Sulphur dioxide is a classic air pollutant usually linked to the burning of fossil fuels.
Greenpeace described the area around the Ventanas plant as "Chile's Chernobyl" following a serious incident in 2018 when around 600 people in Quintero and Puchuncavi received medical treatment for symptoms such as vomiting blood, headaches, dizziness, paralysis of their extremities and strange red marks on children's skin.
Last week, President Gabriel Boric hit out at Chile's record on polluting the environment.
"We don't want any more areas of (environmental) sacrifice," he said.
"There are now hundreds of thousands of people who live in our country exposed to severe degradation of the environment that we have provoked or allowed and, as a Chilean, that makes me feel ashamed."
Pollution accumulated in the area of Quintero and Puchuncavi, home to around 50,000 people, after the government decided in 1958 to convert it into an industrial center that now hosts four coal-fired power stations and oil and copper refineries.
