April 19

In many major cities, a pall of air pollution was created by the global spread of modern industry. Face masks were frequently worn by traffic police in Tokyo during the 1970s. It was estimated in 2002 that 35,000 people in Mexico City were killed from air pollution. By the late 1980s, about half of the Soviet Union's rivers were severely polluted from industrial pollution. The earth is protected by the ozone layer from excessive ultraviolet radiation. Global warming was the most intractable and critical environmental transformation. The atmosphere had begun to be warmed significantly due to the vastly increased burning of fossil fuels, which occurred by the end of the twentieth century.