Humans have polluted the globe with microscopic pieces of plastic, from ocean depths to mountain heights. We have even taken these microplastics into our bodies, with unknown consequences.

Plastic pollution has become commonplace, with images such as a turtle suffocated by a shopping bag, water bottles washing up on beaches, and the gigantic "Great Pacific Garbage Patch" of floating debris.

Millions of tons of plastic, mostly derived from fossil fuels, enter the environment and break down into smaller and smaller pieces.

"We did not imagine 10 years ago that there could be so many small microplastics, invisible to the naked eye, and that they were everywhere around us," said Jean-Francois Ghiglione, a researcher at the Laboratory of Microbial Oceanography in France. "And we could not yet envisage finding them in the human body."

Read next: Microplastics found in fresh Antarctic snow

Scientific investigations are increasingly identifying microplastics in several human organs, including "the lungs, spleen, kidneys, and even the placenta," Ghiglione told AFP.

It may not come as much of a shock that we breathe in these particles present in the air, in particular microfibres from synthetic clothing. "We know that there's microplastics in the air, we know it's all around us," said Laura Sadofsky, from the Hull York Medical School in the UK.

Her team discovered polypropylene and PET (polyethylene terephthalate) in lung tissue, identifying synthetic fabric fibers. "The surprise for us was how deep it got into the lungs and the size of those particles," she told AFP.

Another study published in March showed the first evidence of PET in the blood. Given the tiny sample size, some scientists believe it is too early to draw conclusions, but there are fears that if plastics are found in the bloodstream, they could be transmitted to all organs.

Breathing in plastics 

In 2021, researchers discovered microplastics in both maternal and fetal placental tissue, expressing "grave concern" about the potential ramifications for prenatal development. But the concern is not the same as a proven risk.

"If you ask a scientist if there is a negative effect, he or she would say 'I don't know'," said Bart Koelmans, professor in Aquatic Ecology and Water Quality at Wageningen University. "It's potentially a big problem, but we don't have the scientific evidence to positively confirm what are the effects if any."

One theory is that microplastics are to blame for some disorders that harm human health. Although scientists have just lately discovered their presence in the body, it is possible that humans have been eating, drinking, and breathing in plastics for many years.

Read next: Nowhere is clean: Study finds plastic in supposedly 'clean' air

According to a shocking analysis released in 2019 by the environmental charity WWF, people consume and inhale up to five grams of plastic per week – enough to produce a credit card.

Koelmans, who questions the study's methodologies and findings, believes the amount is more akin to a grain of salt. "A grain of salt every week is still quite something over a lifetime," he told AFP.

While human health studies have yet to be conducted, toxicity in certain animals raises concerns.  "Small microplastics invisible to the naked eye have deleterious effects on all the animals that we have studied in the marine environment, or on land," said Ghiglione.

He went on to warn that the compounds in these materials, such as dyes, stabilizers, and flame retardants, can affect growth, metabolism, blood sugar, blood pressure, and even reproduction.

According to the study, a "precautionary" strategy should be taken, advising customers to buy fewer plastic-packaged products, notably bottles.

Annual production of fossil-fuel-based plastics is set to top 1.2 billion tonnes by 2060, with waste exceeding one billion tonnes, the Organisation for Economic Co-operation and Development said last month. "People cannot stop breathing, so even if you change your eating habits you will still inhale them," said Koelmans. "They're everywhere."