On Thursday, January 7, a chemical explosion at the DOW Chemical Company in North Andover, MA, left at least four employees critically injured. The explosion occurred around 2:30 p.m., according to reports from WHDH.com, among others. Neighbors of the Willow Street plant reported that the explosion was so powerful they could feel it in their homes.

The four injured employees were rushed to Lawrence General Hospital, where they were decontaminated in the parking lot before being brought inside. Within an hour, three were airlifted to Boston for more intense trauma care. According to recent updates, all four remain in critical condition with shrapnel injuries and burns.

CBS Boston reports that five employees were injured, and that four lab workers were able to walk to the ambulance before they were transported. Employees at the site who were not injured were evacuated by bus.

Per WHDH, the DOW facility in North Andover develops materials used in LED light manufacturing. Victims of the explosion were exposed to trimethylaluminium, which Dr. George Kondylis of Lawrence General Hospital described as “a chemical that’s an irritant to the skin and anything it came in contact with.”

According to firefighters, people who live nearby do not need to worry about any chemicals in the air. A bomb squad will reportedly use a robot to detonate some of the remaining dangerous chemicals.

This is not the first time the DOW facility in North Andover has led to a serious workplace injury. In October of 2013, an explosion and fire at the same facility led to the death of 51-year-old production operator Carlos Amaral. According to an official release from DOW, that incident occurred during a procedure involving Trimethylindium (TMI), a different chemical.