Whatever Happened to Toilet Plumes?
In the dark early days of the pandemic, when we knew almost nothing and feared almost everything, there was a moment when people became very, very worried about toilets. More specifically, they were worried about the possibility that the cloud of particles toilets spew into the air when flushed—known in the scientific literature as “toilet plume”—might be a significant vector of COVID transmission. Because the coronavirus can be found in human excrement, “flushing the toilet may fling coronavirus aerosols all over,” The New York Times warned in June 2020. Every so often in the years since, the occasional PSA from a scientist or public-health expert has renewed the scatological panic.
In retrospect, so much of what we thought we knew in those early days was wrong. Lysoling our groceries turned out to not be helpful. Masking turned out to be very helpful. Hand-washing, though still important, was not all it was cracked up to be, and herd immunity, in the end, was a mirage. As the country shifts into post-pandemic life and takes stock of the past three years, it’s worth asking: What really was the deal with toilet plume?
The short answer is that our fears have not been substantiated, but they weren’t entirely overblown either. Scientists have been studying toilet plume for decades. They’ve found that plumes vary in magnitude depending on the type of toilet and flush mechanism. Flush energy plays a role too: The greater it is, the larger the plume. Closing the lid (if the toilet has one) helps a great deal, though even that cannot completely eliminate toilet plume—particles can still escape through the gap between the seat and the lid.
Whatever the specifics, the main conclusion from years of research preceding the pandemic has been consistent and disgusting: “Flush toilets produce substantial quantities of toilet plume aerosol capable of entraining microorganisms at least as large as bacteria … These bioaerosols may remain viable in the air for extended periods and travel with air currents,” scientists at the CDC and the University of Oklahoma College of Public Health wrote in a 2013 review paper titled “Lifting the Lid on Toilet Plume Aerosol.” In other words, when you flush a toilet, an unsettling amount of the contents go up rather than down.
Knowing this is one thing; seeing it is another. Traditionally, scientists have measured toilet plume with either a particle counter or, in at least one case, “a computational model of an idealized toilet.” But in a new study published last month, researchers at the University of Colorado at Boulder took things a step further, using bright-green lasers to render visible what usually, blessedly, is not. John Crimaldi, an engineering professor and a co-author of the study, who has spent 25 years using lasers to illuminate invisible phenomena, told me that he and his colleagues went into the experiment fully expecting to see something. Even so, they were “completely caught off guard” by the results. The plume was bigger, faster, and more energetic than they’d anticipated—“like an eruption,” Crimaldi said, or, as he and his colleagues put it in their paper, a “strong chaotic jet.”
Within eight seconds, the resulting cloud of aerosols shoots nearly five feet above the toilet bowl—that is, more than six feet above the ground. That is: straight into your face. After the initial burst, the plume continues to rise until it hits the ceiling, and then it wafts outward. It meets a wall and runs along it. Before long, it fills the room. Once that happens, it hangs around for a while. “You can sort of extrapolate in your own mind to walking into a public restroom in an airport that has 20 toilet stalls, all of them flushing every couple minutes,” Crimaldi said. Not a pleasant thought.
The question, then, is not so much whether toilet plume happens—like it or not, it clearly does—as whether it presents a legitimate transmission risk of COVID or anything else. This part is not so clear. The 2013 review paper identified studies of the original SARS virus as “among the most compelling indicators of the potential for toilet plume to cause airborne disease transmission.” (The authors also noted, in a dry aside, that although SARS was “not presently a common disease, it has demonstrated its potential for explosive spread and high mortality.”) The one such study the authors discuss explicitly is a report on the 2003 outbreak in Hong Kong’s Amoy Gardens apartment complex. That study, though, is far from conclusive, Mark Sobsey, an environmental microbiologist at the University of North Carolina at Chapel Hill, told me. The researchers didn’t rule out other modes of transmission, nor did they attempt to culture live virus from the fecal matter—a far more reliable indicator of infectiousness than mere detection.
Beyond that, Sobsey said, there is little evidence that toilet plumes spread SARS or COVID-19. In his own review, published in December 2021, Sobsey found “no documented evidence” of viral transmission via fecal matter. This, at least, seems to track with the three years of pandemic experience we’ve all now endured. Although we can’t easily prove that bathrooms don’t play a significant role in spreading COVID-19, we haven’t seen any glaring indications that they do. And anyway, the coronavirus has found plenty of other awful ways to spread.
Just because toilet plume doesn’t seem to be a vector of COVID transmission, though, doesn’t mean you can forget about it. Gastrointestinal viruses such as norovirus, Sobsey told me, present a more serious risk of transmission via toilet plume, because they are known to spread via fecal matter. The only real solutions are structural. Improved ventilation would keep aerosolized waste from building up in the air, and germicidal lighting, though the technology is still being developed, could potentially disinfect what remains. Neither, however, would stop the plume in the first place. To do that, you would need to change the toilet itself: In order to create a smoother and thus better-contained flush, you could change the geometry of the bowl, the way the water enters and exits, or any number of other variables. Toilet manufacturers could also, you know, stop producing lidless toilets.
But none of that will save you the next time you find yourself staring into a toilet’s blank maw. Crimaldi suggests wearing a mask in public bathrooms to protect against not just the plume created when you flush but also the plumes left by the person who used the bathroom before you, the person who used it before them, and so on. You don’t need to have any great affection for masking as a public-health intervention to consider donning one for a few minutes to avoid literally breathing in shit. Sobsey offered another bit of unconventional bathroom-hygiene advice, which he acknowledged can only do so much to protect you: If you find yourself in a public restroom with a lidless toilet, he said, consider washing your hands before you flush. Then “hold your breath, flush the toilet, and leave.”