Hamiltonians should stop putting black plastics, coffee cup lids and polystyrene in their blue boxes since market demand has made them impossible to sell.

These items — as well as beverage cups, bread tabs, coffee pods, bottle caps and multi-laminate packaging — are the "top contaminants" in Hamilton blue boxes, says Emil Prpic, head of the recycling and waste disposal division.

And while some have never been allowed, black plastics, coffee cup lids and polystyrene (a kind of foam) are new additions, he said. This is because of market demand, and largely driven by new regulations in China.

"These markets can get picky and very demanding of what they want," Prpic said.

When people do throw those items in the blue box, workers at the city's Burlington Street sorting plant remove them, which costs time and money. Enough of those items, Prpic said, will spoil a whole load.

Residents can be forgiven for not knowing this. It's a recent change, and the city's "what goes where?" page still tells people to put coffee cup lids in the blue box.

Prpic said the city is stopping short of a full campaign telling people to change their ways. Markets fluctuate fast, he said, and the city never knows when the list of banned material will grow or shrink.

City councillors on the public works committee Monday were surprised too. Some of them have been tossing coffee cup lids in their own blue boxes.

"I know I was still putting the coffee cup lids and black plastic from restaurants into the recycling," said Tom Jackson, Ward 6 councillor. "Those are things that I, and I'm sure I'm not alone in my constituency, have been putting in the recycling."

The change in market demand is largely driven by China. Chinese companies were buying about half the world's unwanted paper and plastic for recycling when the country changed its standards last summer. Now, it's banned 24 types of solid waste.

It has big implications on municipalities, who are left with large stockpiles of material and nowhere to sell it. Hamilton has large piles of polystyrene, Prpic said. With the current market, the city would have to pay to get rid of it.

The only hope, he told councillors, is for some sort of domestic market to grow around recyclable material. But that's slow moving. There are no existing plants, and no existing market, that will replace China.

The new Chinese standards will cost the city $300,000 per year or more, Prpic said. Each year, the city makes about $4.2 million selling its recyclable material.

Polystyrene isn't the only problem area in the market. Last year, the city earned 58 per cent less on selling its recyclable paper material.