You don’t have to be fighting in a war to suffer irrevocable brain damage from one.

That’s the finding from a new United Nations report released this week showing that an estimated 87 million children under the age of 7 have lived their entire lives in conflict zones, an environment so stressful that it has the potential to significantly impact the development of their brains.

The discovery stems from a report released Thursday by the United Nations Children’s Fund (UNICEF), an organization that works to bring resources to children in the 22 countries affected by conflict worldwide. This year the organization released a report showing that one in four of the more than 100 million school-age kids living in these zones are not in school.

Now, on top of the fact that 25 percent of these children are not able to access education, comes the news that even those who do may be suffering irreversible damage to their brains.

Conflict zones create a stressful environment, one that causes severe, prolonged trauma. While some amount of stress is normal to warn the body that danger is imminent, too much stress can be damaging.

“In addition to the immediate physical threats that children in crises face, they are also at risk of deep-rooted emotional scars,” said UNICEF Chief of Early Child Development Pia Britto, one of the main researchers behind the study.

The scars are not metaphorical: Stress actually rewires young brains.

Humans are born with 253 functioning neurons, nerve cells that receive and transmit information. From that point onwards, hundreds of new neurons are formed every second, until there are nearly 1 billion in most adults.

That’s just the number of circuits, but how they are wired depends on what happens in development.

Harvard researchers elaborated on the process in a 2004 paper titled Excessive Stress Disrupts the Architecture of the Developing Brain. In it, they explain how healthy development can be “derailed” by toxic stress.

Happy moments activate positive circuits, stressful ones negative circuits. The circuits that are activated most frequently are strengthened, sometimes to excess. The result is a body and mind that is constantly on high alert, no matter whether actual danger is present.

“Children may feel threatened by or respond impulsively to situations where no real threat exists, such as seeing anger or hostility in a facial expression that is actually neutral,” the paper reads. “Or they may remain excessively anxious long after a threat has been posed.”

Although learning to manage tough situations is critical to a child’s growth, too much stress can have a toxic effect on a young brain. Chronic stress causes a host of physiological responses, raising our heart rate, blood pressure, and cortisol (the stress hormone). Continuous high levels of these have been linked to a variety of behavioral and physical disorders including depression, anxiety, drug abuse, cardiovascular disease, diabetes, and stroke.

In light of these facts, UNICEF’s data is particularly grim. By their calculations, one in 11 children under age 7 has spent the most critical period of brain development in a conflict zone.

Beyond symptoms like depression, damage to early child brain development has been shown to significantly hinder a child’s ability to reap the benefits of education. A 1999 study from Yale University and three other major institutions found that the quality of development near the time a child begins school directly influences their ability to learn.

Among the countries mentioned in the report are Afghanistan, Iraq, and Syria—one of the most dangerous places on the globe for children today. The numbers were calculated using the UN’s Office of the Special Representative and of the Secretary General’s Children and Armed Conflict annual report.

“Conflict robs children of their safety, family and friends, play and routine,” said Britto. “Yet these are all elements of childhood that give children the best possible chance of developing fully and learning effectively, enabling them to contribute to their economies and societies, and building strong and safe communities when they reach adulthood.”