Thankfully my mum is soon coming to visit me with a suitcase full of the best of Britain: chocolate, tea and savoury snacks

My mum, who lives in England, is coming to visit me in Philadelphia soon and I have given her strict instructions on what to bring from the motherland. Namely, deodorant. You can get a lot of things in the US – it is a land of excess – but the deodorant offerings here stink.

Most American women, you see, tend to use stick deodorants. In the UK, meanwhile, it’s far more normal for women to use a roll-on. Or, at least, its far more normal for shops to stock a wide range of roll-ons. I don’t have some sort of degree in deodorant studies, I should probably note; I have not done extensive market research on this matter. This assertion is based on empirical data gathered over a lifetime of deodorant shopping, along with several years of arguing with my American wife about our underarm preferences. She thinks roll-on deodorant is (trigger warning: an objectionable word is about to appear) moist and leaves a wet sensation. I think that stick deodorant feels clammy and flaky. Clearly I am right.

In the data-driven, optimisation-obsessed corner of the internet, everything has its optimal moment, and washing is no exception

One of my favourite things to do in my middle age is lie on the sofa eating crisps while Googling low-effort ways to optimise my life.

Thanks to data-driven obsessives online, I have discovered that there is an optimal time at which to do everything. Exercising in the afternoon , for example, may reduce the risks of early death more than a morning or evening workout. Delaying your morning coffee until two hours after waking improves cognition and energy, according to Andrew Huberman, a controversial neurobiologist and podcaster who has developed a cult following for his health and fitness “protocols”. As for the best time to have a shower? I’m glad you asked. I have done extensive research on the matter – and it’s complicated.

Enlarge / Whether the toilet lid is up or down doesn't make much difference in the spread of airborne bacterial and viral particles. (credit: Peter Dazeley )

File this one under "Studies We Wish Had Let Us Remain Ignorant." Scientists at the University of Arizona decided to investigate whether closing the toilet lid before flushing reduces cross-contamination of bathroom surfaces by airborne bacterial and viral particles via " toilet plumes ." The bad news is that putting a lid on it doesn't result in any substantial reduction in contamination, according to their recent paper published in the American Journal of Infection Control. The good news: Adding a disinfectant to the toilet bowl before flushing and using disinfectant dispensers in the tank significantly reduce cross-contamination.

Regarding toilet plumes, we're not just talking about large water droplets that splatter when a toilet is flushed. Even smaller droplets can form and be spread into the surrounding air, potentially carrying bacteria like E. coli or a virus (e.g., norovirus) if an infected person has previously used said toilet. Pathogens can linger in the bowl even after repeated flushes, just waiting for their chance to launch into the air and spread disease. That's because larger droplets, in particular, can settle on surfaces before they dry, while smaller ones travel further on natural air currents.

The first experiments examining whether toilet plumes contained contaminated particles were done in the 1950s, and the notion that disease could be spread this way was popularized in a 1975 study . In 2022, physicists and engineers at the University of Colorado, Boulder, managed to visualize toilet plumes of tiny airborne particles ejected from toilets during a flush using a combination of green lasers and cameras. It made for some pretty vivid video footage:

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