Officials have cleared a chemical spill that hospitalized at least 37 people, 7 of whom are in critical condition, and prompted school closures Thursday in north suburban Beach Park.

Authorities were called at 4:25 a.m. for reports of a possible vehicle fire near North Green Bay Road and 29th Street, according to Lake County Sheriff’s Sgt. Christopher Covelli. A responding sergeant and deputy arrived to see a what appeared to be cloud of smoke and “were overcome by an airborne chemical” when they got out of their vehicle.

UPDATE: This appears to be an Anhydrous Ammonia spill, which created a dangerous chemical cloud in the area of Green Bay Rd and 29th Street. DO NOT GET CLOSE TO THE AREA! Residents in a 1-mile radius, STAY INSIDE, WINDOWS CLOSED! pic.twitter.com/aR43RO1gJp — Lake County Sheriff (@LakeCoILSheriff) April 25, 2019

Further investigation revealed that a truck had been towing containers of anhydrous ammonia when the chemical began to leak onto the road, causing “toxic plumes” of it to be released into the air, Covelli said. No actual fire had occurred.

Anhydrous ammonia is often used as an agricultural fertilizer and industrial refrigerant, according to the Centers for Disease Control.

Dr. Robert Cohen, a professor of pulmonary and critical care at Northwestern University Feinberg School of Medicine, said in a statement that the chemical can cause severe irritation to mucous membranes, including the eyes, nose, mouth and throat.

“If it is inhaled deeply into the lungs, it can damage the large airways, causing a severe trachea bronchitis,” Cohen said in the statement. “It can also cause significant permanent lung damage if enough of the vapor is inhaled. The damage to the mucous membranes may also be permanent if severe enough.”

Covelli said 37 people, including 11 firefighters, the deputy, the sergeant and a Zion police officer, have been taken to local hospitals as a result of the spill.

One firefighter and six other people are listed in critical condition and had to be intubated, according to Covelli. The rest of the hospitalized people are either in good condition or were already treated and released by Thursday afternoon.

The Beach Park School District 3 announced about 6:10 a.m. that all District 3 schools would be closed for the day.

Due to a chemical spill that happened recently, All Beach Park School District #3 schools will be closed today, April 25th, for the safety or our students and staff. We apologize for any inconvenience that may have on your family. — BPD3 (@BPD3District) April 25, 2019

Just after 7 a.m., Zion-Benton High School in Zion and Prairie Trail School in Wadsworth announced they would be closed for the day as well.

The spill created a dangerous chemical cloud around the area, officials said. Residents within a one-mile radius were advised to stay inside with the windows shut and HVAC systems turned off while HazMat crews respond.

The spill has since been cleared and the shelter-in-place order was lifted by about 10 a.m., Covelli said. The roads were reopened by 11 a.m.

The cause of the spill remains under investigation.