Ferries used by commuters and tourists in the centre of London are spewing out pollution equivalent to hundreds of buses and trucks every day.

Meanwhile, cruise ships docking at places like Tower Bridge pump out as much toxic nitrogen oxide gases and particulates as nearly 700 lorries each.

These vessels – alongside the tour boats, container ships and tugs that use the river every day – can use filthy fuel and engines because they operate under the same rules as trawlers in the North Sea.

Boats can therefore emit up to 100 times the level of sulphur oxide gases (SOx) permitted for cars, vans and buses.

These lax standards mean river traffic is far from complying with the targets set by London’s low emission zone, let along the ultra-low emission zone (ULEZ) coming into force next year.

Decades of reform have seen increasingly tight regulations placed on cars and vans, culminating in the London mayor’s ULEZ – which will stretch to the North and South Circular roads by 2021.

While the contribution of boats to the city’s air pollution crisis is small compared to that of road vehicles, campaigners and experts have warned that river traffic is being left behind – despite the threat it poses to Londoners living near the Thames.

Ralph Hardwick from No Toxic Cruise Port, a campaign group fighting against a new cruise port in Greenwich, said the river had been completely overlooked in Sadiq Khan’s latest plans to clean up the city’s toxic air.

“Nothing on the Thames has really got emission controls, so when the mayor is trying to have ultra-low emissions, it’s just not going to touch the river,” he said.

“You can’t call London a low emission zone when the river is not included.”

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As the mayor’s authority does not extend to London’s rivers, responsibility for the pollution coming from river traffic falls on the Port of London Authority (PLA), and emissions targets are ultimately set by the International Maritime Organisation (IMO).

“It’s a bit of a strange dichotomy where you can have cars that are subject to increasingly strict restriction and then on the river next to them sometimes you have boats subject to restrictions that permit greater levels of emissions,” said Professor Matt Loxham, an air pollution toxicologist at the University of Southampton.

Shipping pollution typically contains high levels of SOx – pollutants that have been linked with a variety of diseases.

Under the sulphur emission control area introduced by IMO in 2015, which covers the Thames, boats must use fuel that contains 0.1 per cent sulphur in efforts to reduce this pollution. This is 100 times more sulphur than is permitted in road diesel.

“SOx are irritants to the airways – a bit like nitrogen dioxide – they can cause inflammation and in people who are predisposed they can exacerbate asthma or respiratory conditions,” said Professor Loxham.

Some London boats, such as the Thames Clippers – high-speed catamarans that carry commuters and sightseers – use gasoil with SOx levels comparable with those found in road diesel.

However, Dr Tristan Smith, a shipping researcher at University College London, estimates the lack of stringency on nitrogen oxide (NOx) emissions means the engines used by these boats produce quantities around 10 times higher, per passenger, than London buses.

Compared to a truck operating under the latest European diesel standards, the figure is closer to 20 times higher.

Measures like ULEZ are intended to keep vehicles that do not meet the strictest emissions standards out of the city. Data suggests 9,500 Londoners currently die prematurely every year due to air pollution, and scientists say the lethal cocktail of gases and particles is having far-reaching effects.

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“It’s not just about asthma, it’s about cancer, heart disease, poor sleep and dementia as well,” said Professor Hugh Montgomery, an intensive care medic at University College London.

However, Martin Garside from PLA said that not only is the Thames a “vital transport artery”, its river traffic serves a key role in actually reducing air pollution in the city.

“With a single barge able to carry the load of 50 lorries, it is central to reducing traffic and pollution on London’s congested roads,” he said.

“Last year more than four million tonnes of cargo was transported by water between terminals on the Thames – equivalent to more than 300,000 lorry movements removed from London’s roads.”

In May the PLA released its own air quality strategy for the Thames, including targets to cut NOx and particulates by half over the next 25 years.

Such measures are already being implemented in the Thames Clippers, but critics have pointed out that even with these changes pollution levels would still far exceed those of road vehicles.

“To me it’s weak – it’s a lot of smoke and mirrors to say they are doing something,” said Mr Hardwick.

Dr Smith said that considering the capacity the road sector has shown for bringing down pollution, the same ought to be done for the river.

“Given that we hybridised the London buses (which handles a lot of the transient emission issues) 10 years ago, and they were much more stringently regulated then and are now, there is no good technological or environmental reason why the river craft shouldn’t at least meet equivalent standards,” he said.

“Instead, we’ve let their regulation be left to be driven by international multilateral negotiations at the IMO.”

As it stands, PLA estimates use of the river accounts for just 1 per cent of the city’s annual emissions – a figure that will rise to around 3 per cent in the next decade due to improvements in road engines and an increase in river traffic.

However, this figure covers the city in its entirety, and concentrations of toxic pollutants around the Thames are likely to be far higher.

Germany’s Nature And Biodiversity Conservation Union (NABU) has conducted assessments of air pollution in port cities around Europe, from Venice to Hamburg, and found universally harmful levels of pollution.

Of particular note were the levels of highly dangerous ultrafine particulates, which were on average present at levels 400 times higher than is considered “safe”. The tiny size of these particles allows them to penetrate deep into the lungs and cause damage.

Dietmar Oeliger, head of transport policy at NABU, said that, although the organisation was yet to measure levels on the Thames, the share of the area’s pollution coming from boats “must be quite high”.

“Along the rivers you tend to have very high concentrations of ultra-fine particles and I’m sure this is the same in London,” he said.

Mr Hardwick said ultimately control over river emissions needs to come from the government.

“They need to step up and say we can’t carry on allowing vessels in UK waters continuing to pollute as they are,” he said.

A Department for Transport spokesperson said: “Shipping is an international industry, so it is entirely appropriate that there is a global approach to regulating shipping through the IMO.”

“This government is doing more than ever before to reduce harmful emissions across all modes of transport.”

Experts point out there are alternatives available that would bring river traffic in line with the standards being applied to the road.

“Belgium now has a hydrogen powered river craft operating (hydroville), and the Netherlands and other European inland waterways are increasingly electrifying river traffic,” said Dr Smith.

According to NABU, a switch to low-sulphur fuel combined with particulate filter and emission-reducing catalysts would virtually eliminate air pollution from boats being used in cities.