Skyscrapers were invented in the US in the 1870s in major cities like Chicago and New York using structural iron and steel instead of masonry. Builders faced challenges with safety standards, wind resistance, ventilation, and transportation within the tall buildings. By the late 19th century, skyscrapers were understood to be steel-framed structures, and between 1902-1931 New York engaged in competition to build the world's tallest building, though the size and height of skyscrapers was controversial due to issues like lack of sunlight and increased density. Rapid urbanization and immigration in the late 19th/early 20th century resulted in widespread slum conditions in major US cities, as tenement housing with poor sanitation became over