Squirt out your face scrub and rub it between your fingers. If the gritty bits aren't natural ingredients, they are most likely made from polyethylene.

In the cosmetics industry they're called microbeads, cleansing beads or exfoliating pearls. University of Auckland marine biologist Dr Mary Sewell just calls them plastic.

And you'll find bottles containing them everywhere, on shelves at the supermarket and in bathrooms all over the world. They're even in the stores in Antarctica, at environmentally-aware Scott Base: though Sewell grudgingly excuses them this error.

"It's not really their fault. People just don't know," she says. "There's no excuse for ignorance, but this material is so small we don't even realise what they're doing to the environment."

Sewell, who has spent her career studying the drifting animals living in plankton, says the fragments of plastic are potentially getting into the sea and into our food chain. She and Auckland student Lisa Fendall have just embarked on the first New Zealand research into what effect the exfoliants might be having on sea life.

It was at the supermarket one day that she realised the possibilities. "I suddenly had this thought that if this plastic is washing down the drains and out to sea, we had a potential new source of pollution. If you look on the supermarket shelf nearly every exfoliating product has this plastic."

Check the label, says Sewell - polyethylene is on the ingredient list, sometimes the main ingredient after water.

"Where, traditionally, you would have seed husk or oatmeal or whatever, in fact they now all contain plastic, and [the plastic fragments are] very small. Sometimes you can tell only by looking at the label, and various scrubs actually advertise that they do have plastic in them, like it's a good thing."

But it might not be. "What has been well-documented with animals such as turtles is that they die because their stomach is filled with plastic, and it could be the same for plankton as well. It's a worry that plastic is getting into the food chain at such a basic level, because [plankton] will be eating them as food. So potentially, this could be a really bad thing."

Sewell and Fendall spent last summer analysing the sizes and shapes of the polyethylene scrubbers found in four supermarket facial exfoliants. Their study, published this month in the international scientific journal Marine Pollution Bulletin, found most of the polyethylene beads in the cleansers were less than 0.1mm long: about the diameter of a human hair. Some were so tiny that they couldn't see them with a naked eye. Fendall says the smallest particle they found was about four microns. Plastic dust.

So small, says Sewell, that we don't even know they're there. "But if you've ever taken one of these products and rubbed it between your hands you'll feel that it's quite rough."

Just the right size for plankton.

Dr Richard Thompson of the University of Plymouth in England has spent 20 years studying the effects of these tiny plastic morsels on sea creatures. In 2004, he tested whether the effects of eating plastic would be the same on marine invertebrates as on larger animals such as sea birds.

He set up an aquarium and fed bite-sized pieces to invertebrates at the lower end of the food chain: lugworms, which feed on sediment at the bottom of the ocean; sand fleas, which consume beach debris; and barnacles, which filter their food out of the water. When the particles lodged in their guts, the constipation killed them. Why should we care about constipated lugworms? Because they're the basis of the food chain that supports all marine life, says Sewell. "Facial cleansers may now be a major source of microplastic pollution in the ocean, and will have immediate and long-term impacts on plankton and filter-feeding organisms at the base of marine food-chains, and further up."

Thompson's former PhD student, Dr Mark Browne of Sydney's Centre for Research on Ecological Impacts of Coastal Cities, has studied the effects of plastic on marine mussels. In his experiment he fed them with particles of plastic slightly smaller than those Sewell found in the cleansers.

The plastic either accumulated in the gut, passed through or moved into blood cells within three days, where it could still be found almost two months later. He says the global importance of this issue is reflected in the fact that the international Society of Environmental Toxicology and Chemistry (SETAC) has asked him to hold a session on the environmental consequences of microplastic at its meeting in New Orleans this November.

Thompson says there's a growing body of evidence that the plastic particles are accumulating in the sea, and evidence for harmful effects. All three scientists cite studies that show plastic in the sea absorbs toxic contaminants such as PCBs, potentially poisoning ocean food webs.

Sewell and Fendall won't say which cleansers they analysed, but they could be any of the huge range lining supermarket shelves - and 95 per cent of those are controlled by the Cosmetics, Toiletries and Fragrances Association of New Zealand.

Executive director Garth Wyllie says there's no cause for concern.

"Even though the terminology and symbols are the same as if you're actually talking about plastics, they are in fact quite a different ingredient in real terms. They are designed to be biodegradable and break down completely - they're not harmful to the environment. "

How long does it take? "The same timeframe as most of the other ingredients in the product."

It's impossible to pinpoint a time, says University of Canterbury Associate Professor in chemistry Alan Happer: but polyethylene is polyethylene. "It stays the way it is. It will break down in the same way that crude oil or anything like that breaks down. It will probably react very slowly - it doesn't break down easily. The smaller the bead, the faster it breaks down, so a bead at 100 microns is certainly going to break down much faster than normal polyethylene." And, of course, the plastic has to reach the ocean.

It doesn't, says Wyllie. "They don't go straight to the ocean in any shape. That's an assumption again, that they're damaging to marine life. Within our sewerage system there are mechanisms to ensure the products are fully dealt to. Those ingredients do not get out into the marine environment. They are taken out of the system before they can get out into the ocean, and the [outfall] is virtually 100 per cent pure."

But Sewell's paper cites international wastewater treatment design studies that show it's possible the plastic escapes. And Browne asks what evidence Wyllie has based his comments on, given there has been no scientific peer-reviewed work that has measured the effect of these plastics on the marine environment, nor their removal from the sewage treatment process.

Of the treatment plants Browne has looked at in Australia, Britain and America, none had mechanisms to ensure microplastic was removed from sewage. "It's reasonable to postulate that small particles of plastic may be transported with waste water and subsequently enter aquatic habitats," says Browne.

Dunedin wastewater treatment manager Iain Satterthwaite says the filters on his treatment plants have screens of two millimeters. "Conceivably, [the microbeads] could get out. They may settle out - if they float they could get taken off the top and end up as scum, which then gets incinerated. Or if it ends up in sludge it gets taken to landfill. I just don't know. It's not something we've ever considered."

A senior process controller at Auckland's Watercare says it's likely the microbeads are being removed at the Mangere plant, as its filters are designed to strain out particles the size of grains of sand. Any captured solids are sent to a sanitary commercial landfill.

"Where they're then released into the environment anyway," says Fendall.

Sewell says that regardless of whether all the plastic in cleansers is reaching the ocean, why do we need to add to it? Microplastic is already bountiful in the sea.

Last year, Browne studied stranded plastic in the Tamar estuary in southwest England and discovered 85 per cent of it was microplastic, less than 1mm in size. In 2004, using archived water samples, Thompson found it had accumulated in the North Atlantic over the past 40 years.

"The trouble is that all the plastic ever produced on the earth is still here in some form," he says. Through the grinding action of waves, the larger pieces that lodge in the stomach and intestines of creatures such as seabirds and turtles get smaller, becoming available to a wider range of creatures.

"But with these cleansers, consumers are directly releasing plastics that are immediately food for sea creatures," says Sewell. "They don't have to break down.

"People are starting to be more environmentally friendly and thinking about carbon footprints and walking to the shops, but maybe we also need to start thinking about how we clean ourselves and what products are going down our drains. A city of Auckland with a million people - imagine how much is washing into the sea each day. And then extrapolate that around the world."