Pulling the toilet -or rather pressing the button on the cistern- is a gesture that we repeat several times a day, but the path that our excrement travels afterwards is much more alien to us.

The water in the toilet and everything that it carries when we empty the cistern flows down pipes and ends in sewers. From there, it continues to treatment plants and, finally, the water, already free of contamination, returns to the natural environment.

However, although it may be unpleasant to visualize it, the truth is that small particles of the feces – which we will call bioaerosols from now on – come off the main body and take another path after renewing the water in the toilet, ascending the pipes up.

Generally, these sprays are harmless and we live better regardless of their existence, but a study published last week in the journal Annals of Internal Medicine suggests that nine people were infected with Covid-19 through this route on different floors of a housing block in the Chinese city of Guangzhou at the beginning of the year.

Avoiding contagion through saliva particles containing the coronavirus has been the backbone of the prevention strategy against Covid-19 since the beginning of the pandemic, but feces or urine also carry the pathogen and its particles are equally contagiouss when inhaled.

“When some of the patients used the toilet and flushed the toilet, the bathtub drain in the same bathroom could have allowed the bioaerosols carrying the virus to leak back into the room and other rooms connected to the system. drained, “the article explains.

But these microparticles infected with the coronavirus not only would have moved through the drainage system of the house itself, but also could have gone to other upper floors through what Chinese scientists have defined as a “chimney effect”.

“Small enough bioaerosols may have been airborne through pipes and vents for hours and been sucked into other bathrooms when suitable conditions were met.”

Contagion through fecal particles it was already registered in the epidemic generated in 2003 by SARS-CoV-1, a close relative of the current coronavirus.

So, 321 residents of a Hong Kong urbanization called Amoy Gardens were infected and 42 of them ended up dying after a single infected visitor with severe diarrhea used a bathroom in one of the buildings, whose ventilation system was in poor condition.

Although no case of contagion as massive by SARS-CoV-2 as that of Amoy Gardens has been identified, the risk exists and, therefore, the scientists authors of the article recommend “Avoid any potential gas leakage from drainage systems into closed spaces” and they underline the “importance of hygiene and ventilation of bathrooms”.