Could masks be making us sick? That’s the suggestion in a Japanese study, published this week in Nature’s Scientific Report’s journal, which looked at bacterial and fungal growth on face masks worn during the pandemic. The results may put you off your tea.

The study looked at the masks of 109 people and shows that bacteria grows in bigger colonies on the inside of the mask compared with the outside. The opposite was true for fungus. Wearing the same mask for a long period of time ‘significantly’ increased the amount of fungus growing on a mask but had no effect on the amount of bacteria.

Every mask bar one – 99 per cent of the masks examined – contained bacterial colonies on the inside and 94 per cent on the outside. The type of mask worn – cloth or disposable – made no difference to the spread of bacteria. On the other hand, fungus was found on the outside of 95 per cent of masks and on the inside of 79 per cent. An interesting finding was that bacterial counts were lower on the insides of masks worn by women. It suggested this may be because they wear makeup and take better care of their skin.

What you eat can be a factor too. The Japanese are fond of a dish called Natto – which is fermented soybeans. Perhaps unsurprisingly, the study participants who consumed the dish regularly had a nearly three times higher count of B. subtilis bacteria on their masks, the same bug used to brew the beans.

Although many of us have now ditched masks there’s a stubborn minority holding out. They perhaps have a false sense of security, given the most convincing transmission benefit of masks occurred when a large percentage of the population wore them – protecting others from transmission rather than themselves. That quid pro quo simply doesn’t work with just a third of us wearing masks now.

Mind you that doesn’t stop the persistence of some organisations when it comes to encouraging mask wearing. Some NHS trusts have brought back mandatory mask wearing. Also, you don’t need to spend long in a London tube station before you’re greeted with ‘please wear a facemask if it makes you feel comfortable’.

The majority of bugs and germs found on the masks weren’t dangerous to humans but several were found that can cause serious harm. Bacillus cereus for example can cause vomiting and diarrhoea, while Staphylococcus saprophyticus can cause bladder infections and Aspergillus is a mould that can cause respiratory issues.

The researchers made clear that many of these risks can be avoided by regularly changing masks. These risks are worrying enough that the authors recommend official advice be changed: people with weakened immune systems shouldn’t wear the same mask twice. As new variants come and go and as cases peak and wane the debate around masks continues. This new evidence, about the hygiene of face coverings, can only help those discussions and the decisions we make.

Michael Simmons is a data journalist at The Spectator