just close the lid before flushing." It's the most straightforward and cost-effective measure. Indeed, past studies have shown that closing the lid reduces the number of particles in the air. However, there is a gap between the lid and the seat, so micro-particles cannot be completely contained.

In a 2024 U.S. study, bacteriophage MS2 was added to toilets as a substitute for harmless viruses, and both household and public restroom-type toilets were examined. The amount of virus adhering to surrounding surfaces showed no significant difference between when the household toilet lid was open and when it was closed. In contrast, cleaning the toilet with disinfectant and a brush reduced contamination inside the toilet by more than 99.99%.

The current 22-study review also concludes that there is little data available on how effective the position of the lid, flushing pressure, and ventilation are individually, and the results are inconsistent.

This does not mean "closing the lid is meaningless." It may help suppress droplets and relatively large particles and is a reasonable low-cost preventive measure that can be easily done. However, it should not be considered an all-purpose barrier and should be combined with handwashing, cleaning, and ventilation.


On social media, "I can't put my toothbrush there anymore" vs. "overreacting"

Videos visualizing toilet plumes with lasers have previously garnered significant attention on social media. The FOCUS Online article is still new, and it's not yet possible to comprehensively cover reactions to the article itself. Therefore, representative perceptions are introduced here from public posts on videos and articles dealing with the same phenomenon. It is important to note that comments on social media are not public opinion surveys but individual experiences and impressions.

 

The most noticeable reaction is straightforward disgust. On Reddit, comments like "I don't want to keep my toothbrush near the toilet" and "This is why I store my toothbrush in a cupboard" were common. When particles that were invisible are visualized with lasers, the emotional response of "gross" often precedes scientific assessments of danger. From those who already have a habit of closing the lid at home, there are many reactions like "That's why I close it before flushing" and "It's easy to do, so I do it."

On the other hand, there are also many posts cautioning against excessive anxiety, such as "Flush toilets have been used for years, and I've never heard of a major outbreak caused by this" and "The presence of particles is different from health damage." Furthermore, there are many comments that use humor to avoid becoming too serious. When dealing with topics about invisible contamination, reactions of disgust, behavioral change, skepticism, and humor tend to appear simultaneously.

The calmest stance in this debate is likely "Closing the lid is a small effort, so I'll do it. However, there's no need to be overly afraid of public toilets without lids." Scientifically, this is close to the boundary between what is currently known and what is still unknown.


Practical measures you can take starting today

So, what should be done at home? The priorities are simple.

First, wash your hands with soap and running water after using the toilet. The basic action of preventing infection through contact is more practically grounded than worrying about what to do with the toilet lid. If you touch your smartphone before washing your hands, you might transfer dirt back to your hands, so it's best to avoid using it in the toilet if possible.

Second, regularly clean areas that hands and droplets easily touch, such as the toilet seat, lid, flush button, lever, doorknob, and the floor around the toilet. If a family member has symptoms of vomiting or diarrhea, clean more frequently than usual and use methods suitable for the causative pathogen. Follow the product labels for detergents and disinfectants, and do not mix different chemicals.

Third, use windows or exhaust fans to exchange air. Although the review alone cannot quantitatively determine the independent effect of ventilation, it is reasonable as general indoor environmental management to prevent particles and moisture from stagnating in a small space. If there is an exhaust fan, operate it for a while during and after use.

Fourth, if the toilet has a lid, close it before flushing if possible. While it won't create a perfect seal, it can at least help suppress larger droplets. If you open the lid afterward, be sure to wash your hands last.

Fifth, do not place items that touch your mouth or eyes, such as toothbrushes, contact lens supplies, or cosmetics, bare near the toilet. Moving them to a cupboard or a storage place that maintains ventilation is reasonable not only to avoid aerosols but also to prevent splashes and droplets during cleaning. However, sealing a wet toothbrush in a container and keeping it moist can lead to other hygiene issues, so keep it in a condition where it can dry.


What is really needed is not "fear," but improvements in design and habits

The figure of 163 centimeters is striking. Hearing that particles from the toilet can reach the height of your mouth or nose naturally makes one uneasy. However, to read the numbers correctly, one must not forget the differences in measurement conditions and the distance from aerosol detection to infection establishment.

The significance of this study lies more in connecting to the next research on how to improve toilet design, flushing water volume, ventilation, and cleaning methods to reduce exposure, rather than introducing new fears into daily life. Especially in medical facilities used by infectious disease patients, living spaces for people with weakened immune systems, and public facilities used by many people in succession, measures that do not rely solely on the presence or absence of a lid are required. Local exhaust, toilet shapes that suppress particles during flushing, appropriate flushing energy, automatic cleaning, and contactless operation will also become important considerations.

The conclusion for households can be more moderate. Close the lid if you can. Choose the appropriate water volume as needed. Ventilate, clean areas prone to dirt, and wash your hands last. Instead of becoming overly sensitive in trying to eliminate invisible particles, continue basic actions that are likely to be effective without strain. This is the most practical way to incorporate the latest research into daily life.


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