From filter systems to car-free cities, scientists and thinkers have different visions for tackling air pollution, the biggest environmental health risk in the world, reports Vanessa Zainzinger
If you took a walk in central London in early March 2020, you might have come across an unconventional sight on Euston Road. For a while, a street sign on the high traffic thoroughfare read EustNO2 Road, in reference to the hazardous pollutant nitrogen dioxide. The label was applied by anti-pollution protest group The Air Team, who we can also thank for other gems of street sign subvertising, such as CanNO2bury Road, Stoke NewingtNO2 Church Street and CroftNO2wn Road.
Euston Road is known to be one of the most polluted roads in the country. In 2018, monitoring network Open Data Camden measured the annual mean concentration of NO2 on the road at more than 80μg/m3. The legal limit for NO2 is 40μg/m3.
‘Most areas of the UK are breaching legal limits for deadly NO2 – limits that should originally have been met in 2010,’ says Andrea Lee, clean air campaigns manager at the NGO ClientEarth. ‘And many parts of the UK experience levels of particulate matter pollution that, while within current legal limits, are over the levels recommended by WHO.’
WHO recognises air pollution as the biggest environmental health risk in the world, with nine out of ten people breathing air containing high levels of pollutants, such as nitrogen oxides (NOx), sulphur oxides (SOx), ozone and particulate matter. Estimates reveal an impact equivalent to 7m deaths every year caused by air pollution worldwide.
In the UK alone, between 28,000 and 36,000 deaths/year are linked to long-term exposure to air pollutants, according
can affect lung function, lead to respiratory infections and aggravated asthma. Maternal exposure to ambient air pollution is associated with adverse birth outcomes, such as low birth weight, pre-term birth and small gestational age births.
People are more aware than ever of the harm poor air quality is causing them, says Lee. According to a 2017 Eurobarometer survey assessing attitudes of Europeans toward the environment, air pollution was the second highest concern after climate change. Many, however, ‘assume that they have to put up with dirty air if they live in towns and cities, and politicians fail to show leadership to protect people’s health,’ Lee says.
While governments are, according to Lee, ignoring the problem, cleaning up the air is a prevalent goal among engineers and innovators. Most commonly, the focus is on traffic. Vehicles are a source of significant levels of NOx and particulate matter, and more relevant to human health than some other sources, because exhaust emissions generally occur in areas where people live and work.
Thomas Delgado, an inventor and businessman, has patented a pollution removal system for traffic junctions; classic emission hotspots. Delgado’s company, Pollution Solution, claims that its cat’s eye-style filters can remove up to 30% of dangerous pollutants – including NOx and fine particles from brakes and tyres – and ‘recycle’ them into clean air at the roadside.
‘We install a series of pods or gratings, flush with the road, that within them have a suction head that pulls in air. It’s connected using pipework under the road,’ Delgado explains. The suction heads are placed as close as possible to the source of emissions, in the middle of the road, giving them the best chance of capturing pollutants. The system uses different kinds of filters: some leverage electrostatic attraction to remove fine particulate matter from the air. Others remove gases with activated carbon. It can either work full-time, or switch on when occupancy sensors alert it to a car pulling up.
Delgado has plans to install the filters at all major junctions in an area of East London, for a start, and is in talks with various councils about piloting the system. At the same time, the company wants to roll out the system for use in drive-through fast food outlets, which are considered a major pollution culprit. Earlier this year, a BBC investigation on the back of Coventry University research flagged levels of NO2 and particulate matter at many times the legal limit, over a two-week period, at ten drive-through KFC, McDonald’s and Costa outlets across the country.
While interest in Pollution Solution – from both the media and the authorities – is high, Delgado sees the disruption of installing it as a hurdle to convincing councils and other clients to install the system. The company tries to make the installation as convenient as possible by merely cutting a small channel in the center of the road, rather than digging up more than is necessary. The system costs about £60,000 to install and is geared to being eco-friendly. The activated carbon can be replaced and recycled, the fan is low voltage, ‘equivalent of having a Hoover on at home,’ says Delgado.
Pollutant mesh traps
Metal-organic frameworks (MOFs), with their energy-intensive synthesis methods, haven’t traditionally been known for being green, but they show great promise for capturing air pollutants. ‘I think they are the zeolites of the 21st century,’ says Martin Schröder, VP and Dean of Manchester University’s faculty of engineering and physical sciences.
MOFs are crystalline porous materials with a structure of both organic and inorganic components that makes them unconventionally tunable. They can be put together like Lego bricks to form a cage-like, hollow structure with a large internal surface area. By making the MOF from different metal atoms and organic linkers, researchers can create materials that selectively absorb specific gases into tailor-made pockets within the structure.
Schröder and his colleagues have been working on a MOF1 that selectively captures NO2. The material, MFM-520, contains zinc in a structure with tiny, bowtie-shaped pores. Each pore can trap two molecules of NO2, with a high adsorption capacity.
The material remains highly selective to NO2 in the presence of moisture and other gaseous pollutants, such as sulfur dioxide and carbon dioxide. And despite the highly reactive nature of NO2, MFM-520 can be fully regenerated, without losing adsorption capacity, for more than 100 cycles by submerging the trapped NO2 in water and stirring—a process that also converts the pollutant into nitric acid, a multi-billion dollar industry with applications in agricultural fertilisers, rocket propellants and nylon.
‘The big thing is that we have been able to make a material that is sufficiently stable to NO2; that is a huge step forward,’ says Schröder. Few porous materials are stable to NO2, Schröder adds. The substance is extremely reactive, caustic, and oxidising. While porous carbons and zeolites may capture NO2, they don’t do it in a way that is fully reversible.
Schröder and his team are working on scaling up the material’s production and shaping it into a filter system or facemask for practical use. He sees its potential to do other chemistries on NO2, such as a platform for catalytic reduction of the substance to dinitrogen.
At present, MFM-520 is not stable to car exhaust catalyst temperatures, making its application on vehicles a distant ambition. But Schröder believes in its potential to develop rapidly: ‘When I first started working [on MOFs] 25 years ago, many of them were unstable to water. If someone had told me that we would be able to make a zinc MOF that is stable to NO2, I wouldn’t have believed it.’ Since then, the materials have evolved to being stable to sulfur dioxide (SO2), ammonia, highly acidic and thermal conditions. ‘So, why not car exhaust catalyst temperatures?’ Schröder ponders.
Going for greenspace
While Schröder looks to the future, Pollution Solution’s Delgado sees his technology as a short-term solution for the pollution crisis. ‘It’s an interim infrastructure measure that mitigates health issues for the foreseeable future,’ he says. In the longer term, there will be no more exhausts to filter, he predicts. ‘We will be phasing out fossil fuel vehicles eventually. But not everybody can buy an electric car tomorrow.’
ClientEarth’s Lee agrees: ‘The long-term solution is clear: we need fewer and cleaner vehicles on our roads.’ Although she salutes innovative methods to tackle air pollution, Lee says solutions for the air pollution problem are readily available now, ‘and well known to decision makers – they just need to be bold enough to put them in place.’
Bhavik Bakshi, a professor of chemical and biomolecular engineering at Ohio State University, US, says that not just decision-makers, but engineers and innovators, too, have been ignoring an obvious solution. In a 2019 study,2 Bakshi showed that restoring native vegetation in urban and rural areas can be a more effective and cheaper method for reducing pollution than introducing technologies.
Restoring land cover near pollution sources removes SO2, particulate matter and NO2 by an average of 27% through interception of particulate matter and absorption of gaseous pollutants, Bakshi’s study found. This figure varied by county and region; land cover in the desert of Nevada, for instance, would be smaller than that in the farmlands of Ohio, because the desert could not grow as much vegetation as farmland. But in 75% of the US counties analysed, it was cheaper to use plants to mitigate air pollution than it was to add technological interventions – such as novel industrial exhaust systems – to the sources of pollution.
‘What we are trying to convey is not necessarily a move away from technological solutions but the consideration of the systems that technology depends on,’ Bakshi says. ‘Engineering has developed with this attitude of wanting to dominate nature. But if we also think about the services that nature provides and take that into account then we can come up with solutions that are better than techno-centric options.’
Bakshi differentiates between levels and types of air pollution that need engineered interference, and those that don’t. ‘We wouldn't’ say, don’t treat the emissions from the coal powered power plant because nature will take care of it. At high concentrations of pollution, technology is a much better solution than nature,’ he says. But in lower concentrations, as in most of the urban landscape, nature - or, better, a harmony between nature and technology - wins. Especially if one considers the added value to human health and wellbeing that greenspaces provide, Bakshi adds.
Cities without cars
Audrey de Nazelle, a lecturer in air pollution management at Imperial College London, UK, envisages a more drastic approach that exiles cars from cities and reclaims the streets for people. Her research has repeatedly found that reducing car traffic and encouraging citizens to walk or cycle beats technological solutions – such as different exhaust filters and switching to electric cars – in its benefits to human health.
When de Nazelle studied3 the health impacts of reducing car traffic in Barcelona’s city centre by 40%, she found that the reduction in levels of air pollution could avoid ten deaths every year. ‘But when we assumed that the car trips would be replaced by walking or cycling, we found that 67 deaths could be avoided every year,’ she says. Another study, in London, compared using purely technological solutions to reach air pollution targets to behavioural solutions that integrate walking and cycling. De Nazelle found 20 times the health benefits in the walking and cycling scenarios.
De Nazelle says we are missing a huge opportunity to take a more holistic approach to the health and well-being of people living in cities. ‘The way we tend to approach any kind of problem is to tackle it one thing at a time. Health is one problem, pollution is another,’ she says. ‘We lack that vision to see streets as public spaces for people.’
Some cities have already announced car-free or car-less visions, including Milan, Copenhagen, Madrid and Paris. Oslo has closed off certain streets in the centre to cars entirely, removed almost all parking spots and replaced them with cycling lanes, benches and miniature parks. Chengdu in China is designing a new residential area in which people will be able to walk everywhere easily, reducing the need for cars.
Eventually, other cities will follow suit, de Nazelle believes. ‘I have a five-year-old son who tells his teachers there won’t be any cars in the future, and they laugh at him,’ she says. ‘But I think that, some day, our children will laugh at us when they hear about the conditions that we live in today.’
Developing chemistries to tackle air pollution is rarely straight-forward. Titanium dioxide (TiO2) nanoparticles, for instance, were hailed as a promising approach to improving air quality some years ago.
When exposed to UV light, they can oxidise organic compounds in the air and reduce the concentration of pollutants.
In 2011, a two-year trial of a TiO2 paint, in London, showed that pollutants were reduced by approximately 65%, in the vicinity of the paint application.
Later, projects in Mexico and Milan began integrating TiO2-coated tiles into buildings. In 2019, a team of engineers combined graphene and nano-TiO2 into a solar-powered catalyst that can pluck pollutants out of the air.
But excitement over TiO2’s potential to solve the pollution problem has been lessened by studies4 indicating that the smog-eating paints release significant amounts of volatile organic compounds (VOCs), such as the carcinogenic formaldehyde, during degradation.
A 2016 report by the UK government’s Air Quality Expert Group found little evidence that photocatalysis could reduce ambient concentrations of airborne NOx; this was disputed in 2018, by a review carried out by Imperial College London, which found that it does.
More research will have to settle the case for or against TiO2 before we trust it more widely to feed on our pollutants.
1 Li et al, Nat. Chem., doi: 10.1038/s41557-019-0356-0
2 Gopalakrishnan et al, Env. Sci. & Tech., doi: 10.1021/acs. est.9b01445
3 Rojas-Rueda et al, Env. Int., doi: 10.1016/j. envint.2012.08.009
4 D Truffier-Boutry et al, Environ. Sci.: Nano, doi: 10.1039/c7en00467b