A Breached Trap Seal Let Drainage Bioaerosols Spread Three Floors in a High-Rise.
Key takeaway.
In a 17-story residential building, this study traced pathogen-laden bioaerosols in apartment air back to the shared drainage system, confirming a fecal-water-air pathway. When a trap seal was breached, sub-micron bioaerosols spread across three floors within 19 minutes through the stack effect. With the seals intact, that vertical pathway did not open, though a single toilet flush still moved bioaerosols into adjacent rooms on the same floor. The drainage system is a real bioaerosol route, and an intact seal is the barrier against the vertical spread.
The study.
Fan and colleagues studied a 17-story residential building using metagenomic sequencing and fluorescein tracers to determine whether the shared drainage system contributes to the bioaerosols people breathe indoors. In apartment air they found high relative abundances of Pseudomonas aeruginosa (around 30%) and Acinetobacter baumannii (around 10%), and detected the fecal indicator Salmonella enterica, evidence of a fecal-water-air chain originating in the drainage system.
The team then tested how those bioaerosols move. When a trap seal was breached, sub-micron aerosols below 0.5 microns diffused across three floors within 19 minutes via the stack effect, with upward deposition 2.5 to 3.1 times greater than downward. The breach of the seal was what opened the vertical pathway between floors.
Even with the water seals intact, a single toilet flush spread 0.3-micron bioaerosols horizontally into adjacent rooms within six minutes, showing that flushing generates bioaerosols that move within a unit. This is the first study to confirm the drainage system contribution to indoor bioaerosols using both genetic sequencing and aerosol-transport measurement.
Key findings.
- The drainage system was the bioaerosol source Metagenomics tied apartment-air bioaerosols to the shared drainage system, with high levels of Pseudomonas aeruginosa and Acinetobacter baumannii and the fecal indicator Salmonella enterica confirming a fecal-water-air chain.
- A breached seal let bioaerosols cross three floors When a trap seal was breached, sub-micron bioaerosols spread across three floors within 19 minutes through the stack effect, with upward transport 2.5 to 3.1 times greater than downward.
- An intact seal was the barrier to vertical spread Cross-floor vertical spread required a breach of the seal; where the seals were intact, that vertical pathway did not open.
- Flushing still moved bioaerosols within a unit Even with the seals intact, a single toilet flush spread 0.3-micron bioaerosols into adjacent rooms within six minutes.
What this means for your facility.
The core lesson is that the building drainage system is a genuine source of pathogen-laden indoor bioaerosols, and that an intact trap seal restricts the upward movement of air and aerosols between floors. When the seal was breached, they crossed three floors in minutes. Keeping every drain reliably sealed is what closes that vertical route.
This is exactly what a waterless trap seal is designed to guarantee. A conventional water trap can be lost to evaporation in floor drains that see irregular use, corridors, utility rooms, mechanical spaces, and low-traffic areas, and a depleted trap is a breached one. The Green Drain one-way silicone valve keeps the drain sealed whether or not water is present, so the seal cannot be lost to evaporation. DTI testing confirmed the valve holds its seal against more than 700 Pa of pressure.
The study also shows that flushing can move bioaerosols within a unit even with seals intact, a reminder that a drain barrier is one layer of protection alongside ventilation and hygiene, not a standalone guarantee. The ASSE 1072-2020 life-cycle test confirmed the GD4 performs identically after 2,500 open-close cycles, and independent SGS testing (Report QDF25-0049810-01) found the GD3 retained over 99.9% of an aerosolized viral surrogate (MS2 bacteriophage) in a controlled bench test.
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Related research.
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Green Drain's waterless trap seal is a supportive engineering control that restricts the upward movement of air and aerosols, backed by independent bench testing. See how it works for your industry.