The discovery means small particles could travel around the body and migrate to different organs.
The effects are not yet known, but tiny plastics are known to damage human cells in laboratory experiments and there are concerns they could affect immune function.
“We have now proven that our bloodstream, our river of life as it were, has plastic in it,” said Heather Leslie, an ecotoxicologist at VU Amsterdam, who helped lead the research in the Netherlands.
The researchers analysed samples from 22 healthy, anonymous donors to search for the presence of five polymers.
The research found that three quarters of subjects had them. The research was published in the journal Environment International.
The overall concentration of plastic particles in the blood amounted to an average of 1.6 µg/ml, which is comparable to a teaspoon of plastic in 1,000 litres of water, or 10 large bath tubs, researchers said.
Polyethylene terephthalate (PET), which is used for packaging, bottles and household containers; polyethylene, used to make shopping bags; and polystyrene were the most common.
The researchers wrote it is “scientifically plausible that plastic particles may be transported to organs via the bloodstream”.
“The human placenta has been shown to be permeable to 50, 80 and 240nm polystyrene beads and likely also to microsized polypropylene.”
But the effects are not yet understood.
Researchers said the study, the first of its kind, must be followed up to understand how widespread the problem is and how harmful plastics may be to our health.
“With this insight we can determine whether exposure to plastic particles poses a threat to public health,” said Marja Lamoree, an analytical chemist with VU Amsterdam.
A 2019 report from the WHO found microplastics, defined as any plastic debris less than five millimetres in length, in tap and bottled water do not appear to pose a risk to human health.
It said larger particles, and most smaller ones, pass through the body without being absorbed.
Microplastics are found everywhere.
Tiny particles are present in tea bags and even snow, with airborne particles finding their way to alpine environments.
Research in 2020 showed that the average person eats five grams of plastic each week, the equivalent of a credit card.
That fact was among findings of a study by the World Wildlife Fund, carried out by the University of Newcastle, Australia.
The same report revealed that some people consume 2,000 tiny pieces of plastic each week, about 21 grams a month.