40 | New Scientist | 7 December 2019
we ingest more than 68,000 particles a year
from this source. If there was any doubt that
we regularly ingest them, a study last year put
that to rest. It looked at samples of human
faeces from around the world and found
microplastics in all of them.
But how much we swallow and to what effect
is unknown. The ones in our faeces are actually
the least of our worries, says Heather Leslie of
Vrije University Amsterdam, because they are
simply passing through. What we really need
to worry about is whether microplastics
damage the lining of the gut, or are absorbed
into the bloodstream.
According to Ingeborg Kooter of the
Netherlands Organisation for Applied
Scientific Research (TNO) in Utrecht, whatisn’t clear. “Do they have an effect on lungs?
The honest answer is that there is not much
knowledge,” says Fransien van Dijk at the
University of Groningen in the Netherlands.
Ditto the risk from microbes that colonise
plastics. According to Ana Maria de Roda
Husman of the Netherlands National Institute
for Public Health and the Environment in
Bilthoven, microorganisms grow extremely
well on microplastic surfaces and are
“probably” vectors of bacterial and viral
disease, as well as antibiotic resistance genes.
Chemical toxicity is also murky. According
to an analysis by the European Food Safety
Authority, one portion of mussels can deliver
small doses of known toxins including
polychlorinated biphenyls (PCBs), polycyclic
aromatic hydrocarbons (PAHs) and bisphenol
A (BPA). However, it found that the quantities
are negligible compared to what we ingest from
other sources. But this is just one analysis. The
sheer diversity of compounds that could be
dumped in our bodies by microplastics means
we have still only scraped the surface.
As for biological effects, animal studies
have been done but – no surprise – they
aren’t very informative. Most involve aquatic
organisms and, although they often find
toxic effects, the experiments typically use
very high concentrations of clean, spherical
microplastics that aren’t representative of
real-world exposure.
Despite these uncertainties – or perhaps
because of them – scientific authorities have
been quick to damp down speculation of a risk
to public health. SAPEA concluded that “we
have no evidence of widespread risk to humanSOUR
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Number of microplastics we eat in a
year from particles falling on foodwe know about the behaviour of small particles
suggests that uptake across the gut wall is likely
to be very limited, with only 1 to 2 per cent of
the smallest nanoplastics getting across. That
may not seem much, but smaller nanoplastics
are more likely to be hazardous. “They can
more easily pass the membranes and enter
blood circulation and maybe even pass the
blood-brain barrier or the placenta,” says
Vethaak. “In principle.”
The problem is that when you get down
to the really tiny particles, we have no idea
how many we are exposed to or have in our
bodies. As far as we know, plastic breaks down
into smaller and smaller pieces so it is a fair
bet to assume that they exist at nanoscales.
Yet analytical methods aren’t sensitive
enough to detect them in the environment
on this scale. In fact, according to the SAPEA
consortium report, we can’t confirm that
nanoplastics even exist.
It is a similar story for inhaled plastics.
The upper airways are good at clearing out
small microplastic particles, says Kooter, but
nanoplastics might penetrate deep into the
lungs and possibly cross into the bloodstream.
Even if they do, the biological consequence700
billion
Number of microplastic particles
generated in an average house per yearhealth from [micro and nanoplastics] at
present.” The World Health Organization noted
that humans have ingested microplastics for
decades with no signs of serious health effects.
Routine monitoring of microplastics in
drinking water isn’t necessary, it said.
Scare over? Far from it. This relaxed stance
surprises and dismays many researchers
working in the field. “I’m a critic,” says Vethaak.
“It would have been better if they said, ‘Sorry,
the data is very limited, we don’t know
anything about these very small particles,
we need more research’.”
“I think we’re making a certain error of logic
called the ‘appeal to ignorance’ fallacy,” says
Leslie. “Absence of evidence [of harm] is not
evidence of absence. The last thing we need
now is another large-scale, long-term threat
to human or ecological health.”
Leslie is one of a group of researchers
urgently working to reduce that absence of
evidence. Earlier this year, she received funding
from ZonMw, the Netherlands Organisation
for Health Research and Development,
to find out whether microplastics enter
the bloodstream. Hers is one of 15 quick-
turnaround projects funded by ZonMw to
the tune of €1.8 million. These will report their
results next year and those with interesting
findings will be given more money to follow up.
The research is an attempt to shed light on
five key concerns over micro and nanoplastics
and health: risk from food, risk from
inhalation, effects on the immune system,
whether the particles reach the brain or cross
the placenta, and their potential as carriers of
pathogens. The projects are all ongoing, but
some presented interim results at the Plastic
Health Summit in Amsterdam in October.
Although preliminary, they don’t make for
comforting reading.
Take a project led by Nienke Vrisekoop at the
University Medical Center Utrecht on how the
immune system responds to microplastics.
She says previous research has shown that in