The Howrah Bridge in the Indian city of Kolkata is at risk of collapsing because the steel at its base is being corroded by human saliva.

Pedestrians who have been chewing betel leaf, areca nut and slaked lime have been spitting on the bridge's steel hangers, causing them to weaken.

The red stains left by the paan mixture - a mild stimulant chewed throughout India - are caked around the metal casings of the bridge.

Authorities say the paan-laced spit has reduced the thickness of the bridge's hangers from six millimetres to three millimetres.

In an attempt to save the bridge, port engineers have come up with the idea of covering the steel with fibreglass.

Several years ago authorities were forced to begin regular clean up of bird droppings which were also corroding the bridge.

It has also been damaged before from a number of car and boat accidents.

The cantilever suspension bridge opened in 1943 and spans more than 450 metres across the Hooghly River.

It is thought to be the busiest of its kind in the world, with more than 100,000 vehicles crossing a day and more than 150,000 pedestrians pounding its walkways daily.

ABC/BBC