HANOI, Vietnam — Hanoi has no horizon. Blocky apartment towers dissolve into gray fog in the Vietnamese capital, as barges carrying sand inch down the Red River toward makeshift jetties. At street level, the city blurs as if it's covered in film. The air stings your eyes and smells of chemicals, like chlorine but not quite. When the sun does punch through, it hangs like a red beach ball against the silver sky. This winter, Hanoi topped global air pollution charts, not once, but repeatedly, exposing the environmental cost ofVietnam's explosive growth, and briefly earning it the title of the world's most polluted city. In January, the average air quality index in the city of almost 9 million people was breaching the "hazardous" threshold of 300, shrouding its skyline in fog and prompting warnings from health officials. And in March, the city recorded levels of hazardous small particles known as PM2.5 that were more than 24 times the World Health Organization's recommended limits. Schools closed, work slowed and N95 masks were the norm. "I struggled a few weeks ago. The air was so bad, and the nature of my work means that I have to be outside," Mã Thị Dung, 50, a peanut seller in Hanoi's Old Town, said in an interview this month. "I cannot hide indoors in the air-conditioning. I had a cough for two weeks that I couldn't cure, and cycling on my bicycle is particularly difficult when breathing is tough." The WHO estimates thatmore than 60,000 deathsper year in Vietnam are linked to air pollution. In alive ranking on Saturday by IQAir, a Swiss air monitoring company, Hanoi ranked 13th on a list of the most polluted major cities. The fog hanging over Hanoi isn't just pollution, but a byproduct of growth that has lifted Vietnam's economy while fueling its environmental struggles. Since 2018, Vietnam's gross domestic product has grown an average of 5% to 7% per year, far more than larger economies such as the United States and China, due in part to its infrastructure boom. Lower labor costs and a skilled workforce have made the Southeast Asian nation an attractive alternative for companies shifting production from China such as Apple and Nike, turning it into a regional manufacturing hub. As Vietnamese people relocate to cities for work, the scale and speed of urban expansion is reshaping swaths of rural land. Private developers such as Vinhomes, Vietnam's largest real estate firm, are building new cities on the edges of Hanoi and Ho Chi Minh City, formerly known as Saigon. "It is a very busy time for us now. We have no inventory left to sell. It's all been bought, and we're rushing all over town to deliver supplies to our customers," said Hoang Văn Hữu, 38, a manager and construction worker at Thuỳ Đá Concrete. Constructed almost entirely from concrete, developments such as Ocean Park and Grand Park span hundreds of acres, including enough land to build at current rates for the next 30 years. Foreign companies are also investing, with theTrump Organization breaking groundlast month on a $1.5 billion luxury residential development outside Hanoi. Concrete defines the built environment here, with elevated highways, metro lines, prefab towers and entire cities poured into place. Globally, concrete is used more than any other substance except water, at an estimated 30 billion tons a year. Cement, the ingredient that binds concrete together, accounts for 8% of global carbon emissions, according to a report by the London-based think tankChatham House, more than all air travel combined. Vietnam uses more cement per capita than any country outside China, and almost double that of the United States. Cement factories, batching plants and construction sites generate both carbon emissions and harmful PM2.5 particles, the same pollutants behind Hanoi's hazardous air. With its dense population and booming infrastructure, the city has become a flashpoint for these problems. According to Vietnamese media, traffic accounts for more than 50% of Hanoi's air pollution, followed by industrial activity at 30% and construction at 10%–15%. Concrete is embedded in all three. Roads enable the constant movement of trucks, mixers and motorbikes. Construction zones blanket neighborhoods in dust. Cement kilns and material processing sites draw heavily on coal-fired power and emit their own clouds of pollutants. Nguyễn Thị Mỹ, 74, who works at Chợ Châu Long Market, said Hanoi's air in the winter is "terrible." "For older people and people with health conditions the effect is much worse. They have trouble breathing," she said. As for herself, "I didn't suffer this year, I am strong," she said. "My work keeps me active — all the ladies here in this market are strong." The high levels of pollution in Hanoi "cause substantial short- and long-term effects," said Nguyen Thi Kim Oanh, director of the Air Quality Nexus Center at the Asian Institute of Technology in Thailand. Meteorological conditions in the winter hinder the dilution of local emissions, she said, while monsoons bring more pollution from neighboring countries. "In other seasons, the air quality is comparable to other cities," Kim Oanh said. Vietnam has introduced reforms, adopting stricter vehicle emissions standards and pledging that 50% of buses and taxis will be electric by 2030. The monitoring of infrastructure-based pollution is improving, and national campaigns have encouraged cleaner household fuels and better waste management. Vietnam is also a signatory to the Paris Agreement, committing to reach net-zero carbon emissions by 2050 by curbing coal reliance and improving energy efficiency in industry, including cement production. But the gap between policy and lived reality remains wide. In practice, development continues to move faster than regulation, and environmental protections are often secondary to economic priorities. Cement kilns still burn coal. Construction zones still pour dust into crowded neighborhoods. And the skyline keeps rising.