Toilet flushes release pathogen-laden aerosols that linger in breathing zone

When you flush a toilet, you set off a chain reaction that scientists are only now beginning to fully understand. Researchers at Flinders University examined 22 studies on what happens in those seconds after the water swirls down, and what they found challenges a basic assumption most people hold about bathroom safety: that closing the lid solves the problem.

The turbulence of a flush—the force and volume of water moving through the bowl—creates aerosols that become suspended in the air at the height where adults breathe. These particles can linger for at least 20 seconds. More forceful flushes, with higher water volumes, release more of them. The aerosols carry biological material: microbes from the water itself or from contamination already present in the bowl. Among the pathogens that can travel this way are legionella, influenza, and norovirus. The picture for COVID-19 transmission through toilet aerosols remains unclear, with conflicting evidence in the literature.

Lira Adiyani, a PhD student and lead author of the review published in Science of the Total Environment, explained the mechanics plainly: turbulence releases aerosols into the surrounding air, and some studies have detected these particles at the exact height where people inhale them. The more microbes present in the toilet water, the more bioaerosols escape. Adiyani still recommends closing the lid as a precautionary step, though she acknowledges it does not completely prevent aerosols from escaping. The research found that the effects of bathroom ventilation and whether the lid was open or closed remained unclear—a gap that suggests the picture is more complicated than common practice assumes.

Harriett Whiley, an environmental health professor and co-author, offered reassurance for healthy people in their own homes, but with an important caveat: the risk sharpens considerably for immunocompromised individuals. For people with weakened immune systems, opportunistic pathogens present in water—particularly recycled or rainwater systems—can cause serious illness or prove fatal. Whiley's team is now focused on protecting vulnerable populations through better design in hospitals and aged care facilities. The goal is to achieve effective flushing with less volume and less turbulence, reducing the aerosol burden without sacrificing sanitation.

The concern extends beyond the toilet itself. Lotti Tajouri, an associate professor in molecular genetics who studies public bathrooms, frames the entire space as a contamination zone. Every surface—the door handle, the floor, the lid, the walls—harbors microbes left by previous users. When you flush with the lid up, Tajouri describes it as an "explosion of aerosols," a plume that sprays the user directly. Closing the lid may reduce direct exposure to that plume, but it does not eliminate the microbes already coating every surface in the room. The sequence of contact—touching the lid, flushing, touching the door, washing hands, touching the taps, using the hand dryer, touching the exit door—creates multiple pathways for microorganisms to transfer to skin and, eventually, to the mouth or eyes.

Tajouri's research on mobile phone use in toilets adds another layer. Between 50 and 80 percent of people use their phones while in the bathroom, rising to nearly 100 percent among younger generations. Those phones become contaminated with fecal microorganisms—salmonella, campylobacter, and various bacteria and viruses—which then travel wherever the phone goes. The implication is stark: the bathroom is not an isolated risk but a node in a network of transmission that extends into daily life.

The Flinders review highlights a gap in scientific attention. Indoor aerosols have been understudied despite their clear role in infection transmission. Their small size allows them to penetrate deep into the respiratory tract when inhaled, creating a direct pathway to infection. As water security pressures mount and emerging infectious diseases continue to surface, the design of bathrooms—in homes, hospitals, and public spaces—has taken on new importance. The research suggests that infection control strategies and building design standards may need to account for what happens in those 20 seconds after a flush, particularly in settings where vulnerable people gather.