Embedded in the sleek lines and elegant cantilevers of modern buildings sprouting up in city centers is a huge amount of pollution.

Whether it’s the distinctive Shard skyscraper in London or the World Trade Center in New York, modern structures rely on cement and steel. And while those materials may look benign, making each involves chemical processes that throw off tons of carbon dioxide in addition to the energy they consume.

Construction is responsible for 11% of global carbon emissions, according to a report from C40, a group of leading metropolitan authorities. Those figures underscore how efforts to rein in greenhouse gases will touch almost every part of the economy. After concentrating on the energy industry, policymakers now are scrutinizing buildings as a major source of pollution, prompting companies from steelmaker ThyssenKrupp AG to cement supplier LafargeHolcim Ltd. to look for greener alternatives.

“Everyone’s aware that we use electricity and energy to heat our homes and turn our lights on, but they might not have thought that loads of energy is needed in all the materials required to build the building in the first place,” said John Barrett, professor at the school of Earth and environment at Leeds University in northern England. “It’s a massive issue.”

A significant part of a building’s lifetime carbon footprint is locked into the materials used in the structure. Those “embodied emissions” are not regulated by anyone, according to the U.K. Green Buildings Council, which brings together 400 designers, construction companies and property developers.

One example it noted was the Leadenhall Building in London’s financial district, known for its cheese grater silhouette. Its construction required emissions of 92,210 tons of carbon, the council said in a report citing data from the building’s developer, British Land Co. That’s about the same as the annual emissions of 20,000 cars. About 60% of that came from steel and cement.

By switching to greener versions for the materials that go into construction and improving efficiency, lifetime emissions for buildings can be slashed 44% by 2050 if local governments and mayors push through ambitious climate rules to reduce the harm cities have on the environment.

“Material efficiency is the real win,” said Christina Lumsden, an energy consultant at the construction company Arup Group in London. “Low carbon cement as a real potential solution.”

Those are the findings of a report by C40, designers Arup Group Ltd. and Leeds University, which examined where emissions are coming from in 96 of the world’s largest cities. C40 is a network of cities committed to fighting climate change. Michael Bloomberg, the founder and majority owner of Bloomberg LP, is president of the group’s board of directors.

Emissions from making the cement, steel and rubber materials that are used in construction make up 60% of a building’s emissions taken across extraction, transport and on-site, the report said. Almost half of the source emissions from buildings come from fossil fuel extraction and electricity.

Steel beams support many commercial buildings and an increasing number of homes. Making them is one of the most polluting industrial processes.

A number of steps in the manufacturing process put CO2 into the air, the most significant being when iron is extracted from its ore in a blast furnace. Heated to more than 1,000 degrees Celsius (1,832 Fahrenheit), the furnace draws in both iron ore and carbon, often in the form of coal. Baking the two together makes pure iron plus carbon dioxide.

For each ton of steel produced, about 1.83 tons of carbon leak into the atmosphere, according to the World Steel Association. That’s more than the emissions from the average U.S. car driven from New York to Salt Lake City and back, based on data from the U.S. Energy Information Administration.

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Modern skyscrapers have thousands of tons of steel. London’s 95-floor Shard has 500 tons of the metal in its 66-meter spire alone, according to its Montreal-based structural engineer, WSP.

Companies such as ThyssenKrupp are looking at using hydrogen instead of coal to purify iron ore, which would slash emissions. That would require making big quantities of hydrogen, which requires energy. Another solution is finding ways to use less steel or other materials.

“It might require 10 to 20 years before we really see the roll-out,” Andrew Purvis, director of safety, health and environment at the World Steel Association, said by phone from Brussels.

Cement making is another massively polluting process. It requires baking limestone, or calcium carbonate, at more than 1,400 degrees Celsius until it turns to a material called clinker, the key ingredient that makes concrete harden. The kilns used to break carbon out of the limestone release CO2. They also need a great deal of energy to operate.

Each ton of cement produces about half a ton of CO2, according to the Global Concrete & Cement Association. About 13 tons fit in the average cement truck. Cement alone is responsible for about 7% of the world’s carbon emissions, the IEA estimates. And many modern buildings are using more cement.

While the old World Trade Center buildings were supported by external steel columns, the new one finished in 2016 has a concrete core rising to the uppermost floors in the 104-story structure, the engineer WSP says on its website. About 200,000 yards of concrete were used, enough to build a sidewalk from New York to Chicago, according to its developer, the Port Authority of New York & New Jersey.

Lafarge, based in Switzerland, and companies such as Wagners Holding Co. in Australia are looking at ways to reduce the amount of clinker in cement and also to find substitutes like fly ash or blast furnace slag.

“We have done a lot as a sector, but acknowledge more has to be done to achieve the targets in the Paris Agreement,” said Claude Lorea, cement director at the Global Cement & Concrete Association. “We are working hard to reduce our direct emissions.”

Policymakers are looking for solutions, and industry and research groups are coming up with suggestions. Their challenge is to balance the environment against the needs of the population for more infrastructure. They’re confronting forecasts that pollution from building materials will only expand.

“In Southeast Asia, India and Africa most buildings that will exist in 2050 haven’t been built yet,” said Markus Berensson, research manager at the C40 cities program. “It is a critical opportunity that many cities and nations should start considering at this point.”

Cement production is likely to rise 12% to 23% by the middle of the century, according to a report by the International Energy Agency assessing alternatives for the industry. It had one scenario showing cement producers could cut emissions 24% by 2050 while boosting production. Another IEA report on carbon emissions calculated that emissions from iron and steel doubled since 2000.

By using existing buildings more efficiently and avoiding new construction, London could save over $11 billion over the next five years, the C40 report said. One example: choosing wood instead of cement or steel could cut deliveries of materials to building sites by 60%.

“Solutions currently available in the market are capable of delivering low or zero carbon buildings,” the report authors said. “Architects and engineers need to propose them, surveyors need to cost them, and clients need to procure them.”

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