The hygiene mythbuster

You find me in Japan where I might just have found the best means yet for the NHS to waste further millions in the futile pursuit of total environmental sterility.

I thought tales of loo seats that lift automatically as you walk in then – once you’ve finished – pressure wash and blow-dry your bits before flushing all by themselves were the stuff of urban myth. Not so. They also deodorise, disinfect and even play sounds to mask any audible evidence of a call of nature. The ‘dual cyclone’ flush is no empty marketing phrase either; if you shift your weight and inadvertently set the darn thing off before standing up you will experience its full horror. I could quite get used to the heated seats though.

NHS Trusts seem to fit into two camps when it comes to loo seats. There are facilities that insist we lower the lid before flushing to prevent the aerosol spread of germs and those that have removed lids completely lest we touch them and get germs on our hands.

Studies have shown that toilet flushing does indeed create aerosols that can spread all sorts of nasties over quite some distance. But despite these practical studies, epidemiological data (i.e. how infections spread) to quantify any risk is practically non-existent. It’s quite difficult to separate contamination via a toilet flush aerosol from the myriad of other ways bugs can get spread about the place.

Nonetheless, limiting the spread of germs from toilet flush aerosols would seem to be a good idea even if there isn’t ‘smoking gun’ scientific evidence this has actually made people sick. But in the absence of data we need to look at basic biological plausibility. Could limiting the aerosols produced by flushing loos help prevent the spread of infections?

Flushing out the truth about aerosols

The most recent research shows you get between 150,000 and 2 million aerosol droplets generated per flush. Yes, people really do measure such things. Over 95% of those droplets are smaller than 2μm across (that’s two thousandths of a millimetre) and over 99% are smaller than 5μm. This is important for two reasons: first, even the smallest aerosol droplet is big enough to carry bugs and second the length of time an aerosol droplet spends in the air (before falling out of it) varies depending on its composition and size. This has been studied extensively in the context of workplace exposure to aerosols of different types. Some other variables such as air turbulence creep in but in essence particles sized at 10µm stay in the air for about three minutes, 5µm particles for about eight. 

But the overwhelming majority of aerosol droplets created by flushing a loo are smaller than 2µm. These will take 4 to 5 hours to settle and anything smaller than 1µm will not settle at all but stay airborne indefinitely. So, while it might seem desirable to limit such droplets leaving the toilet, germs will escape no matter what, even were someone to contrive a loo seat that could hermetically seal itself for hours after each flush.

The absolute risk from aerosols produced by flushing is negligible and leaving the seat down may reduce that very small number further. From a microbiological perspective everything is covered by a fine patina of animal stool anyway but it’s important to consider relative risk here: any threat from aerosolised germs pales into insignificance next to the risk associated with poor hand hygiene. If food handlers or healthcare workers get that bit wrong no £1,000 loo seat will add karma to your khazi…

Seán Derrig is scientific director at Chemex International