Ohio officials on Wednesday said an explosion at a chemical tank at a North Toledo company released a chemical into the air overnight and prompted an evacuation of nearby residents. Glycol methacrylate, an industrial coating product, was released into the air after Tuesday night’s explosion, Toledo Mayor Wade Kapszukiewicz told reporters. One person, a first responder, was injured and taken to the hospital, he added, but officials believe the injury is not serious. Officials do not know what caused the explosion.

An employee at the company — MC Tank Terminal — called 911 to alert authorities to the explosion, Kapszukiewicz said. He added that the company is headquartered in Arkansas, but that executives are en route to Toledo to assess the situation. Kapszukiewicz explained that when the chemical was released in the explosion, it became a gas, but when it settles back into the tank, it will turn back into a liquid. Dense fog and haze hindered visibility in the city of about 265,000 people, and local news stations reported smelling a plastic-like odor.

“That is the chemical whose plume we are monitoring, and we are concerned enough about the health impacts to have taken two actions overnight,” the mayor said. The city had already issued an evacuation order last night for everyone living within a half-mile of the incident — about 300 people. It was later expanded to a one-mile radius, upping the number of those who needed to evacuate to almost 1,000. All residents who live within a three-mile radius have been put under a shelter-in-place order and have been instructed to keep windows shut, Kapszukiewicz said Wednesday.

As far as city officials know, all residents have been evacuated, they said Wednesday. He did not say when those orders will be lifted. Residents should check with local businesses and schools within the evacuation zone to see what plans are for Wednesday. Some schools were closed Wednesday, and others delayed opening.

There is still concern that “the tank is emitting this chemical,” he said, “and it has the potential within 12 hours of the incident to be explosive.”Because of this, first responders and hazmat teams could not approach the tank until 9:30 a.m. At that point, they will determine whether the chemical can be contained. “The hope is that at that point the risk of explosion will have expired, and that efforts can be made to contain and eliminate the hazard,” Kapszukiewicz said.

He noted that those in and around the evacuation zone might react to the chemical in the air — they might taste or smell something unnatural, or their eyes could water. Kapszukiewicz called this a “real concern.”But residents should take note of whether those exposures are dangerous or simply annoying. “There is no question that we’re all kind of smelling and sensing, that there’s something there,” Kapszukiewicz said. “That doesn’t necessarily mean that it is something that is harming us immediately. There are levels of exposure to something like this that are unpleasant but not necessarily dangerous to human health, and that entered into our decision-making in terms of where the shelter-in-place orders took place.”

The chemical is a long-term carcinogen, Karim Baroudi, the Toledo health commissioner, said. But he added that there is “no immediate danger” related to Tuesday night’s release. Baroudi said those who feel irritation from the chemical should shower to wash it off their bodies. Kapszukiewicz thanked all community agencies that helped respond to the event, including those who assisted in evacuating residents and setting up shelters.

He said his administration has been in touch with both the state and the U.S. Environmental Protection Agency. “We understand the seriousness of what has happened here,” Kapszukiewicz said. “That’s why we took the actions that we did. That’s why we’re going to continue to monitor and communicate with the public throughout the day.”

Kapszukiewicz acknowledged the fog hanging over his city Wednesday morning and noted that while the chemical release was a factor, the weather also caused natural fog that morning. Videos posted to social media and verified by NBC News show hazy skies as residents in the danger zone drive with their hazards on. Street lamps and car lights are visible through the fog as a line of cars appears to try to leave the area.

Some videos appeared to show buses and emergency vehicles in an empty parking lot, surrounded by the fog. Another video, taken from a distance, shows thick plumes of smoke gathering in the air. The smoke appears to be coming from one building. The tank at MC Tank Terminal is a “transport hub,” Michael Posadny, the special operations deputy chief in charge of hazmat, explained. It keeps products on site briefly before shipping them to the next location. “In a city like Toledo, with industrial history, events like this, as unpleasant and unwelcome as they are, sometimes happen,” Kapszukiewicz said. “And you know, we have gotten through events like this before. We’ll get through them again.”