It's a simple contraption -- an iron frame, foldout handle and four rubber wheels -- but in Hong Kong the old-fashioned handcart is what keeps the city rolling.

In the shadow of skyscrapers, Hong Kong's working class trolley pushers transport everything from crates of live seafood to appliances, financial documents, furniture and mail.

But among the street cleaners, market traders and removal men, it is probably the city's elderly scavengers who best highlight how vital handcarts are to the city.

Lee Cheung-Ho, 78, spends all day pushing her cart, and says she even goes out when there is a typhoon.

"I have to go out and make a living," she told AFP without stopping. "It helps even if I can only earn a few dollars."

The estimated 10,000 scavengers in Hong Kong collect cardboard, tin cans and scrap metal, selling it on for a few dollars to recycling centres to ship to mainland China.

With Hong Kong's landfill sites set to reach capacity by 2020, this is seen as one way of addressing a looming environmental crisis.

But scavenging is also a means for many of the city's poor to scrape by.

The financial hub, known as one of the world's wealthiest cities, last month said that 1.31 million of its citizens were living in poverty.

Almost one in five is classified as poor and for the elderly the proportion rises to one in three, according to government data.

The scavengers AFP talked to fell well within that bracket, earning as little as HK$20 (US$3) a day.

Handcarts can be seen everywhere in the city. On a 10 minute walk around one neighbourhood, an AFP reporter counted 103, many stacked head-high with cardboard, others rattling with empty office water cooler cannisters, one carrying an air conditioning unit, another a fridge-freezer.

Adam Bobbette, an expert on the urban environment at Hong Kong University, said handcarts were at the smallest end of a transport scale that includes the city's vast cargo ships, trade lorries and red taxis.

And they are a necessity in a city with narrow streets and alleyways, and restricted parking.

"One of the best ways to think about trolleys in Hong Kong is as a kind of elementary unit within the broader urban metabolism of the city," he said.

"That older way of life has found a way to persist within the city because of its functional value.

"Should the trolley be celebrated within Hong Kong? Absolutely."

But rather than being celebrated, some people complain that the trolleys, often locked to railings like bicycles, take up space on the pavement and could cause accidents. They are often pushed along on the side of roads, forcing traffic to carefully overtake.

There were 11 accidents involving pushcarts recorded in the first half of this year, according to a transport department spokeswoman.

And of two trolley-related fatalities last year, one cart pusher was hit by a bus and the other by a taxi. Both were in their 70s.

The spokeswoman did not offer any praise for the handcart when asked about its value to the city.

"Pedestrians using a handcart... should take extra care to protect their own safety and the safety of others," she said in a statement.

The future of the handcart may not be under threat but like much else in Hong Kong it is still subject to the influence of mainland China.

Sparks flew as Mackey Li, 46, welded the axle of a broken trolley in his workshop.

Li says his business -- inherited from his father and running for 60 years -- is the last on the island where handcarts are built from scratch.

But his son and daughter are both at university and will not be following in their father's footsteps -- meaning that when he retires in a few years most handcarts will come from the mainland.

"Most of the trolleys are made in mainland China nowadays -- but they don't last very long, he told AFP.

"They can beat you by selling the handcarts at a very cheap price."

He added that without trolleys the city would be clogged with even more traffic.

Hong Kong is already choked with pollution -- much of it from roadside emissions -- that kills more than 3,000 residents a year, according to a study by Hong Kong University.

But Li shrugged off suggestions the handcart could symbolise the city's wealth gap, one of the world's greatest.

"It's just a tool," he said. "And even rich people need to carry things."

Nearby, a 90-year-old woman scavenger with a bent spine collected two bin liners of cardboard from a coffee shop, where a cappuccino cost more than her daily earnings.

But she told AFP that pushing the cart was also a chance to do some exercise and fill her time.

"What would I do if I didn't go out?" she asked, looking up through eyes clouded grey with cataracts.

"Do you just want me to stay at home and wait to die?"