By Stan Deresinski, MD, FACP, FIDSA

Clinical Professor of Medicine, Stanford University

Dr. Deresinski reports no financial relationships relevant to this field of study.

SYNOPSIS: Although not definitive, evidence is consistent with aerosol spread of SARS-CoV-2 in an apartment building as a result of transport through the drainage system to apartments directly above.

SOURCE: Kang M, Wei J, Yuan J, et al. Probable evidence of fecal aerosol transmission of SARS-CoV-2 in a high-rise building. Ann Intern Med 2020; Sept. 1. doi:10.7326/M20-0928. [Online ahead of print].

After visiting Wuhan, China, all five members of a family living in a 15th floor apartment in Guangzhou, China, developed COVID-19 in late January 2020. Days later, couples living directly above them in apartments on the 25th and 27th floors became ill. A review of surveillance videos in the building showed no evidence of contact between any of the three clusters, and no one among the two couples had traveled or knew of coming in contact with anyone with COVID-19. However, the three apartments shared the same waste pipes.

Among many air and surface samples collected from within the apartment tower, the only ones in which SARS-CoV-2 was recovered were in the implicated 15th floor apartment as well as from a vacant bathroom on the 16th floor directly above. The investigators released a tracer gas into the floor drainpipe of the 15th floor apartment, and the gas was detected in the bathrooms of the implicated apartments on the 25th and 27th floors. The investigators concluded this event was the result of aerosolization of fecal material containing SARS-CoV-2 within the drainage plumbing system of the Guangzhou apartment.


In 2003, SARS-CoV caused an outbreak in the Amoy Gardens housing complex in Hong Kong, affecting 321 individuals, 42 of whom died. Apartments in China commonly include floor drains with underlying U-shaped traps that are water-filled. Yu et al reported the Amoy Gardens outbreak was the result of the traps in the index apartment drying out and, thus, allowing aerosolization of virus particles upward, possibly abetted by an exhaust fan, through the air shaft into apartments above.1 Flushing toilets has been demonstrated to generate aerosols.

Kang et al concluded the Guangzhou event spread similarly. However, since the bathroom floor traps had been cleaned in the interval, they could not determine if the traps had lost their protective water. Nonetheless, the authors proposed the “bioaerosolization of wastewater mixed with urine, feces, and exhaled mucus originating from index patients is suggested to be the source of infectious bioaerosols in this outbreak.”

Although the role of aerosols in transmission of SARS-CoV-2 has been disputed, the accumulated evidence indicates this is an important mode of infection spread. The degree to which this is a risk at a distance remains somewhat uncertain but undoubtedly is a function of the density of aerosol particles and the duration of exposure. Also disputed is the role of the high SARS-CoV-2 RNA loads in the feces of COVID-19 patients since the detection of transmissible virus (as opposed to just residual RNA fragments) has been a rarity.

Although bathroom drain traps are rare in the United States, other bathroom drains may include U-shaped traps. These traps should not be allowed to dry out (such dry out is possible if these traps are meant to drain sinks or bathtubs that go unused). Also, maintain bathroom hygiene and ventilation. Surfaces in toilet areas used by COVID-19 patients can be contaminated with SARS-CoV-2. At one Wuhan hospital, aerosol samples were more highly contaminated in a small, poorly ventilated patient toilet room than were patient care areas.2


  1. Liu Y, Ning Z, Chen Y, et al. Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals. Nature 2020;582:557-560.
  2. Yu IT, Li Y, Wong TW, et al. Evidence of airborne transmission of the severe acute respiratory syndrome virus. N Engl J Med 2004;350:1731-1739.