CHICAGO (CBS) — A firefighter is among seven people in critical condition after an ammonia spill in north suburban Beach Park sent a total of 40 people to the hospital on Thursday. Several schools closed for the day as a precaution, and people in nearby homes were told to stay inside for several hours after the spill.

Around 4:30 a.m., the Lake County Sheriff’s office received a 911 call about a possible vehicle fire at Green Bay Road and 29th Street. When two deputies arrived, they discovered ammonia leaking from at least one of a pair of two-ton tanks being hauled by a tractor.

The deputies were overcome by ammonia fumes, and had to back away from the scene, according to Lake County Sheriff’s Sgt. Chris Covelli. Firefighters initiated a hazardous materials response to contain the spill, pouring water on the ammonia to dilute the chemical. Covelli said firefighters wearing protective gear rescued several people who were on the ground near the leak.

Using the village’s reverse 911 system, Beach Park officials notified anyone living within a one-mile radius of the spill to stay indoors, keep their doors and windows closed, and turn off any air conditioning or ventilation systems, according to Covelli. That shelter-in-place order was lifted at about 10 a.m.

A total of about 25 ambulances were on the scene, shuttling victims to the hospital. Authorities were still receiving 911 calls as of 9 a.m.

“This is a very dangerous chemical that can cause unconsciousness, and worst-case scenario death,” Covelli said.

Victims who inhale even small amounts of anhydrous ammonia experience burning of the eyes, nose, and throat. Inhaling higher doses cause coughing or choking. Exposure to high levels of anhydrous ammonia can cause death from a swollen throat or from chemical burns to the lungs.

The cause of the leak was under investigation. Anhydrous ammonia is often used as a fertilizer in soil.

This is the third such spill in Illinois this year. The first spill happened on April 10 in Easton; the second was Wednesday afternoon in Farina. Since 2013, there have been on average about a dozen spills statewide each year.

At least 40 people were were taken to hospitals after inhaling ammonia fumes, including 11 firefighters, two Lake County Sheriff’s deputies, and one Zion police officer.

Seven victims were in critical condition, including one firefighter.

Several other victims have since been released from the hospital.

Sylvia Cruz said she lives close to the spill, and was driving with her three children at the time. After police stopped her and told her to turn around because of the spill, she started feeling dizzy and having trouble breathing, so she called for an ambulance.

“My son was also very scared, and started shaking. He said his stomach was hurting, so I’d just rather be safe than sorry,” she said. “That was very scary, especially fear for the life of my kids.”

They were released from Vista Medical Center later Thursday morning.

“The boys and the baby are doing just fine. We all got checked, our vitals. My oxygen’s a little low, so they gave me a little bit of oxygen. Other than that, we’re doing just fine,” Cruz said.

Several nearby schools also have closed for the day, including all schools in Beach Park School District 3, all schools in Zion Elementary School District 6, Zion-Benton Township High School, and Prairie Trail School.

Police and fire crews made door-to-door checks of about 100 homes near the spill.

“This is a silent gas, so to speak. People can inhale this and not know, and become quickly unconscious. So we’re checking on their well-being,” Coveilli said.