In the 1960s, Japan imported about 90% of its crude oil from the Middle East, and most of the oil had a high sulfur content.
The companies in the petrochemical complexes initially used heavy oils refined from this crude oil, and which therefore had high sulfur levels. As the issue of air pollution became more prominent, in addition to raising stack height, companies began converting to heavy oils with low sulfur content or the use of gas and other fuels.