The teabags, which are often pyramid-shaped, also release 3.1 billion nanoparticles, which are even smaller.
The bags, which were introduced by tea manufacturers around 10 years ago, use polypropylene as a sealing plastic—but when heated with boiling water, they are releasing billions of particles of plastic into the cup, say researchers from McGill University in Canada.
They reviewed four commercial teas and tested the impact of the plastics on a water flea, or Daphnia. The fleas swam more erratically and even their body shape started to change.
But nobody is sure what effect they have on people's health, although the levels released by the teabags are thousands of times above current safety levels. The McGill researchers fear drinking numerous cups of tea made from plastic bags could cause behavioural and developmental problems.
"There's been very little research done on human health and toxicity of microplastics, especially on ingesting at these levels," said lead researcher Nathalie Tufenkji. The World Health Organization has estimated that two pints of water in plastic bottles has tens to hundreds of particles—far below those seen with plastic teabags.