=biology =materials =polymers =suggestion
Can you guess what car brakes and microfiber fabrics have in common?
Microplastics have become
something of a meme among some millenials and zoomers. The microplastic
memes have a different feel than concerns about global warming. It feels
like global warming is a problem that people could still maybe do something
about, but if microplastics are bad...they're already everywhere. So the
memes are more...nihilistic.
Mice fed microplastics
end up with some in
their blood. Microplastics have also been
found in human blood. So, how bad is that?
Typical levels of
microplastics in human blood are ~2 ug/dL. Typical blood lead levels in the
US are ~0.6 ug/dL. Are microplastics even 1/10 as bad as lead? Probably not,
so you should probably be more concerned about lead. (Ban leaded avgas now!
Legalize car engines in aircraft!) But that could still be worth doing
something about.
In any case, you have to ask: how
are those microplastics getting into blood? I don't think they're just
permeating the intestinal lining; they may be small, but they're not that
small. (Also, blood is pressurized, and osmotic pressure isn't relevant for
such particles.)
My guess is that microplastics are being carried
into blood by macrophages
"eating" them. The
intestinal lining has a lot of macrophages, and they can eat stuff that's
small enough. Bacteria are generally <2 um diameter and <10 um long; I
wouldn't expect macrophages to often eat anything >10 um in diameter.
Some people didn't have detectable microplastics in their blood, despite
almost certainly being exposed to them. If microplastics are taken in by
macrophages, immune system variation would explain that.
Hopefully,
those macrophages would then go somewhere innocuous to die and leave their
microplastics, such as inside the gut, the epidermis, or the appendix. In
that case, you might see some microplastic accumulation in the appendix.
What concerns me more than people
eating microplastics is inhalation.
Only small
particles can get deep into lungs - ones <10um in diameter, especially
<2.5um. But what happens afterwards when they do?
When someone
inhales small sand
particles, what happens isn't just that they get into the lungs and stay
there forever. The particles get eaten by macrophages, but they're sharp so
they poke through the membrane containing them, which causes those
macrophages to die in the lungs, which causes various problems including
scarring.
Only a small fraction of microplastics in intestines would
get eaten by macrophages, but if small particles get into the lungs, they
all get eaten by macrophages.
Particulate pollution is
associated with systemic oxidative stress. There are multiple theories
about this, including general immune system activation, but what I think is
this:
When macrophages eat particles, they try to break them down with
hypochlorite and nonspecific proteases. When the particles are inert - like
graphitic carbon, or polyethylene - they don't break down, and the
macrophages keep generating hypochlorite indefinitely for as long as they
live. Some hypochlorite always gets out and causes problems, and then you
have oxidative stress - which is, of course,
bad.
Some scientists tried making people inhale carbon particles with radioactive
labels, and the particles were
detected in the bloodstream within minutes.
In this sense,
microplastic particles would be about as bad as inorganic carbon particles
from eg cars - and those are more common than microplastic particles, but
microplastics would be relatively more common indoors.
In the air,
most plastic particles get degraded by UV light and oxidation. In water or
soil, some are stable for a very long time. Polyester particles in water
will degrade eventually, but polyethylene won't. However, polyester seems to
be the microplastic type that humans are most exposed to, and I think that's
because most plastic fibers are polyester. If you rub polyethylene with
sandpaper it will make some small particles, but it's easier to break off
small fibers - in a clothes dryer, for example. Note that some older studies
looked at microplastic levels in tissue by using hot NaOH to dissolve
everything organic, but that also degrades PET, so it wasn't noticed as
much.
Particle inhalation is mainly concerning for particles <10um in
diameter. Polyester microfiber is defined as fibers with widths less
than...10um. But people like microfiber. The smaller the fibers, the easier
they bend, the softer the fabric. The smaller the fibers, the better they
insulate. Big polyester fibers are stiff and uncomfortable.
OK, so is
there something less concerning that could be used instead? Yes,
poly(butylene succinate) fibers would work well enough for clothes, and they
can be biodegraded quickly enough. Unfortunately, for the same reason
they're biodegradable, they would also get somewhat degraded by washing
machines. Maybe ribbon-shaped polyester fibers would make less
microplastics, something like 100um x 5um. I'm not sure what properties
clothes from such fibers would have, but it seems worth considering.
Now do you see what car brakes and microfiber fabrics have in common? That's right: wear of manmade items producing particles that can be inhalation hazards causing problems largely mediated by macrophages, and a complete lack of consideration for that issue in design or regulation.
Here are the main current types of brakes. All of them produce hazardous
types of particles when used.
What could brake pads be made of
instead to avoid this issue? As you would expect from
basic molecular toxicology, particles of
iron oxide and magnesium oxide are relatively innocuous when inhaled.
(Steels with nickel or chromium are much worse.) So, brake pads could be
made of MgO particles in mostly-pure iron. Pure iron is weaker than steel,
but that shouldn't be a big issue if it was combined with MgO.