Travelling around the globe, largely unseen, are some of the world's worst toxic pollutants. And they're present, in varying amounts, in all of us.

In early December more than 120 countries will meet to decide the world's defence against PCBs, DDT and ten other Persistent Organic Pollutants (POPs).

In this week's Earth Report we look at how POPs have affected communities in Sweden, Brazil, Zambia, Thailand and Canada.

Southern Sweden

In the 1920's the process of electrifying the world needed cheap, inert, insulation. Poly chlorinated biphenyls, or PCB's, were hailed as a universal answer. Decades later, their toxic risks were discovered but not before millions of tonnes were installed in electrical equipment all over the globe.

It was in Sweden that the PCB problem was discovered in 1966. PCBs had leaked from ageing transformers and found their way into the Baltic Sea. Here, quickly absorbed by plankton, they rapidly made their way up the food chain into fish consumed by humans. Coastal communities were ingesting PCBs without even knowing it and women were passing them on to their unborn babies.

Researchers had found, perhaps for the first time, a link between environmental cause and effect.

In 1972, Sweden banned PCBs and removed every trace of the chemical from their transformers. Yet the Baltic still has PCBs and other POPs - only this time - originating from other sources abroad. It's clear that POPs are a global problem.

Rondonia, Brazil

Here, one pollutant is proving much harder to clean up.

DDT, an extremely toxic and long-lasting chemical, was used for years as a crop pesticide and, importantly, to control the spread of malaria-carrying mosquitos.

In Brazil, where Malaria is wide-spread, DDT played an important role. Simply banning DDT without providing an alternative would have created a national health disaster.

Now, Brazil has come up with two new ways of combating malaria. Caught early enough, malaria can be treated with a course of cloraquin pills and biodegradeable synthetic pyrethrum, sprayed over affected homes, can deter mosquitos. What's more, research into how people catch and develop malaria and the development of biological larvicides may yield yet more ways to halt the disease.

But this isn't the end of the story for DDT. A worldwide ban when malaria is on the increase would cost lives and isn't yet possible. For the timebeing, DDT is here to stay.

Victoria Falls, Zambia

Banned in the West, pesticides such as DDT, Aldrin, Dieldrin, Endrin and Heptachlor were once seen as a boon for growing food in poor countries.

Agrochemicals like these were donated to farmers as aid. But farmers received little training in their safe use and many preferred not to try them. As a result, stockpiles of these chemicals have been left unattended ever since.

The German aid agency GTZ discovered the dangers and found 360 tonnes at seven sites all over Zambia. A $1.2 million clean-up successfully removed these obsolete chemicals - but not before untold amounts had leaked into local drinking water supplies.

Zambia is but one country where Western governments are trying to make amends for a policy that gave farmers agrochemicals, but that had unforeseen consequences. The clean-up may have started in Zambia, but thousands of tonnes of dangerous pesticides remain across Africa - a legacy from continents away.

Bangkok, Thailand

For many countries doing anything about the next pollutants is going to be difficult.

Created unintentionally in combustion, especially industrial burning, are dioxins and furans, thought to be some of the most lethal substances known. Even minute amounts are highly carcinogenic. What makes them so difficult to control is that nobody knows how they're formed, so they're particularly difficult, and expensive, to treat.

Even with the most complicated western technology in place it's quite a challenge to ensure there is no dioxin release from an incinerator. For the type of waste disposal available for much of the world, we may just have to accept that dioxins and furans are being released into our atmosphere.

Nunavut, Canada

In the once-pristine Arctic north of Canada, POPs arrive uninvited to threaten the Inuit way of life.

More than half of all Inuit women have levels of POPs above those regarded as safe by the World Health Organization.

POPs have found their way into the Inuit population through their largely fish and seal diet and passed on to babies through breast milk.

Living in harmony with their environment has sadly not protected the Inuits from the toxic problems generated by other nations.

The POP Treaty, Johannesburg, South Africa

In early December the United Nations Environment Programme will bring together some 120 countries to decide the fate of the worst POPs.

After five years of build up, the last barrier to agreement is how the rich countries of the West are going to help those that can't afford, or don't have the expertise, to deal with POPs. In order to get these countries on board, the West must play a proactive role in the implemention of hazardous waste clean-up programmes and provide the technical expertise for sourcing alternatives. Just how much money is available, and how it's going to be provided, is the last stumbling block.

But even if all POPs were banned tomorrow it would take generations to see any real benefit in terms of dramatically different levels in people's bodies. POPs are a global problem, requiring a global solution, which is why global actions must be taken now.