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WATCH: Nielsen ‘not familiar’ with how toxic stress harms separated families

Homeland Security Secretary Kirstjen Nielsen told a House committee Wednesday that she was “not familiar” with the idea of toxic stress, which can be the result of intense, repetitive or prolonged adversity, particularly in children.

Mental health experts have warned migrant children who were separated from their parents at the U.S.-Mexico border are at risk of developing the condition — defined as stress that is “intense, repetitive or prolonged,” without an adult’s intervention.

Toxic stress has been linked to a number of traumatic situations such as when a child is placed in an orphanage, survives a natural disaster or lives in poverty, a war zone or a refugee camp.

WATCH: Kirstjen Nielsen testifies on border security, future of border wall

Rep. Lauren Underwood, D-Ill., questioned Nielsen about the practice of separating immigrant children from their parents on Wednesday during a House Homeland Security Committee hearing on border security.

Underwood asked Nielsen several yes or no questions about whether DHS took the physical and mental health of children into consideration when it began separating families at the border.

Nielsen said she was unaware of the term toxic stress, and also did not know that it can change a child’s developing brain. Nielsen interrupted the line of questioning repeatedly to interject that kids should be with their families, but that parents also shouldn’t break U.S. law.

“Children need to be with their families. We need to fix the system to enable them to do that,” Nielsen said.

READ MORE: How the toxic stress of family separation can harm a child