In May 2023, Popocatepetl threw enough ash into the sky that Mexican authorities raised the Volcanic Alert Traffic Light to Yellow Phase 3, a warning for ashfall, minor pyroclastic flows, and the possibility of moderate to strong explosions. The volcano stands 5,452 meters tall and sits within 120 km of some 27 million people. Roughly 15 km from the crater, in four communities in the Puebla municipalities of San Nicolas de los Ranchos and Tochimilco, health workers kept seeing patients. A team led by Sagrario Lobato of the Puebla state health research department went looking for the surge in ash-related illness that the eruption seemed certain to produce.
They did not find one.
The researchers pulled every outpatient visit at state health service clinics in those four communities from May 1 to May 31, 2023: 1,213 patients in all. Nearly seven in ten were women, and the largest age group was 40 to 64. Then they sorted the top twenty reasons for those visits into two piles. One pile held conditions a prior review of the published literature had linked to volcanic ash: noninfectious respiratory problems, gastrointestinal complaints, conjunctivitis, noninfectious dermatitis, and mental health conditions like stress and anxiety. The other pile held everything else, which served as a comparison profile.
Fourteen percent of patients landed in the ash-linked pile, most of them with noninfectious respiratory trouble. Exactly one person came in with anxiety. The remaining 86 percent were there for the ordinary business of a rural clinic: managing type 2 diabetes, controlling high blood pressure, respiratory infections, well-child checkups, and prenatal care.
Measuring an exposure nobody was measuring
The harder problem was working out what people were actually breathing. Neither municipality has an air quality monitoring station. So the team built an estimate instead, combining readings from Puebla's state monitoring network with atmospheric dispersion equations, a Gaussian plume model, wind and eruption data from NOAA's HYSPLIT platform and Mexico's national disaster prevention center, and elevation points pulled from Google Earth. They used Kriging, a geostatistical method that fills in values between measured points based on how spatially similar nearby readings tend to be, to draw a continuous map of where the ash went.
The modelled peak concentrations fell off sharply with distance: 818 parts per million at the crater itself, 660 ppm over San Nicolas de los Ranchos, and 540 ppm over Tochimilco.
Then they compared daily particle levels against daily patient counts using Spearman's correlation, a test for whether two things rise and fall together. The correlations between ash exposure and the ash-linked conditions were positive but weak to moderate, and none reached statistical significance. Study group versus PM2.5 came in at rho = 0.21 (P = 0.24); versus PM10, rho = 0.24 (P = 0.18). The one significant result was odd: PM2.5 tracked the control profile, the conditions supposedly unrelated to ash. The authors flag that themselves as likely spurious, a coincidence of two lines drifting in similar directions on a chart.
What the authors think might explain it
Lobato and colleagues offer several possibilities, all of them tentative. Their clinical data included no baseline chest X-rays and no follow-up over time, which means early pulmonary fibrosis, often silent until it is advanced, could be accumulating invisibly. They also suggest adaptation. Communities beside Popocatepetl have lived with persistent ash for generations and have built household and community practices around it, and the volcano is woven into Mesoamerican cultural identity.
The authors float a biological version of that idea too: hormesis, in which a moderate stressor triggers responses that make an organism more tolerant of the next dose. They are careful about it. Hormesis was not measured in this study, they write, and remains speculative. They propose it as a direction for future work rather than a conclusion.
One caveat runs through everything. This is an ecological study, meaning it compares group-level averages rather than tracking individuals. Ambient particle readings stand in for what each person breathed, and no negative control group was possible, since the volcano never stops. The authors say plainly that the associations should be read with caution.
Why it matters
An estimated one billion people, about 14 percent of humanity in 2015, live within 100 km of an active volcano, and the epidemiological evidence on what that does to them is thin. Most of those communities, like these four, have no air quality monitor anywhere nearby.
That is arguably the most portable thing here. The team assembled a workable picture of ash dispersion from public wind data, free satellite terrain, and dispersion math, then tested it against real clinic records. They go further and suggest a low-cost improvement: recruit residents to collect ash in PET bottles fitted with funnels over the course of a week, weigh the deposits on a microbalance, and check the numbers against international volcanic ash advisories.
A null result is not a clean bill of health. The authors are explicit that their findings say nothing about the volcano's more violent hazards, the pyroclastic flows and ballistic projectiles that come with magma movement, and that preparedness for those needs to keep improving. Their recommendation is to fold baseline chest X-rays and ongoing patient monitoring into early warning systems, so that the slow damage a single month of clinic visits cannot see would not stay invisible.