“ I
HATE housework. You make the beds, wash
the dishes and then six months later you
have to start all over again”. Adherents of
this Joan Rivers quote got a fillip back in
1989, when David Strachan, then an
epidemiologist at the London School of
Hygiene and Tropical Medicine, proposed his hygiene
hypothesis. He suggested that modern lifestyles, with
their lack of unhygienic exposure, mean we catch fewer
infections in early childhood, and that this predisposes
us to developing allergies.
Some of the evidence is fascinating. Children who
grow up on farms seem to be healthier, as are children
whose parents wash dishes by hand rather than using a
dishwasher, and those who have a dog. Meanwhile, a
2014 paper found that children are less likely to develop
precursors to asthma if, during their first year, they’re
exposed to particles from cats, mice and cockroaches.
Many of us have embraced the idea of a link between
a little bit of dirt and good health. There’s just one
problem: things probably aren’t that simple. In recent
years, thousands of studies have linked changes in the
microorganisms living inside our bodies to everything
from allergies and asthma to obesity, depression, and
Alzheimer’s disease. This emerging understanding of
what’s known as our microbiome suggests that, rather
than there being a clear distinction between “clean”
and “dirty”, we have a complicated relationship with
bacteria. Being exposed to some kinds is good for us, to
others not. And figuring out which is which isn’t easy.
One thing is becoming clear, however: it’s diversity
that counts. A 2015 study of nearly 400 infants in
Germany hinted as much, when researchers found no
direct link between their chances of developing asthma
or allergies and their caregivers’ personal and home
cleanliness. What did make the difference was their
exposure to bacteria. Several studies show that
healthier people tend to host a wider range of microbes.
YAWestern lifestyles may have adverse effects on thisNATATEVOSIAN/ISTOCK^ PHOTOThe coronavirus pandemic has reminded us of the importance of healthy immune systems
to help us ward off infection. In two extracts from the new New Scientist Essential Guide:
Human Health, we examine ways of optimising our immune response. Overleaf we examine
claims made of some dietary supplements, but first we investigate a troubling question:
has an obsession with killing off all bacteria using heavy-duty cleaning agents actually
made us more susceptible to certain conditions?
Essential Guide Extract
diversity, which was found to be 40 per cent lower than
that detected in an isolated, traditional hunter-gatherer
society. And a 2013 study found that infants whose
parents picked their dropped dummy up off the street,
sucked it clean, and then handed it back to their child,
were less likely to develop asthma or eczema. The
organisms in their saliva were also different from the
ones inhabiting infants whose parents would never do
such a thing, prompting the researchers to suggest that
the protective effect is down to the transfer of
beneficial microbes from parent to child.
There is now also evidence for why living on a farm is
so good at reducing allergies – the bacterial
components in farm dust suppress inflammatory
immune responses. This seems to indicate that the
hygiene hypothesis is in need of revision. Possibly it is
not so much infections as exposure to a wide variety of
bacteria that benefits our immune systems.
When we are exposed to these beneficial microbes is
also crucial. The most important time seems to be early
childhood. According to current thinking, the cut-off
may be as early as three. So, if you’re the parent of a very
young child, it is probably very important to take them
outdoors and let them play on the ground.
By the time we are adults, our microbiome may be
largely set, so any suggestions that shirking the
chores – or your personal hygiene – could improve your
health, is spurious, and potentially even harmful.
For example, there are about 17 million cases of food
poisoning a year in the UK, caused by microbes
including Norovirus and Campylobacter. Many cases
are picked up in restaurants, but a study of 18 European
countries suggests that nearly a third of food-borne
outbreaks occur inside our own homes.
Filth has consequences. About 50 per cent of
chickens in the UK carry Campylobacter, the country’s
most common cause of food poisoning, and the
bacteria can easily transfer from the raw meat to your
hands or a chopping board. Seen in this light, >