(Reuters Health) - Police killings of unarmed black people are associated with worse mental health for African-Americans across the country, even when they have no direct connection to the deaths, a U.S. study suggests.

Each year in the U.S., police kill more than 300 black men and women - at least a quarter of them unarmed, researchers note in The Lancet. African-Americans are more than three times as likely as white people to be killed by police and more than five times as likely to be killed while unarmed.

Beyond the immediate impact for victims and families, however, research to date hasn’t provided a clear picture of the spillover effect these killings can have in the black community.

For the current study, researchers examined survey data from more than 103,000 black adults, collected between 2013 and 2015, to see how often they reported days on which their mental health was “not good” in the previous month. The study team also looked at data on police killings in participants’ home states in the past 90 days.

On average, participants reported 4.1 days of poor mental health. But researchers found that each additional police killing of an unarmed black person in the past 90 days before the survey was associated with 0.14 additional days of poor mental health among African-Americans who lived in the same state as the victim.

“To people who may be suffering from poor mental health in the wake of police shootings, our study says you are not alone,” said lead study author Jacob Bor of the Boston University School of Public Health.

“There is an urgent need to reduce the incidence of police killings of unarmed black Americans,” Bor said by email. “But there is also a need to support the mental health of black people and communities when these events occur.”

African-Americans are exposed to an average of four police killings in their state each year, the study found. Extrapolating the results from the study to the entire population of 33 million African-American adults in the U.S., researchers estimated that police killings of unarmed black people could contribute to 55 million excess poor mental health days annually.

Overall, almost 39,000 of the black survey participants were exposed to one or more police killings of an unarmed black person in their state during the study.

The largest effects on mental health occurred in the one to two months after killings, with no significant effect on psychological wellbeing for people surveyed before killings occurred.

Researchers also didn’t find police killings associated with any shifts in mental health among the white people participating in the same surveys.

The study wasn’t a controlled experiment designed to prove whether or how exposure to shootings might directly impact mental health for people with no connection to the victims or their families. Another limitation is that researchers lacked data on whether survey participants were aware of police shootings and on the severity of any specific mental health issues they experienced.

Even so, police violence is widely considered a form of structural racism, and it’s not necessarily surprising that police killing unarmed black Americans is experienced negatively by black Americans and perceived as a form of injustice that is difficult to escape or prevent, said Dr. Rhea Boyd, author of an accompanying editorial and a pediatrician at the Palo Alto Medical Foundation in California.

This type of systemic racism has been linked to so-called toxic stress - wear and tear on the body from chronic exposure to traumatic experiences - which can lead to changes in the brain, immune function and metabolism that contribute to physical and mental health problems.

“While the evidence presented in The Lancet did not identify the pathophysiologic pathway by which police violence causes population mental health impairment for black Americans, evidence that such an impairment is indeed caused by police killing unarmed black Americans opens (the) question of the operative biochemical pathway,” Boyd said by email.

“Because of the relationship between racism and toxic stress, future research should explore how police violence, as a vicarious exposure to racism, may be toxic to the functioning of organ systems and thus the health of black Americans,” Boyd added.

SOURCE: bit.ly/2uOEUtL The Lancet, online June 21, 2018.