It has survived West Bengal's monsoons, being rammed by a barge and a daily flow of some 60,000 vehicles. Now the engineers who maintain the historic Howrah bridge over the Hooghly river in Kolkata say a new enemy has appeared: the half-chewed mouthfuls of betel leaf, areca nut and slaked lime spat out by pedestrians.

Key struts that support the girders of the bridge – one of the biggest such cantilever structures in the world – have already lost half their protective metal casing to the acids contained in paan, as the mixture is known, engineers say.

There are few public structures in India that are without the tell-tale red stains from paan, which for thousands of years has been chewed throughout India as a mild stimulant, a palate cleanser and breath freshener.

A series of studies have highlighted the health risks of the tonnes of spit and chewed leaf spilled each day on India's already decrepit infrastructure, pointing out that the habit increases the risk of infectious disease as well as causing environmental damage, but have had little impact on its popularity.

Forensic specialists said the lime used in paan was a particular problem as it has powerful corrosive properties.

"A safety audit [of the bridge] is now being planned to find out the effect of constant spitting," one senior official told the Calcutta Telegraph. "The 6mm bases are now around 3mm each. If corrosion continues at such a rate, we would need to decommission the bridge for repairs."

Amithab Chatterjee, an engineer working on the 500-metre-long bridge, said: "It may not be a problem now but it may become a very big problem. The study will find out."

Efforts to crackdown on paan spitting are under way across the country. In Mumbai, the commercial capital, commuters caught spitting on trains were recently cornered by activists armed with buckets of water and mops and told to clean up. Other initiatives have been launched in the central city of Pune and in the poor northern state of Bihar. In the capital, Delhi, authorities have started a publicity drive aimed at cutting down on the practice, as well as public urination, before this autumn's Commonwealth Games.

In London Brent council recently launched a campaign, saying it spent £20,000 a year cleaning paan stains from pavements.

An estimated 50,000 pedestrians a day use the 67-year-old Howrah bridge, built under British imperial rule.

Police officers stationed near it said they routinely fined pedestrians caught spitting but a lack of manpower meant that fully protecting its structure was impossible.