Acid pollution in the atmosphere from industry has fallen dramatically since man-made acid pollution took off in the 1930s and peaked in the 1960s and 70s. In the 1970s, both Europe and the United States adopted the ‘The clean air act amendments’, which required filters in factories. As a result, acid levels are now almost back to the level that it was before the pollution started with industrialization in the 1930s.
http://news.ku.dk/all_news/2016/09/acidity-in-atmosphere-minimised-to-preindustrial-levels/
