Stress doesn't just live inside your brain.
The invisible chemical signature of stress and worry can be found all over your body – in your blood, in your saliva, and even in the hair upon your head.
But just how far do these chemical markers of stress extend? Reading a saliva sample is one thing, but if we zoom out, is it possible to detect biological stress at the population level?
In a new study, scientists showed that you can do this, and it's all thanks to human sewage.
"Wastewater surveillance offers a complementary, non-invasive approach to population health surveillance by quantifying aggregated biological and chemical markers excreted by communities," a team of researchers from the University of Queensland in Australia explains in Nature Water.
Wastewater analysis can be used to track all sorts of things, revealing secrets about how viruses spread, how superbugs spawn, and how human products end up contaminating the marine environment.
An emerging area in this field of research is using wastewater analysis to measure biomarkers of stress, which is what scientists investigated here.
In the study, the University of Queensland team analyzed wastewater collected from an Australian university campus (serving 30,000 people), comparing sewage collected during the teaching semester with samples from an examination period, and also examining wastewater from a nearby general community.
Results showed that during exam weeks, levels of two oxidative stress biomarkers – 8-OHG and 8-OHdG – rose higher on the university campus, compared to the non-examination period, although only the increase in 8-OHG was statistically significant when results were compared with levels seen in the general community.
"A clear and temporary spike in oxidative stress levels was observed during the examination period [aligning] with the expected physiological and psychological burden of academic assessments," the researchers explain.
"Together, these findings suggest that examination periods represent a robust, population-level stressor that can be objectively detected through wastewater-derived biomarkers."
But it wasn't just stress biomarkers that fluctuated.
The exam period was also linked to higher levels of antidepressant chemicals in the wastewater, suggesting greater amounts of amitriptyline, citalopram, fluoxetine, and venlafaxine may have been consumed on campus over that time – but only the increase in amitriptyline was found to be statistically significant.
Despite the statistical uncertainty, the researchers think the relative increases seen on campus hint at heightened antidepressant use during the exam window.
"The consistent direction of change across multiple antidepressants and metabolites suggests a possible examination-period increase in antidepressant consumption within the university campus, although the evidence remains suggestive rather than conclusive," the team writes in their paper.
"This pattern is broadly consistent with the well-established association between academic stress and worsening depressive and anxiety symptoms among university students."
Other substances, including ADHD medications, showed no significant exam-related increases, while markers associated with caffeine consumption and artificial sweeteners actually appeared to decline relative to the general community, which might seem a bit surprising during a busy exam period.
Alcohol consumption, less surprisingly, also appeared to be temporarily suppressed during exam time, with wastewater measurements suggesting a rebound after the exams concluded.
The researchers acknowledge several limitations with their study, including that the toilets on campus aren't just used by students, with university staff, contractors, and visitors all contributing to the "mass load of target biomarkers".
But they say sewage analysis is an important tool for detecting changes in stress-related biomarkers and substance use among students, giving us an objective tool that can help us monitor fluctuations over short timeframes.
"These findings provide a proof of concept for using wastewater surveillance to monitor dynamic stress-related responses in university populations," they write, "[establishing] a foundation for future work to determine whether the magnitude, duration, or recurrence of wastewater-derived biomarker changes has clinically or pathologically meaningful implications, beyond traditional measurement tools."
The findings are reported in Nature Water.
This article was fact-checked by Rebecca Dyer and edited by Rebecca Dyer. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.