What does school smell like? For misty-eyed nostalgics, it might be the aroma of school dinners or dusty books. For cleared realists, it might be the stench of manky toilets in the infants or body odour (by that, I of course mean the whiff of teenage pupils clad in man-made fibres, not that of the ever-fragrant staff).

But in the wake of the coronavirus pandemic, there is only one answer that anybody in a school can give: the defining smell is that of Dettol (or Cif or Cillit Bang, or some equally effective alternative product with a brand name that is at once powerfully clean-sounding yet totally inexplicable).

It’s with good reason, naturally: blitzing door handles and toilets four times per day, scrubbing floors every 30 minutes and wiping down tables every 15 minutes is how schools have been advised to attempt to prevent the spread of Covid-19 (OK, the number of times may be exaggerated but you get the picture).

And surely no teacher would question such practices: everyone wants to stay safe.

Except…plenty of school staff will have in their minds a half-remembered theory that children must be exposed to germs in order to develop healthy immune systems.

When people put forward such an idea, they might be invoking - whether they realise it or not - the so-called “hygiene hypothesis” first set out in 1989 by British epidemiologist David Strachan. The concept suggested that not being exposed in childhood to micro-organisms, such as bacteria and moulds, damages the proper development of our immune systems, and that improved hygiene and cleanliness could be the cause of rising rates of allergies, such as hay fever, in the developed world.

Could it be true? In the cleaning frenzy of the pandemic era, can schools be “too clean”? In the drive to splash the Flash, could schools actually be harming the health of children in the long term?

No place like microbiome

Jack Gilbert, a professor in the department of pediatrics at the University of California San Diego (UCSD), quite literally wrote the book on the benefits of germs to children - he is the author of Dirt Is Good: the advantage of germs for your child’s developing immune system. He studies the microbiome: the genetic material of all the microbes - including bacteria, fungi and viruses - that live inside and on the human body.

Despite being firmly pro-dirt, Gilbert says the current pandemic cleaning measures are unlikely to be an issue.

“A child’s microbiome will not be negatively impacted by washing their hands or wearing a mask,” he explains. “Proper nutrition, healthy outdoor activities…and interacting with pets, are going to have a huge [beneficial] impact, whereas wearing a mask, washing your hands and interacting with sterilised surfaces in a school is going to have virtually no impact at all on your child’s microbiome health.”

Does Gilbert’s colleague Richard Gallo, Irma Gigli distinguished professor in the department of dermatology at UCSD, share that view? He previously led research, widely reported in the media, which purportedly found that being “too clean” can impair the skin’s ability to heal.

“Current recommendations for handwashing [in the pandemic] do a good job of balancing the need to kill pathogens and not damage the skin or its microbiome,” he says. “If your skin is becoming red or itchy, that is the sign you’re doing too much [handwashing] and damaging the microbiome.”

This may have teachers nervously looking at the pink hands of their pupils. But Gallo says the “real risk of being too clean” is that harsh sanitisers used on surfaces come into contact with the skin or are inhaled.

So nothing to worry about then? Actually, there is: the idea of being too clean is itself a danger. Plenty of academic researchers call for the so-called “hygiene hypothesis” to be abandoned, saying there is no evidence to suggest that “hygiene” is the cause of increased rates of allergies. They add that such an idea is dangerous if it means people allow disease-causing pathogens freer rein through poor food hygiene and lack of handwashing and cleaning.

A key voice in this debate has been Sally Bloomfield, honorary professor at the London School of Hygiene and Tropical Medicine and chair of the scientific advisory board of the International Scientific Forum on Home Hygiene (IFH). Bloomfield laments that journalists often use the terms “hygiene” - defined by the IFH as “actions aimed at protection against infectious diseases” - and “cleanliness” interchangeably, thus giving rise to the “too clean” misconception.

The IFH’s approach “is to develop and promote “targeted hygiene”, which simply means that, to be effective in protecting against infection, our hygiene practice needs to be targeted at the times and in the places that matter to stop the spread of harmful microbes”, Bloomfield says.

The organisation has released a YouTube video on “the nine moments for hygiene when cleanliness really matters”, including when you’re using the toilet or changing a nappy; when coughing, sneezing and blowing your nose; and while eating with your fingers (see box, below).

There’s salient material in the last “moment” for primary school staff, in particular, who will have marvelled at younger children’s determination to eat with their fingers foods that cannot feasibly be eaten without cutlery, such as yoghurt, ice cream and baked beans (or maybe even a combination of all three).

Another critic of the so-called “hygiene hypothesis” is Elizabeth Scott, professor and associate dean at Simmons University in Boston and co-director of its Center for Hygiene and Health in Home and Community. She also advocates a “targeted hygiene” approach.

Achieving “pathogen reduction…without indiscriminate elimination of potentially beneficial microbes from the human and environmental microbiomes”, is crucial, her research argues. If a targeted hygiene approach helps reduce levels of exposure to infectious agents, fewer people will need to seek antibiotic treatment - thereby helping to tackle the global problem of antibiotic resistance, she says.

What does targeted hygiene look like? Much like the message in the IFH’s video, Scott says that a “targeted cleaning and disinfection” method, focusing on frequent-use touch surfaces, such as door handles, taps, keyboards and lunch tables, should be combined with hand washing and/or hand hygiene using alcohol-based sanitisers.

If you’re going beyond those hygiene measures in attempting to be coronavirus free, you’re probably doing too much. Bloomfield says that making sure a school Covid-secure is simply about trying to block the potential routes of infection via the air, hands and surfaces. “It’s not about trying to keep the school deep cleaned,” she stresses.

So, the official scientific advice is that we need to scrub out the notion that it is possible to be “too clean” and instead carefully target cleaning efforts on the high-risk surfaces where it needs to go.

And on the issue of the old unwritten law among staff that you never, ever eat any food prepared by the children in class? The scientists are quiet on that one, but I think you probably already know the answer.

John Morgan is a freelance journalist

This article originally appeared in the 4 September 2020 issue