Pittsburgh 's smoke pollution , seen in the 1890s as a sign of prosperity , was recognized as a problem in the Progressive Era .
Pittsburgh 's smoke pollution , seen in the 1890s as a sign of prosperity , was was cleared up in the 1930s -- 1940s .
Pittsburgh 's smoke pollution , seen in the 1890s as a sign of prosperity , was recognized as a problem in the Progressive Era and was cleared up in the 1930s -- 1940s .