They're in our oceans, soil, air, snow - and even in your cup of tea.

Plastic tea bags are shedding billions of shards of microplastics into their water, according to a new study.

Researchers at McGill University in Canada analysed the effects of placing four different commercial tea bags into boiling water.

They found that a single bag releases around 11.6 billion microplastic particles, and 3.1 billion even smaller nanoplastic particles, into the cup - thousands of times higher than the amount of plastic previously found in other food and drink.

Plastic tea bags are shedding microplastics into people's drinks. (Getty)

The health effects of drinking these particles are unknown, according to the researchers, who called for further study into the area.

The team removed the tea from inside the bags to prevent it from interfering with the results, before boiling the bags in water to simulate the tea-making process.

Scientists have found microplastics in various foods, but less research has been done into whether they can be shed into water during the brewing of tea and other hot drinks.

Many tea producers use polypropylene to seal their bags.

Microplastics can leach from teabags into cups, an alarming study has found. (iStock)

Humans eat an average of 5 grams of plastic each week, according to a separate study earlier this year - the equivalent of a credit card's weight in plastic.

In its first review of the health risks of plastic in tap and bottled water, the World Health Organization (WHO) said last month that microplastics "don't appear to pose a health risk at current levels," but the key finding came with a big caveat - the review said available information was limited and more research was needed on microplastics and how they affect human health.