The study, led by Bruno Cesar Silva Rocha of Fluminense Federal University together with colleagues from several Brazilian institutions, drew on monitoring data from the Rio de Janeiro State Environment Institute and hydrological records from Brazil’s National Water and Basic Sanitation Agency. Rather than relying on a single station or a short snapshot, the researchers assembled a multi-station, multi-year dataset and subjected it to a battery of statistical tests designed to separate genuine long-term signals from year-to-year noise. The result is a detailed portrait of how particle concentrations behave across seasons, locations, and meteorological regimes in a city whose air quality has been studied intermittently since the early 2000s.

The statistical toolkit was deliberately robust. To detect long-term monotonic trends, the team applied the Mann–Kendall test, a non-parametric method widely used in environmental science because it makes no assumptions about the underlying distribution of the data. Where trends were estimated, they used the Theil–Sen estimator, which calculates the median of all pairwise slopes and is far less sensitive to outliers than ordinary least-squares regression. Seasonal and inter-period comparisons relied on Mann–Whitney tests, spatial differences among monitoring stations were evaluated with Kruskal–Wallis tests, and relationships with meteorological variables were probed using Spearman rank correlations. Together, these methods allowed the researchers to interrogate the dataset without forcing it into assumptions it could not support.

The headline numbers are stark. Annual PM10 concentrations exceeded the World Health Organization’s annual guideline of 15 micrograms per cubic meter at every monitoring station and in every year analyzed. For the finer PM2.5 fraction, which penetrates deep into the lungs and enters the bloodstream, concentrations at the Volta Grande station surpassed the WHO guideline of 5 micrograms per cubic meter in all years of the record. In 2024 alone, 36 percent of PM10 measurements exceeded the Brazilian national standard of 35 micrograms per cubic meter, and annual means reached 44 plus or minus 22 micrograms per cubic meter at the Conforto station and 44 plus or minus 12 micrograms per cubic meter at Siderville. For a pollutant class with no established safe threshold, these figures represent a sustained public health exposure gap.

Perhaps the most consequential finding, however, is what the trend analysis did not show. Despite fifteen years of data, neither PM10 nor PM2.5 exhibited a statistically significant long-term monotonic trend. For PM2.5, the Theil–Sen slope was a slight negative 0.346 micrograms per cubic meter per year, but with a p-value of 0.0866 the result fell just short of conventional significance. In practical terms, the air in Volta Redonda has not measurably improved over a decade and a half. In a city where earlier studies dating back to 2004 had already flagged metallurgical activity as a dominant influence on air quality, the absence of a downward trajectory suggests that emissions controls, industrial restructuring, and meteorological variability have so far failed to bend the curve.

What the record does show clearly is seasonality. Concentrations of both pollutants were significantly higher in winter than in summer, with p-values of 0.002 for PM10 and 0.001 for PM2.5. This pattern is characteristic of southeastern Brazil, where the dry, cooler months bring stable atmospheric conditions, temperature inversions that trap pollutants near the surface, and reduced wet deposition of particles. The meteorological correlations reinforce this interpretation: PM10 showed a strong negative correlation with precipitation, with a Spearman coefficient of minus 0.66, and a negative correlation with temperature of minus 0.46. Rain scrubs particles from the air, and warmer, more turbulent conditions help disperse them, so it follows that the cleanest air arrives with the summer storms and the dirtiest with the winter stillness.

To trace where the pollution comes from, the researchers turned to two complementary visualization and modeling techniques. Pollution roses, which plot pollutant concentrations against wind direction and speed, helped identify the conditions under which elevated readings occur. More ambitiously, the team performed concentration-weighted trajectory analysis, a method that combines back-trajectory modeling of air parcels with measured concentrations to map the geographic regions most likely to contribute to pollution at the receptor sites. The CWT results pointed to potential contribution regions predominantly to the west and northwest of the monitoring stations, implicating transport pathways that funnel industrial and urban emissions toward the city rather than away from it.

The spatial dimension of the analysis also matters. Kruskal–Wallis tests revealed significant differences among monitoring stations, confirming that pollution exposure in Volta Redonda is not uniform but shaped by proximity to industrial sources, local topography, and the valley geography of the Paraíba do Sul river basin. Residents living near the steel complex or downwind of prevailing transport routes experience markedly different air than those in other neighborhoods, a reality that has implications for how health risks are assessed and where interventions should be targeted. Earlier epidemiological work in the city has linked air pollution to cardiovascular hospitalizations and low birth weight, and studies have documented elevated nickel levels in the air and urine of residents near steel industry facilities and waste deposits.

The health stakes of these findings are difficult to overstate. Fine particulate matter is classified as a carcinogen by the International Agency for Research on Cancer, and a large body of evidence links chronic PM2.5 exposure to cardiovascular disease, respiratory illness, and premature mortality. Recent global analyses covering hundreds of cities have shown that heat and air pollution act jointly on mortality risk, a concern that grows as climate change intensifies both winter stagnation events and summer heat waves. For Volta Redonda, the combination of chronically elevated particle levels and a flat long-term trend means that the health burden documented in earlier studies is likely ongoing, borne disproportionately by those living closest to industrial sources.

The study also arrives at a moment of regulatory transition in Brazil. In 2024, the National Environment Council issued Resolution CONAMA 506, establishing new national air quality standards and guidelines for their application, updating a framework that had remained largely unchanged since resolutions from 1990 and 2018. Against this evolving regulatory backdrop, the Volta Redonda record provides exactly the kind of baseline evidence that policymakers need: it demonstrates that even as standards tighten, actual concentrations in industrial cities remain far above both national limits and WHO guidelines. The researchers have made their processed datasets, summary tables, and R scripts available as supplementary material to support reproducibility, and the underlying raw data remain publicly accessible through state and federal agencies. Whether the next fifteen years show a different story will depend on whether the statistical flatness documented here is finally replaced by a slope that matters.

Subject of Research: Long-term trends and seasonal variability of PM10 and PM2.5 air pollution in an industrial Brazilian city

Article Title: Long‑term trends and seasonal variability of particulate matter (PM10 and PM2.5) in Volta Redonda (RJ), Brazil: a 15‑year temporal analysis

Article References: Rocha, B. C. S., Cardoso, J. P., de Medeiros Soares, J., da Silva, L. J., de Mello Nascimento, N., Gioda, A., & Bernardes, M. C. (2026). Long‑term trends and seasonal variability of particulate matter (PM10 and PM2.5) in Volta Redonda (RJ), Brazil: a 15‑year temporal analysis. Environmental Monitoring and Assessment, 198(11), Article 1170. https://doi.org/10.1007/s10661-026-15988-6

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15988-6

Keywords: air quality, particulate matter, PM2.5, PM10, Volta Redonda, Brazil, industrial pollution, Mann–Kendall test, Theil–Sen estimator, seasonal variability, concentration-weighted trajectory, environmental monitoring