The horrifying truth about closing the loo lid when you flush

It’s a well-known, although not always well-observed, piece of hygiene advice – close the loo seat lid before you flush, or you risk potentially infectious particles of faecal matter being spread around the bathroom in a fine, invisible mist.

A 2022 study by the University of Colorado in Boulder, US, appeared to confirm the received wisdom.

Researchers used lasers to illuminate the ‘plumes’ of mist that spray from the bowl when it is flushed, finding that these can shoot up 1.5 metres high within eight seconds. The aerosolised loo water also spread horizontally.

The worry is that if the last person to use the bathroom had a highly infectious illness, such as norovirus or hand, foot and mouth – which is known to be spread in faeces as well as via other forms of transmission – the next user could unwittingly be exposed to bacteria or viruses in the air and other surfaces, especially in public loos.

And at this time of year, when many illnesses run rampant, caution is more important than ever.

‘When you flush, traces of whatever you deposited into the pan will be present in the spray that is created with the water flush,’ says Dr Primrose Freestone, an associate professor in clinical microbiology at the University of Leicester.

‘Human faeces can carry a range of transmissible potential pathogens: Campylobacter, Candida, Cryptosporidium, Enterococcus, Escherichia coli, Salmonella, Shigella, Staphylococcus, Streptococcus and Yersinia bacteria – as well as viruses such as norovirus, rotavirus and hepatitis A and E, and Covid-19 to name just a few.’

The Colorado study, published in the journal Scientific Reports, did not test how much the spray was reduced when the lid was shut before flushing.

But new research suggests that even closing it does not solve all these pathogenic problems.

In the study, published in the American Journal of Infection Control, researchers at the University of Arizona found that although putting the lid down somewhat reduced the spray, it did not significantly prevent the areas around the loo becoming contaminated.

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Not surprisingly, the lid and seat were the most contaminated areas in all cases, although there was also some contamination seen on the walls and floor.

‘The infection risk would be reduced [by putting the lid down] as the spray trajectory from the flush is reduced, but you now have a contaminated lid to clean,’ says Dr Freestone.

‘So wipe the lid with tissue, then spray with an anti-infective surface spray or bleach; allow to dry naturally.

‘As you are working on the lid, cleaning the seat and flush handle would be a good idea. And then wash your hands thoroughly.’

But even if you don’t manage all this, it is this last instruction – to wash your hands – that is key.

After all, no study has yet proved that these plumes can definitively lead to infection, as it’s the dose of faecal pathogens – the number of germ cells – that determines whether an infection is established, says Dr Freestone.

And while ‘faecal matter contamination in the environment [termed sewage] is regarded as a major source of human infection’, it’s not known if there is enough of any one pathogen ejected from a loo flush to cause illness.

Instead, infections ‘usually come from eating contaminated food or hand-to-mouth transfer of bacteria as a result of faecal contamination of hands, food and objects or surfaces’, explains Dr Freestone.

It’s dirty hands that spread contamination on to surfaces, and other hands that then pick up those bacteria or viruses and transfer them into our mouths or eyes (when we rub them).

‘Handwashing is therefore very important but not everyone washes their hands properly after going to the bathroom, so anything those unwashed hands touch can become contaminated with faeces,’ warns Dr Freestone.

‘Hands that have just wiped bottoms might then press the lever of a soap dispenser, or turn on a tap, and turn off [with now clean hands] the same [now contaminated] tap.’

So at home, clean all bathroom surfaces regularly with disinfectant, and be religious about your own hand hygiene at all times to avoid infecting yourself.

You can also alleviate any worries about potential infection from loo flush plumes by cleaning towels, bath mats and wash cloths frequently.

As Dr Freestone explains: ‘In an open bathroom the spray from the loo could reach towels – how far that spray goes depends on the water pressure and how close the towels were to the loo.’

On the basis that the Colorado study showed the spray from an open loo could reach 1.5 metres high, and the spray can move with a breeze, most small to medium-sized bathrooms could have towels in the potential vicinity of the plume.

‘To kill the germs on bathroom or hand towels the NHS advises washing in a laundry detergent at 60C after every day of use,’ adds Dr Freestone.

‘This stops towels smelling musty and ensures they are properly sanitised, important if towels are shared or someone in the house has a transmissible infection.’

And finally: one more reason to keep your things tidy and stowed away in the bathroom cabinet rather than leaving toothbrushes, makeup and washcloths out on the counter top.

‘Keeping your toothbrush under cover is a good idea, as it stops it being potentially contaminated by any flying insects crawling over it and depositing germs, dust settling on it, or contamination from random droplets from the loo flush spray,’ adds Dr Freestone.

‘The NHS advises rinsing your toothbrush with hot water after every use and allowing it to air dry between uses.’

Mum was right about putting things away neatly after all!