For decades, the majority of the Twin Ports lived under the threat of a concentrated chlorine spill, which could have spread over 10 miles and sickened up to 128,000 people in an unlikely, though technically possible, worst-case scenario.
Then in 2006, Western Lake Superior Sanitary District decided to do away with the risk altogether.
"While chlorine is undoubtedly an effective method of disinfection, wastewater treatment plants nationwide are shifting away from its use due to the dangers chlorine can pose when stored on site or during transportation," former WLSSD Executive Director Kurt Soderberg said in a statement at the time.
It cost $1.5 million and 18 months to build a new disinfection process, but it meant an end to the storage of up to 220,000 pounds of chlorine at the facility along the St. Louis River in Duluth.
According to Environmental Protection Agency filings, the move eliminated the worst-case possibility that a 55-ton rail car could send a full tank of chlorine into the atmosphere in 10 minutes - at least from those tank cars traveling to WLSSD.
That wasn't the only chlorine used in high quantities in the region.
Seven facilities in and around the Twin Ports use or store enough hazardous chemicals to pose a danger to the greater public, according to EPA records. So while the Husky Energy refinery handles the most dangerous chemical, hydrogen fluoride, it isn't the only risk in the region.
Chlorine still threatens thousands of people at three locations in Duluth and Superior, with the highest risk at the Cloquet Pump Station No. 2 on Knowlton Creek Boulevard. Under the worst-case scenario spelled out in EPA documents, a sudden release could spread three miles and affect more than 19,000 people.
As far as flammable risks, nearly 9 million pounds of propane could go up in a vapor cloud explosion and spread 1.7 miles in a worst-case scenario at the Depropanizer and Storage Facility at 2600 E. 21st St. in Superior. That would affect up to 5,600 people, according to EPA files.
The News Tribune requested access to the documents and was able to review them in Duluth with the help of the Department of Justice, the EPA and the U.S. Marshals Service. They are not publicly available without making arrangements due to security concerns.
The worst-case scenarios are used for emergency planning purposes and are considered extremely unlikely. It would take the failure of many safety systems and certain weather conditions for them to occur.
The EPA documents also detail alternative release scenarios that may be more realistic in scope. Under those scenarios, the highest risk to the public would come from a chlorine gas leak at the Knowlton Creek pump station that could spread half a mile and injure up to 450 people. Fire accidents considered more likely would not travel far enough to be a risk to a nearby residential population.
When WLSSD handled chlorine, its worst-case scenario involved a railcar spill. It's entirely possible such a risk remains in the community, though federal law and security concerns make it hard to say when and where.
Railroads are not required to file risk management plans detailing worst-case scenarios and affected populations. They must track highly hazardous materials and be able to quickly tell the Transportation Security Administration where high-risk railcars are.
For the general public outside emergency responders, the Federal Railroad Administration says "the railroad has no obligation to provide this information, which is considered proprietary or for some commodities, classified by the railroad as 'security sensitive information,' only released to an entity on a 'need-to-know' basis."
First responders in the area say they know the risks and are well-prepared to handle them. The Duluth Fire Department is home to one of the state's regional hazardous materials teams, covering incidents across Northeastern Minnesota.
"It's very specialized training, and there's a lot of specialized equipment that goes with that," said Mark Herman, acting assistant fire chief. "That helps us create a safer community here and provide a quicker response."
Superior also maintains an emergency response team that covers the northern part of Wisconsin.