The epidemic of severe acute respiratory syndrome (SARS) in Hong Kong in 2002-2003 was linked to plumbing design by the World Health Organisation (WHO), which speculated that the virus spread through a faulty plumbing system. The WHO concluded that defective U-trap seals allowed the disease to be carried through the plumbing in the Amoy Gardens building complex but could not provide evidence for the interconnectedness of the sanitary plumbing system and airflow communication pathways.

Now a new report has provided evidence of the link between plumbing and the SARS outbreak, highlighting the risk of illness as a result of transmission routes within sanitary plumbing systems.

Since the 1800s the prevention of contaminated air seeping into habitable space has been a key concern in the design of building drainage and vent systems. Complex drainage venting schemes were developed to ensure local air pressure at the appliance trap was never sufficient to deplete the trap through applied suction pressure or to drive through the trap into habitable space by the action of positive system pressures.

The appliance waste was connected downstream of the trap to vent pipework that eventually led to an open termination above roof level, imposing atmospheric pressure as a terminal boundary condition.

Engineers adopted this passive ‘if it exists vent it’ approach but the method had to be modified as buildings became increasingly complex.

There are now a range of protections and local active controls, including air admittance valves and variable volume containment devices, being used to limit local air pressures and protect the trap seal. The acceptability of these has been part of a long-running debate but it is now acknowledged that an ‘engineering only’ approach to understanding the inherent risks in system operation is insufficient.

This was highlighted during the SARS epidemic in Hong Kong, when the WHO and local investigator reports found dry U-traps were the primary cause of fatalities in Amoy Gardens, which was seen as a ‘super-spreading event’. A lack of maintenance had allowed floor drains in bathroom areas to become dry, resulting in a loss of sealing function that enabled the aerosolised virus to enter different apartments in the building from the sewerage system. At the time no biological evidence was presented to support this hypothesis.

Now though a new research report, Pathogen cross-transmission via building sanitary plumbing systems in a full scale pilot test-rig published in the Public Library of Science ‘PLOS One’ scientific journal, has backed the WHO result. It tested the WHO hypothesis and found evidence that pathogens can be transmitted within airflows and deliver an infective exposure dose via empty U-traps.

The researchers built a sanitary plumbing system to represent two floors of a building, with simulated toilet flushes on the lower floor and a sterile chamber with extractor fan on the upper floor. Cultures of a model organism were flushed into the system in volumes of 6 -20L to represent single or multiple toilet flushes, while air and surface samples were cultured on agar plates and assessed qualitatively and semi-quantitatively.

The report found flushing from a toilet into a sanitary plumbing system generated enough turbulence to aerosolise pathogens and typical sanitary plumbing system airflows were enough to carry aerosolised pathogens between different floors of a building. Empty U-traps allowed aerosolised pathogens to enter the chamber encouraging cross-transmission and all parts of the system were found to be contaminated post-flush.

Defective conditions that allow pathogens to be carried on airflows within sanitary plumbing systems are not uncommon and the authors say greater consideration needs to be given to this mode of transmission. While plumbing may not be high on everyone’s agenda, the findings in this report prove the important role plumbing systems can play in disease control.