The suspect, Theresa Monique Jefferson, 33, followed another woman into the store and assaulted her, county police spokesman Shawn Vinson said in an email. She later turned herself in and was arrested.
The two women knew each other and were involved in a continuing dispute, Vinson said.
The victim's boyfriend has a child with Jefferson, who is from the Lansdowne-Baltimore Highlands community of Baltimore County, police said.
Jefferson has been charged with first- and second-degree assault, theft of items worth less than $100 and malicious destruction of property, police said. She is being held at the Baltimore County Detention Center on $350,000 bail.
Victim information was still being determined Saturday evening, Vinson said.
The Baltimore County Fire Department's fire and medic units, who arrived on the scene just after 11 a.m., requested the hazardous materials team a short time later because of complaints about watery eyes and other injuries, said spokesman Glenn Blackwell.
One person was taken to the Wilmer Eye Institute for a potentially serious injury, while 18 others were taken to area hospitals, including Baltimore Washington Medical Center, Harbor Hospital, Northwest Hospital Center and St. Agnes Hospital, Blackwell said.
The store at the Lansdowne Station shopping center was evacuated and closed. The store reopened at 2 p.m. after employees cleaned up and ventilated the area, said Walmart spokeswoman Dianna Gee. Three employees suffered minor respiratory problems, she said.
"This is obviously not the type of behavior that we would expect from people at our stores," Gee said. "And we want to apologize for any inconvenience this may have caused other customers."
Baltimore County officials said ammonia had been thrown as well. Mixing ammonia and bleach can create a toxic gas that could lead to choking or other breathing problems.