A review of 22 flush studies puts aerosols in the adult breathing zone for over 20 seconds

Flushing creates turbulence, and the turbulence throws particles out of the bowl into the room ^[13]. Each one measures nanometres to micrometres across, usually invisible, and small enough to reach deep into the respiratory tract once inhaled ^[13][14]. What they carry is whatever was deposited in the bowl plus whatever was living in the water itself ^[14].

Eleven of the 22 pooled studies seeded the bowl on purpose with viruses, bacteria or fluorescent surrogates; the other eleven sampled under unseeded conditions ^[5][22]. The pool also mixes seated pans, squat pans and urinals at a range of flush volumes ^[6], and concentrations varied significantly from one study to the next ^[9]. The flush-volume association is drawn across that mix. In my view it is still the most useful number in the paper, because flush volume is the one variable a building owner selects, with the condition that the same review found bowl seeding tracking concentration just as clearly ^[8].

The tallest detection, 163 centimetres above floor level, comes from a single study, about 4.5 percent of the included literature ^[11][21]. That is a maximum someone measured, not a central estimate. The suspension time is given as over 20 seconds, which is a lower bound ^[7].

Air-sample data and reported aerosol and bioaerosol concentrations were conditions for a study to enter the review ^[4]. Illness was not one of them. The link from bioaerosols to infections, coronaviruses among them, comes from separate work in the International Journal of Hygiene and Environmental Health ^[15].

On lids, the authors suggest a closed one may influence dispersal and reduce the number of aerosols at breathing height ^[18]. Adiyani went further in a statement. On the available evidence, she said, she would recommend closing the toilet lid before flushing where a lid is available, as a precaution to minimize potential inhalation exposure, while noting that "it does not completely prevent aerosols from escaping" ^[19].

The team says the review highlights important gaps in current research, and that working out where and how aerosols are distributed after flushing could inform safer toilet designs ^[16]. It also says the dispersal cannot be eliminated entirely ^[12]. The practical list the researchers offer is regular cleaning, adequate ventilation from an exhaust fan or an open window, and selecting a model with lower flush volumes when possible ^[17].