On Friday, the EPA approved Arizona’s 10 year maintenance plan for Douglas to keep the level of sulfur dioxide, a toxic gas, low.

The levels of sulfur dioxide in the Southern Arizona city were deemed unsafe in 1978, largely due to The Phelps Dodge Douglas Reduction Works smelter. Once the plant closed in 1987, the amount of gas dropped dramatically.

Because sulfur dioxide in the air can aggravate asthma, cardiovascular diseases and respiratory illness, the EPA remains involved in areas that violate the clean air standard for decades afterward.

Friday’s plan was a continuation of a 30-year process to ensure the level of sulfur dioxide remains within public health standards. The initial plan outlined efforts to lower the toxic gas, which included monitoring the amount in the air. Although they produce a tiny fraction of the sulfur dioxide once released by the smelter, APS’s Fairview Generating station and a nearby concrete producer have limits on their sulfur dioxide emissions.

Several other areas in Gila, Greenlee, Pima and Pinal counties were once violating EPA’s clean air rules but have since reined in pollution.

However areas near Hayden and Miami have unsafe levels of the toxic gas in the air and the state is actively working with the EPA to meet public safety standards.

EDITOR’S NOTE: This story has been updated to clarify the scope of the mediation plan and current state of sulfur dioxide emissions in the area.