gotitas diminutas que no caen, sino que flotan. Y algunas llegan más alto de lo que parece.
What flies out when you flush
Each flush is a small storm inside the bowl, and part of its spray does not fall: it stays afloat.
- Some droplets reach the height of an adult’s face and stay airborne for at least 20 seconds.
- Where it concentrates the most is near the toilet, at seat level.
- Greater flushing water volume and more microbes in the bowl lead to more aerosols forming.
- Closing the lid diverts the jet to the sides, but does not trap it completely.
- No one has yet measured how many infections this might cause: hence hospitals are the priority.
That is what a systematic review published in Science of the Total Environment describes, by researchers from Flinders University and the University of Adelaide in Australia. The team, led by PhD student Lira Adiyani and environmental microbiologist Harriet Whiley, gathered 22 studies on the aerosols formed when flushing and analyzed which factors make them more or less likely to occur. Their central conclusion is simple to state: some of those studies detected aerosols at the height of an adult’s breathing, and those particles can remain suspended for at least 20 seconds after the flush.
What exactly is an aerosol (and why does it matter that it stays afloat)
An aerosol is essentially a collection of droplets or particles so small that the air keeps them suspended for a while instead of letting them fall. The larger droplets that splash the rim of the bowl follow gravity and land within seconds on nearby surfaces. The finer ones, by contrast, behave almost like smoke: they travel with air currents, rise with turbulence, and can reach the nose and mouth of someone standing beside the toilet.
When those droplets carry microorganisms present in the water of the bowl, specialists speak of bioaerosols. That is where the health interest lies. If a microbe travels in a drop that is inhaled, the toilet flush becomes a potential exposure route that does not depend on touching anything. Something akin to what happens with respiratory viruses in enclosed rooms, and one of the reasons it is wise to ventilate indoor spaces.
A flush is a small, violent wave inside a bowl: part of its spray doesn’t fall, it stays afloat.
The physics of the invisible jet
Turbulence is key. When the flush starts, the water enters with force, strikes the porcelain walls, and mixes chaotically with the surrounding air. That agitation disrupts the water surface into a multitude of fragments and launches a column of droplets upward, what physicists call a plume.
A few years ago, engineers at the University of Colorado Boulder managed to make this phenomenon visible with lasers and cameras in a public flushometer toilet, the lidless model common in many American restrooms. They measured a flow speed of over 2 meters per second capable of lifting aerosols up to 1.5 meters in 8 seconds. That experiment already made clear why it is unwise to flush with the lid up.
The new review goes a step further. It does not stay at a single toilet or a single laboratory: it compares measurements from very different contexts to determine which variables actually matter.
What weighs most: how many microbes there are and how much water is flushed
The 22 studies in the review are very diverse in their approach. Some were conducted in laboratories, deliberately seeding microorganisms or harmless substitutes in the bowl water to be able to track them later in the air. Others measured what happens under normal conditions, without adding anything. Despite this diversity, two fairly consistent patterns emerge.
The first concerns what is in the bowl. The higher the concentration of microbes in the water, the more bioaerosols are generated. “The more microbes there are in the toilet, the more bioaerosols can be released,” summarizes Adiyani. It seems obvious, yet it has a practical consequence: regular toilet cleaning is not only a matter of aesthetics. The same applies to other corners of the bathroom, such as the sink, which also accumulates microorganisms with ease.
The second pattern is hydraulic. Flushing with greater water volume is generally associated with more aerosols. More water means more energy and turbulence, and therefore more droplets launched into the air. That is why the authors suggest using the short flush option on dual-flush toilets when that suffices.
The studies also agree that the concentration of aerosols is higher near the toilet itself, especially at seat level. The zone of greatest exposure isn’t the face of the person standing, but the immediate vicinity of the bowl. Still, the fact that some particles rise higher means inhalation remains a risk to take into account.
The most exposed zone isn’t the face of the person standing, but the immediate surroundings of the bowl.
And the lid? It helps, but it isn’t a seal
Here the review adds a nuance that unsettles a widespread belief. Several individual studies observed that closing the lid before flushing changes the direction of the aerosols: instead of rising straight up, they escape to the sides through the gaps between the lid and the bowl. The lid, therefore, does not enclose the cloud. It deflects it.
Moreover, when the researchers analyzed the data set as a whole, neither the position of the lid nor the bathroom ventilation showed a statistically significant association with aerosol concentration. That does not mean closing the lid is useless. It means the available studies are too heterogeneous, with different toilets, sampling methods, and microorganisms to measure its effect with precision.
With that caution in mind, the authors still recommend closing the lid as a precaution, along with regularly cleaning the toilet, washing hands after use, and keeping the bathroom well ventilated. These are cheap actions that reduce exposure even if they do not eliminate it.
Exposing yourself is not the same as getting sick
It is wise not to turn a spray of droplets into a threatening cloud. If an aerosol reaches the height of the face, that does not automatically mean infection will occur. For that to happen, a pathogen would have to be present in sufficient quantity, viable, and reach a susceptible person. Nobody among the studies reviewed quantifies how many infections come from this route, and that remains the big open question.
That is precisely where the most useful contribution of the work lies. In addition to synthesizing the evidence, the team produced probability distributions with the available data to feed the so-called quantitative microbial risk assessments. These are the models public health experts use to estimate how much risk a given exposure represents and to decide which measures are worth taking. And they point toward a clear objective: the design of the toilet itself.
Whiley stresses that more research is needed to develop design and engineering solutions that reduce toilet bioaerosols, especially in high-risk settings like hospitals. This is no accident. In a hospital, vulnerable people, resistant microorganisms, and shared bathrooms collide, turning hospital infections into a serious public health concern.
Lowering the lid and using the short flush costs little; redesigning the toilet is the task still pending.
A small gesture, an open question
Next time you press the button, you might recall that the toilet is not a closed container, but a small source of mist. There is no alarmism: the evidence describes a plausible exposure route, not a domestic epidemic. But there are sensible habits worth adopting: close the lid, choose the short flush, clean regularly, ventilate, and wash your hands. Meanwhile, science still has duties to perform: measuring the real risk and designing toilets that simply spray less upward.
