There’s clear evidence that racial discrimination negatively affects the health of people of color over the course of their lives. It’s associated with depression, anxiety, and psychological stress; it increases blood pressure; and it has been shown to weaken the immune system. However, few studies have linked single discriminatory events to immediate health effects. Now, data from a first-of-its-kind study suggest a racist attack could raise a person’s stress biomarkers almost immediately.

“The big question mark, for me, has always been, how does this happen? What’s the black box that’s in the middle of discrimination, stress, and health disparities?” says Tiffany Yip, a developmental psychologist at Fordham University who was not involved with the study. “I think that this paper addresses that mechanistic question.”

For the proof-of-concept study, Soohyun Nam at Yale University’s School of Nursing and her team collaborated with Black churches and their communities to recruit 12 Black people between the ages of 30 and 55 living in the northeastern United States.

After accounting for the participants’ baseline stress levels, the research team adapted standardized survey questions about discrimination and microaggressions—such as whether they believed they had been mistaken for a service worker because of their race—and asked participants to share any occurrences of these experiences through a smartphone app. The method, known as ecological momentary assessment (EMA), has previously been used to study physical activity and behavior—such as alcohol intake reduction or smoking frequency. But this is one of the first studies correlating stress biomarkers and racist experiences using this precise monitoring technique.

Researchers also asked the participants to describe their mood five times a day over the course of a week using the same phone app. To measure their biological response, participants spat into a tube four times a day over 4 days and froze the samples until research staff collected them. The researchers then had the samples analyzed in the lab to measure levels of cortisol, a hormone released during emotional distress, and alpha amylase, an enzyme that breaks down sugars and is secreted in stressful situations.

Among the participants, racial discrimination was a clear stressor, even against other stressors, such as having a fight with a spouse or financial distress, the study found. Cortisol levels almost doubled in participants’ saliva the morning after they reported experiencing racial discrimination, such as being called slurs, the team reports today in PLOS ONE. Microaggressions, on the other hand, seemed to increase cortisol levels on the very same day. Even outside any racist incidents, instances in which participants reported bad moods were associated with an increase in alpha amylase during the same day.

This is the first study to measure both of these biomarkers simultaneously and link them to racism in real time, says Nam, who validated the approach in an earlier study. Although these results can’t prove that experiencing racism caused these biomarkers to spike, having high levels of cortisol over time has been linked to hypertension, bone loss, and type 2 diabetes. The study confirms that “racial discrimination has a detrimental effect on physical and mental health,” Nam says. “[There’s] no question that the subtle experience of racial discrimination, like microaggressions, actually does matter.”

Yip agrees: “Irrespective of the magnitude, these stressors have health implications.” Last year, her group showed daily discrimination, such as being treated with less respect than others, seems to affect sleep in adolescents. “We’re beginning to really unpack at the very basic physiological level the impact of racism,” she says.

Elizabeth Brondolo, a psychologist at St John’s University, says the “compelling” new study offers important insights into the timing of discrimination-related impacts of stress.

Nam acknowledges the study’s sample size is small, but says the cost of saliva testing and the time and effort required from participants can make larger studies impractical. Nam hopes future studies will look at how participants cope after a racist exchange for clues to mitigating the negative health effects of those episodes.

Both Yip and Brondolo would like to see whether the results hold up in a larger sample, with participants of different ages and living in other regions of the country.

Nam is now planning a large-scale project to further study how nonbiological factors such as environment and life experiences affect health. Her study will focus on monitoring in real time how racial discrimination, lifestyle behaviors such as diet and sleep, and biomarkers of heart health and inflammation affect a multiracial group. In June, she received a National Institutes of Health grant to use EMAs to study how environmental and lifestyle factors—such as discrimination, social support, and type of neighborhood—influence daily glucose levels in people of different races and ethnicities with type 2 diabetes. Continuous glucose monitoring is already a mainstay of type 1 diabetes care but combining this age-old approach with the novelty of real-time monitoring of nonbiological factors that may also play a role in fluctuating glucose levels is novel, Nam says.

“I’m excited that the field is moving in this direction,” Brondolo says. “This is how you can understand the mechanisms of how life stressors—including discrimination—affect day-to-day functions, both on a psychological and physiological level.”