Chris
Woodford

Angels with dirty faces: How air pollution blackens our buildings and monuments

by Chris Woodford

Last updated: 13 December 2023.

This is an extra chapter from Breathless that we cut at the last minute due to lack of space.

Breathless Book Cover - large

The Taj Mahal, the Villa d'Este, the Acropolis, the Colosseum, Westminster Abbey, Notre-Dame, the Leshan Buddha, the Sphinx—the list of world monuments damaged or degraded by air pollution is long and lamentable. On the other hand, if it weren't for all those praying cathedral angels with their horribly dirty faces, hideously worn to gargoyles by the corrosive effects of passing air, perhaps we wouldn't notice pollution at all, much less do anything about it?

I was lucky enough to grow up in the green, bucolic countryside, where the milk (remarkably) sometimes arrived by horse and our school bus paused each morning to let cows hoof across the road. My home turf was also home to Abraham Darby and Josiah Wedgwood, Richard Arkwright, Matthew Boulton and James Watt. The heart of England, where I grew up, was the crucible of the global Industrial Revolution and the dirty birthplace of modern air pollution—and the stains of industry took a very long time to disappear.

A century or two after those august names made their mark, as children, we'd chug off on train-treat day trips to Birmingham, the nearest big city, and still, at that point, very much a centre of manufacturing industry. But each time we went, I found myself dumbstruck by the sight of streets so black they could have been Great Fire survivors. At that age, I couldn't comprehend the connection between the filthy diesel trains and buses we'd travelled on and the sad, sooty faces of these towering, historic structures. How could a building be not just a little bit grubby, but as black as if it the whole thing had been chiselled from coal? It was astonishing to me then and it still is today, several decades later: what does it take to turn an entire building so black that the rain can't scrub it clean? Of course, I wasn't the first to think these thoughts—and I had no idea, at that point, that air pollution was to blame. [1]

People have noted with dismay the disturbing, corrosive effects of filthy air on historic buildings ever since the Industrial Revolution brought industrial-strength pollution to countries like Britain and the United States in the 18th and 19th centuries. Way back in the 1850s, Scottish pollution chemist Robert Angus Smith pointed out that 'the stones and bricks of buildings, especially under projecting parts, crumble more readily in large towns where much coal is burned... I was led to attribute this effect to the slow, but constant, action of acid rain.' Getting on for two centuries later, thanks to revolutionary—but polluting – technologies such as power plants and diesel engines, the problem of crumbling, blackened buildings has spread right around the world. From India to Italy and from the United Kingdom to the United States, the degradation of ancient materials, like stone and marble, and modern ones, such as steel and concrete, has become a truly global problem. And in combination with other global problems, like climate change, it could make us homeless in history: culture-less as well as breathless. [2]

A world alas

When it comes to pollution, historic buildings and monuments are our coal-mine 'canaries', our limestone litmus tests, if you will: day by day, decade by decade, they reveal a dangerous erosion and dirty degradation analogous to the long-term harm filthy air is doing when it sucks inside our lungs. It would be fascinating to know whether the cosmopolitan citizens of historic cities, like Rome and Athens, are more conscious of air pollution, and more bothered about it, than those living in equally polluted but more modern places where the effects of toxic gases like ozone and nitrogen dioxide are essentially invisible. There's no shortage of places where scientists could carry out such a study; here are just a few of them.

A teardrop on the face of time

From Princess Diana to Jackie Kennedy, Bill Clinton to Oprah Winfrey, and Mark Zuckerberg to Tom Cruise, everyone—but everyone – has been photographed at the Taj Mahal. The finest surviving example of Mughal architecture, it was built over a 22-year period in the mid-17th century by emperor Shah Jahan as a poetic memorial to his wife Mumtaz Mahal, who had died in childbirth. Jahan said the mausoleum 'creates sorrowing sighs and makes sun and moon shed tears from their eyes.' According to a very dubious legend, he ordered the craftsmen who carved the place to have their thumbs cut off and their eyes gouged out so they could never again see or behold beauty such as they'd just created. The poet Rabindranath Tagore was right when he called it 'A lone teardrop on the face of time', but the tears now are for the building itself.

Though the Taj Mahal still looks bright white in a carefully filtered selfie, decades of air pollution have poisoned the marble facade, turning it, on closer inspection, a greeny bile yellow; it's cracked and stained with black and brown soot from traffic, factories, artisan workshops and home fuel burning. It's easy to see why: average levels of PM10 particulates in Agra (the city in the state of Uttar Pradesh where the Taj is located) are as high as 196 micrograms per cubic metre—almost 10 times the World Health Organization's recommended limit (fine PM2.5s are also 10 times the limit).

Taj Mahal by Brain Gratwicke

Photo: The Taj Mahal: A teardrop on the face of time. Photo by courtesy of Brian Gratwicke, published on Wikimedia Commons under a Creative Commons (CC BY 2.0) Licence.

Visiting in 2000, President Bill Clinton summed things up perfectly when he said: 'Pollution has managed to do what 350 years of wars, invasions and natural disasters have failed to do. It has begun to mar the magnificent walls of the Taj Mahal. I can't help wondering that if a stone can get cancer, what kind of damage can this pollution do to children?' (That question still has no firm answer two decades later.) By 2010, warnings of the building's imminent collapse, due to rotten wooden foundations, had started to sound like a tired routine. In 2018, the crisis had become so severe, the damage to the building so embarrassing, that India's Supreme Court demanded, in a flourish of histrionics, that the government properly restore the Taj or knock it down altogether. Neither of these things is currently in prospect.

All sorts of other solutions have been tried, none of them lastingly effective. For over a quarter of a century, there's been a vast exclusion zone around the Taj where no new foundries, tanneries or other factories could be built, but existing ones are still there, still polluting. Cars and buses are also banned within a 4-km (2.5-mile)-wide radius; visitors have to take horse-drawn carts, bicycle rickshaws, electric buggies or make their way by foot. But pollution, as we've already seen, doesn't respect arbitrary human boundaries; the wider problems persist—and new ones are always looming. Further afield in Agra, thousands of trees have been felled to make way for recent roads, railways and other developments—a double-whammy that allows even more filth to drift towards the Taj. On top of all this, chronic lack of maintenance has also taken a toll. According to the Times of India, while the building earned $11 million from visitors between 2013 and 2016 alone, a mere $1.6 million was spent in the same period for upkeep and repairs.

Meanwhile, India's conflict between respecting its ancient heritage and encouraging modern development extends to other historic parts of the nation. Ahmedabad, for example, the former capital of Gujarat, was named the country's first ever UNESCO World Heritage City in 2017 in a fanfare of celebration. But thanks to ever-growing traffic, endless construction sites and unstoppable urbanisation, it's also one of India's top five dirtiest places and the ninth most polluted city in the world: PM2.5 levels are sometimes 30 per cent worse than at Agra. Ahmedabad is home to 3,000 buildings of priceless historic value, but with no proper air-quality monitoring system or action plan to tackle air pollution, how many of them are destined to end up just like the Taj Mahal? [3]

Rome under siege

Rome, to Byron, in the late-18th century, was the 'city of the soul'. He might not recognise the place two centuries later, certainly not from the fuming words penned by a group of assorted Italian intellectuals in a joint letter to UNESCO in 2016: 'We can no longer stand this ugly, wounded, vulgar, dirty, raped, invaded, besieged Rome.'

Some of their complaints are far from new. Rome has long been one of the world's most poetic cities, but for almost all its history, it's also been one of the most polluted. Coughing under 'heavy skies', wheezing in 'foul air', Romans were certainly familiar with air pollution in ancient times. But thanks mostly to traffic pollution, modern Romans—the citizens of places like Rome and nearby Tivoli, home to the famous Villa d'Este water gardens – are still struggling with dirty air to this day.

Like an ancient fight to the death, Rome's gladiatorial battle between priceless heritage and peerless pollution is taking no prisoners. According to recent studies by the Institute for Environmental Protection and Research (ISPRA) and the Institute for Conservation and Restoration of Heritage (ISCR), some 3,600 stone monuments and 60 bronze objects are now at serious risk of deterioration from pollution. And some of that amazing legacy is wearing away fast (in monumental terms, at least). The experts have discovered that Rome's marble is eroding at an average rate of 5–6 microns per year, while its bronze is slipping away somewhat more slowly (a more sedate 0.3 microns per year). Even these frightening observations are nothing new. As far back as 1964, The New York Times noted how 'the Colosseum, the Arch of Titus and many frescoes have been damaged recently' by air pollution. Such reports are now so common, so very old hat, that their frightening implications—the irreplaceable loss of some of the world's oldest historic buildings—barely bother us at all.

Have the Romans learned anything in the five decades since? They've tried many things. They've shunted traffic away from key sites like the Colosseum. They've banned cars, bikes and scooters from the city for hours at a time. They've tried the classic ruse of letting odd- and even-numbered registrations into the city on different days—but the regulations aren't well enforced and canny Romans simply buy second-hand cars with alternative numbers instead. By 2024, they will have finally banned diesel cars from the centre of the city entirely—a much bigger deal than it sounds when you consider that, until recently, around two thirds of the cars bought in Italy were diesel-powered.

What of the city's heritage in the meantime? Restorations sponsored by big business seem to be holding the worst pollution at bay, in a few, select places at least. At the Colosseum, dedicated conservationists spent three years, and a €20 million donation from a shoe company, turning back the grim tide of ambient filth using nothing more sophisticated than toothbrushes. Stripped of black soot, green algae and two millennia of assorted grime, the fabulous historic travertine smiles in the sunlight once more. [4]

Athens attacked

Athens, like Rome, is another historic city where air pollution has been locked in ground combat with thousands of years of priceless heritage for many decades. Poets once fell over themselves to praise the place: Milton described the city, home to the sacred citadel of the Acropolis, as the 'mother of arts and eloquence'; Shelley likened Athens to light, rising 'like sunrise from the sea'; Byron wrote of how the city had stolen his heart.

Exhaust pollution damage on the Parthenon in Athens, Greece

Photo: The columns of the Parthenon in Athens, Greece have been blackened by vehicle pollution. Athens is one of the world's most traffic-polluted cities. Photo by Michael M. Reddy courtesy of U.S. Geological Survey.

Sadly, decades of tourist-powered traffic pollution and things like soot from home fireplaces and wood burners have changed all that – with all too predictable impacts on the city's heritage. Britain and Greece have a long-running diplomatic spat over a large collection of sculptures known as the Elgin (Parthenon) Marbles, which originally formed part of the Acropolis. In the early 1800s, Thomas Bruce (Lord Elgin) removed some of the sculptures and brought them back to his home country, England, where they've been displayed in London's British Museum almost ever since. Though Elgin's actions have often been described as 'vandalism', the sculptures preserved in London are actually in far better condition than the ones left behind in Greece, which have been steadily eroded by pollution to the point where some features have been all but erased. But the Greeks did take note. In 1979, after being seriously eroded, a group of Caryatids (column supports shaped like standing females) from the Erechtheion temple were finally replaced with replicas and shunted off for safety to a local museum.

People have been warning of this kind of damage for decades. As far back as 1975, a United Nations report commented that four decades of pollution had done more damage to the heritage of Athens than the previous four centuries of steady erosion put together. According to Professor Constantine Trypanis, then the Greek Minister of Culture and Science, quoted by The New York Times at the time, it was worse even than that: 'The wear and tear in one year today is equal to the wear and tear of three centuries! It's an extraordinary thing, and it can't continue.' Several years later, the same paper quoted one government official's opinion that the Acropolis 'will cease to exist within 30 years if something is not done.' Thankfully, that didn't come to pass. Stung by such warnings, the Greeks have tried all the usual remedies, including opting for low-sulphur fuels and banning traffic from Plaka, the old quarter of Athens, to protect the Acropolis above.

And yet the problems persist. Today, average levels of PM10 particulates in Athens are twice WHO guidelines—that's twice as bad as London and 50 per cent worse than Paris. Ground-level ozone concentrations in Athens are sometimes among the highest in the world.

Where Rome's conservationists have been turning to toothbrushes, in Athens, they've been more hi-tech. After comparing 40 different methods of cleaning and restoration, they've found high-precision lasers more help for scanning and recording fine details and blasting off centuries of encrusted filth. But constantly cleaning the worst effects of air pollution is no substitute for stopping it altogether—and some damage is irreversible. [5]

Coal takes a toll

China's addiction to coal takes a toll in some unexpected ways, but you might not expect ancient statues—permanent stone structures that seem to withstand the centuries—to be high on the list. You'd be wrong. The world's tallest ancient statue, the vast, 19-storey (71-m/233-ft) high figure of a Buddha carved into a red sandstone cliff near Leshan, in China's Sichuan province, has been steadily eroded by pollution mostly from coal-fired power plants. In the early 2000s, it seemed almost like a miracle when the Leshan Buddha started crying—except that the lines streaming down his ancient face were coal-black streaks from air pollution; later, his nose, forehead, ears and torso turned black as well. Like the crumbling architecture of Rome, Greece and many other places, the Buddha had happily survived over a millennium of natural wear-and-tear before human damage—including a botched 1960s restoration—blackened his face. Today, expensive cleaning and repair jobs are needed every few years. The Buddha underwent further restoration in 2001, 2007 and again from 2018 to 2019. But with coal-powered industry still fundamental to China's economic might, it's only a matter of time before the conservationists, with their chisels and toothbrushes and laser beams, need to pay yet another visit. [6]

Leshan buddha by Min Zhou

Photo: The coal-stained face of the Leshan Buddha. Photo by Min Zhou , published on Wikimedia Commons under a Creative Commons (CC BY 2.0) Licence.

Polluted parks

In the relatively youthful United States, where such ancient buildings and monuments don't exist, heritage is still under threat in less obvious ways—as the country's 400 national parks amply demonstrate. If calm, wild places like these are where people go to escape the strains of the city—air pollution among them—they may well be wasting their time. An astonishing 2019 report by the National Parks Conservation Association (NPCA) found that hazy skies caused by air pollution are a concern in 96 per cent of parks, where taking in spectacular views is all-important. The air is unhealthy to breathe (at least some of the time) in 85 per cent of parks and ozone pollution is a 'significant concern' in 22 per cent. According to Theresa Pierno, President and CEO of the NPCA: 'Nearly every single one of our more than 400 national parks is plagued by air pollution ... When people think of iconic parks like Joshua Tree or the Grand Canyon, they think of unspoiled landscapes and scenic views. I think they would be shocked to know that these are actually some of our most polluted national parks. Air pollution is also posing a health risk to some of the 330 million people who visit our parks each year, as well as the communities who surround them.' [7]

World Heritage Sites degraded

A small selection of World Heritage Sites, iconic buildings and other historic sites affected by air pollution. Source of data: UNESCO.

Ship to shore

Valletta, the intimate historic capital of Malta, has been a World Heritage Site since 1980. With 320 monuments packed into just 55 honey-coloured hectares (135 acres), it's one of the most densely historic places on the planet. It's also one of the more threatened, thanks to the million or so tourists who flock to the city each year.

Many arrive on towering cruise ships that puff clouds of fumes into the air. Ships, chugging out of sight and scrutiny, are one of our most underestimated sources of air pollution: what belches from their smokestacks is a thick mix that includes sulphur dioxide, particulates and nitrogen dioxide. According to European campaign group Transport and Environment, in one recent year, the 47 huge liners owned by the Carnival Corporation alone produced ten times as much SO2 as Europe's 260 million cars put together. All told, ships pour out twice as much sulphur dioxide as road transport and 80 times more than planes. Why does that matter if ships are steaming along in the middle of the ocean? It matters because they spend a surprising amount of their time moored in ports or drifting near crowded coasts. The European Environment Agency estimates that 60,000 people die early each year due to shipping and cruise line emissions, while the International Maritime Organization (IMO) has calculated that SO2 emissions from ships alone will be responsible for 570,000 premature deaths between 2020 and 2025. That's largely because the huge, dirty diesel engines inside ships feed on low-grade, 'bunker fuel' that can contain up to 3,500 times more sulphur than car or truck diesel. (Thankfully, from January 2020, a long-running initiative called the International Convention for the Prevention of Pollution from Ships, MARPOL, has introduced tougher restrictions, effectively reducing the amount of sulphur in shipping fuel by about 85 per cent.)

Meanwhile, back in Valletta, historic buildings are also dying before their time. The entire city is made from a soft, porous kind of limestone formed from globigerina, which sea-salt in the air (natural pollution) and cruise ship emissions (a less natural kind) are nibbling away. A report by the government of Malta estimates the total adverse cost of ship pollution (in health and environmental impacts) at some €24 million a year. But tourism is worth €1.8 billion a year to the country—75 times as much—which explains why, according to the Times of Malta, that particular report was quickly and quietly 'shelved'. [8]

A ship reduces its emissions by hooking up to a shore-based power connection using giant cables on the dockside.

Ship to shore—shore to ship. One way to reduce the impact of a ship's emissions on a historic site like Valletta is to switch off its engines when it's in port, and use cables like these to hook to shore-based power instead. Picture by Aja Bleu Jackson courtesy of US Navy and DVIDS.

Material breach

If you can drive a truck at a building and barely leave a scratch, how come something as innocuous as air pollution—a drifting solid, liquid or gas—can, in the long term, do more harm? Because materials that are tough enough to resist challenging physical damage can't necessarily withstand sustained chemical attack. The damage air pollution does to historic limestone or marble or the steel inside modern reinforced concrete is essentially all about chemical reactions that slowly unravel the molecules from which they're made. From the rubber in our tyres (which cracks in ground-level ozone) and the glass in old windows (which sulphur dioxide attacks) to the leather and paper of books in our libraries (nibbled away by acid, formed from sulphur dioxide in the atmosphere), almost every material you can think of will be damaged by air pollution sooner or later. Today, the two main issues are corrosion (by chemical action) and soiling (by the deposit of sooty particulates). [9]

Stone

Time for a quick chemistry lesson. Historic buildings like the Parthenon are made from limestone and marble, broadly speaking, calcium carbonate, which acid in the air will feed on. It might be sulphuric or nitric acid (sometimes both), but, for the sake of simplicity, let's just consider the sulphuric part. It forms when sulphur dioxide reacts with moisture and oxygen in the atmosphere to make sulphuric acid—once known as 'vitriol' (the noxious stuff you had to be sure not to spill in chemistry lessons in case it burned holes in your clothes or skin). When limestone (calcium carbonate) and sulphuric acid come together, they make gypsum (calcium sulphate, or more technically, hydrated calcium sulphate – it has some water locked up in its structure) and carbon dioxide. [10]

You can get an idea why that's a problem by looking at something called the Mohs hardness scale, which is a simple way of comparing how hard or soft things are that's used by scientists who study materials. At one end of the ten-point scale, we have super-hard diamond (10) and the tungsten carbide used in drills and saws (9). Limestone and marble live in the middle of the scale at about 3–4, so they're not particularly hard, as materials go; our stone buildings are a softer touch than we realise. But gypsum is another story, creeping in at the lower end of the scale on a Mohs hardness value of about 2, just above talc (at Mohs 1, just about the softest material you'll ever find). In other words, the chemical reaction that turns calcium carbonate to calcium sulphate is also a physical transformation that makes semi-hard stone measurably softer.

Several other factors encourage buildings to crumble. Gypsum crystals are bigger and bulkier than carbonate ones, so the conversion involves expansion, stress and cracking. They're also much more soluble in rainwater. Meanwhile, any water that does get inside porous stone can cause what's called 'frost weathering': it freezes in cold weather, expands and jacks open existing cracks (just as winter water cracks the pipes in your home), causing a vicious circle of cracking, water ingress, crumbling and decay. Although gypsum is naturally white, its crystals sponge up sooty particulates from the atmosphere forming a black and crumbly crust. This 'spalls' (flakes away) from the surface of a building under attack, exposing fresh stone underneath that undergoes further attack in its turn. [11]

Concrete

Modern buildings, in modern cities, are less susceptible to acid attacks because they're 'moulded' out of reinforced concrete. It's a kind of artificial stone made from gravel of various sizes, plus cement and water, all toughened up with strong, steel reinforcing bars, known as rebar, 'locked' inside when the concrete is poured around them. Technically, ordinary concrete is a composite material in which crystals of cement hydrate (calcium-silica-hydrate) strongly bond the aggregates. Reinforced concrete is, in effect, a composite of a composite—steel and ordinary concrete fused together so we get the best of both worlds: a material you can load in many different directions without it collapsing. In theory, this makes concrete buildings resistant to attack from acid rain; in practice, things aren't so simple. In Hangzhou, one of the Chinese cities most polluted by acid rain (and it's harshly acidic here, with a pH of 4-5), erosion of modern concrete buildings is estimated to be causing $2 million of damage per year. [12]

Reinforced concrete can disintegrate in the environment through a variety of mechanisms that take place inside it. First, there's a built-in defect: in what's informally known as 'concrete cancer', alkalis in the cement slowly react with silicates in the aggregates to form bloated crystals that can make the concrete crack apart from within. That lets in acidic moisture, which rusts the steel rebar holding everything together. Rust, too, takes up more space than the metals from which it forms, so we get a kind of feedback loop where the cracks widen, more water gets in, the rusting process gets worse and the cracks widen even more. Second, water trapped inside concrete can freeze, expand and cause cracking (just as it does in rock and stone). Finally, acids in rain can erode the concrete itself, worsen the cracking and exacerbate the rusting of the steel rebar inside.

I won't go into all the chemical reactions but, broadly, what happens is similar to concrete cancer. The hard minerals in the cement are converted to soft things like gypsum and other calcium-based minerals (such as anorthite) that take up roughly twice as much space. So we get further cracking apart inside the concrete, further water and acid-rain incursion and yet another vicious circle leading to a steady loss of strength and, eventually, complete failure. Things like the use of deicing salts (on concrete road bridges), which seriously corrode the rebar, make matters worse. The more acidic the rain, the worse the problem gets; the more load the building, bridge or other structure has to carry, the quicker it's likely to fail. The net result is that concrete is much more vulnerable to erosion in a polluted urban environment than architects and engineers ever supposed when they were splashing it around in such quantities during the 20th century.

Metals

You might think durable metals like steel, aluminium and copper would be relatively resistant to air pollution, but you'd be wrong. Iron-based steels obviously rust (turn to iron oxide), but pollution makes matters considerably worse. Among the first to investigate this was one J. Hudson, in 1943, who compared how steel specimens rusted (and lost weight) when he left them in 20 different locations around the world for a year. Using the dry inland atmosphere of Khartoum, Egypt as his baseline, because it had least effect of all, he found an unpolluted area in Sweden was three times worse, rural Pennsylvania 25 times worse and polluted areas of Britain anything from 55 to 100 times worse. So air pollution, in this test, sped up corrosion by a factor of as much as 100. [13]

Steel is often protected from rusting by galvanizing it (coating it with zinc—as in old-fashioned trash cans), but even galvanised steel will rust relatively quickly if the concentration of sulphur dioxide in the air is high enough. And though aluminium is one of our best, relatively rustproof materials, thanks to a layer of protective oxide that seals its surface, it too will corrode in high enough concentrations of air pollution. In the material world, nothing, it seems, is safe from pollution—not even metal.

Erasing the future

We live through time, striding through the present, eyes to the future, anchored always by the past and the lessons it's taught us. History, quite literally, civilises us—our legacy of inspired invention and achievement marks us out from animals, mere beasts, and it's something we can't afford, carelessly, to lose. Air pollution helps to wipe millions of people from the planet year upon year, but, as we've seen in this chapter, it also has the potential to erode and erase human history. Books, statues, stone cathedrals, marble monuments—these singular historic records can be wiped clean by the slow corrosion of dirty air, albeit less obviously than our own distracted lives take the time to note.

And the war that air pollution is waging against civilised human history is constantly shifting to new fronts. For much of the 20th century, the major problem was sulphur dioxide emissions from power plants and home coal burning; historians and engineers focused on how acid formed from those emissions was eroding limestone and marble monuments like the ones we've explored above. Today, pollution has mutated into a different kind of killer; nitrogen dioxide, ozone and particulates, notably generated by traffic, are the biggest problem both for the health of humans and for the cities they built around them. Now we have to worry about nitric acid rain, the stains of diesel soot, the cracks appearing deep inside modern lumps of concrete and how novel contemporary pollutants can erode the no less novel plastic polymers in cutting-edge buildings. There's also air-pollution 'mixology' to consider: for example, how toxic cocktails of nitrogen dioxide and sulphur dioxide cause stone corrosion at a faster rate than either one alone, and how sooty carbon particulates appear to accelerate ('catalyse') the chemical degradation of marble. We also have to consider the complicating effects of climate change. [14]

This, last, may prove to be the biggest challenge to our built environment in the not-too-distant future. Even without the help of pollutants like sulphur dioxide and nitrogen dioxide, rain (and other moisture in the air) is slightly acidic thanks to carbon dioxide. Steadily increasing concentrations of carbon dioxide—the gas that humans have been hurling out in ever-greater quantities since the start of the Industrial Revolution—amplify this effect. Recent research by Australian civil engineering Professor Mark Stewart and colleagues found that, by the end of the 21st century, acidification of this type could increase the risk of concrete damage by up to 15 per cent. And given that every modern bridge, tunnel, skyscraper, dam and nuclear power plant uses concrete to one degree or another—over half of modern China is built from it, according to one recent study —that must be a cause for concern. [15]

In some places, problems could occur much sooner. In coastal regions, for example, where there's already plenty of salt (sodium chloride) in the air, chloride ions corrode the steel rebar in reinforced concrete markedly faster. According to prize-winning research by Matthew Eckelman and Mithun Saha of Northeastern University, that could lead to significant surface failures of this kind as early as 2025, shortening the normal life of a modern concrete building from about 75 years to as little as 50. With so many concrete structures erected in the second-half of the 20th century, when weaker concrete was more the norm, that starts the clock on a ticking time-bomb. Today, we worry about the corrosive effect of air pollution on the Taj Mahal and the Acropolis; perhaps tomorrow, we'll be more alarmed by climate-powered cracks in China's 38 atomic energy plants or the 90,580 dams in the United States, the average age of which is already 56 years. [16]

Hungry Horse Dam in Montana, courtesy US Bureau of Reclamation.

Dammed if we do? What effect will air pollution have on dams like this one, Horse Dam on the Flathead River, Montana, USA, built in 1953? Picture courtesy of US Bureau of Reclamation.

References

    • Air and Rain: The Beginnings of a Chemical Climatology by Robert Angus Smith. London: Longmans, Green, and Co., 1872. p444.

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