Moss biomonitoring rests on a simple but powerful biological principle. Bryophytes such as Hypnum cupressiforme lack a protective waxy cuticle and true roots, so they draw their nutrients and water directly from the atmosphere rather than from soil. Their feather-like, densely branched shoots act as passive filters, intercepting airborne particles and dissolved contaminants and retaining them on and within their tissues. Because the plants accumulate pollutants over time without shedding their leaves in the way vascular plants do, the concentration of metals measured in a moss sample reflects integrated atmospheric deposition over the preceding growing seasons rather than a single moment in time. This makes mosses ideal for mapping pollution across wide territories where networks of high-volume air samplers, which are expensive to install and maintain, simply do not exist.

The Bosnian survey collected samples of Hypnum cupressiforme from 44 sites spanning both urban and rural environments across the country. At each location, the researchers harvested moss tissue following standardized biomonitoring protocols, carefully handling the samples to minimize contamination while preserving the signal of genuine atmospheric deposition. The samples were then analyzed for eight elements of environmental concern: cadmium, chromium, copper, iron, manganese, nickel, lead and zinc. Concentrations were quantified using flame atomic absorption spectrometry, a well-established analytical technique in which the sample is ashed and dissolved, then aspirated into a flame where free atoms of each metal absorb light of element-specific wavelengths, allowing precise quantification. To complement this bulk chemical measurement, the team also employed scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, a technique that images individual particles adhering to the moss surface and identifies their elemental composition, revealing not just how much metal was present but in what physical and chemical form.

The statistical treatment of the data was deliberately rigorous. Because environmental concentration data rarely follow a normal distribution, the researchers applied non-parametric tests, supplemented by Spearman rank correlation to detect associations between pairs of elements, principal component analysis to reduce the eight variables into interpretable source-related factors, and hierarchical cluster analysis to group sampling sites and elements according to similarity of their elemental fingerprints. This combination of methods is now a standard toolbox in geochemical source apportionment, allowing investigators to distinguish, for example, elements that co-occur because they share a common industrial source from those that merely coincide by chance.

The results paint a nuanced picture of pollution in Bosnia and Herzegovina. Iron and nickel concentrations in the moss exceeded the median values reported for comparable European moss surveys in both urban and rural settings, suggesting that the country’s geology and its industrial heritage continue to leave a measurable mark on atmospheric deposition. In contrast, cadmium and lead remained at or below European benchmark levels, a finding consistent with the broader decline in lead pollution across Europe following decades of leaded-petrol phase-out and industrial restructuring. Perhaps the most striking result was what the researchers did not find: no statistically significant differences between urban and rural sampling sites for the elements measured, with p-values exceeding 0.05. This absence of an urban-rural gradient points toward diffuse, regionally distributed atmospheric deposition, influenced in part by long-range transport of pollutants across borders, rather than a pattern dominated by isolated city-level emission hotspots.

Principal component analysis and hierarchical cluster analysis resolved the eight elements into three coherent source components. The first group combined copper, zinc, lead and iron, interpreted as a crustal or geogenic association overlaid with localized anthropogenic enrichment, a signature typical of areas where natural soil-derived particles mix with emissions from combustion, wear of vehicle components and small-scale industrial activity. The second group, comprising chromium and nickel, was classified as geogenic-industrial, reflecting both the weathering of ultrabasic rock formations common in the Dinaric region and contributions from metallurgical processing. The third grouping, containing cadmium and manganese, behaved differently: enrichment factor analysis suggested that these two elements did not share a common source, with each following its own divergent pathway of emission and deposition. Manganese, for instance, is influenced by crustal dust as well as specific industrial uses, while cadmium is typically associated with combustion processes and non-ferrous metallurgy.

The scanning electron microscopy work added a physical dimension to the chemical story. Examination of the moss surfaces revealed abundant aluminosilicate and mineral particles consistent with crustal origin, likely derived from resuspended soil and road dust lifted into the air by wind and traffic. Alongside these, the microscopy identified metal-bearing particles potentially associated with traffic emissions, particularly at urban sites, where brake and tire wear, exhaust residues and road dust produce distinctive morphologies and elemental signatures. This dual analytical approach, combining bulk concentration data with particle-level characterization, strengthens the interpretation of the statistical results by providing direct visual evidence of what is actually landing on the biomonitor.

The study situates Bosnia and Herzegovina within a wider European tradition of moss surveys. Since the pioneering work of Rühling and Tyler in Scandinavia in the late 1960s, moss biomonitoring has grown into a coordinated international program under the UNECE Convention on Long-Range Transboundary Air Pollution, with successive surveys mapping heavy metal and nitrogen deposition across dozens of countries. Neighboring Serbia, Albania, North Macedonia and Croatia have all maintained active moss monitoring programs, and previous work in Bosnia and Herzegovina itself, including studies using the epiphytic lichen Hypogymnia physodes and earlier moss surveys in collaboration with international partners, had laid groundwork. The new research, however, provides a comprehensive national in situ assessment that explicitly contrasts urban and rural deposition patterns, filling a gap in the regional dataset.

The confirmation that Hypnum cupressiforme performs well as a biomonitor in the specific climatic and geological conditions of the Western Balkans carries practical significance for policy. Air pollution remains a major public health burden in Europe, and the European Environment Agency’s assessments consistently rank airborne particulate matter among the leading environmental health risks in the region. Countries with limited instrumental monitoring capacity, which includes much of the Balkans, face difficulties in complying with reporting obligations and in designing targeted abatement strategies. Moss surveys offer a cost-effective complement: a single sampling campaign can cover a national territory at high spatial density, producing maps of deposition that reveal gradients and hotspots invisible to sparse fixed stations. The technique’s limitations, including sensitivity to local factors such as canopy throughfall, moss vitality and the influence of soil splash, are well documented, and the authors’ adherence to established sampling and cleaning protocols addresses these concerns.

Looking forward, the findings support the continued integration of Bosnia and Herzegovina into national and European biomonitoring programs, enabling temporal trend analysis in future survey rounds. Repeated surveys at five-year intervals, as practiced in several neighboring countries, would allow the country to track whether iron and nickel deposition declines alongside industrial modernization, and whether diffuse regional pollution responds to continental-scale emission controls. The study also underscores the value of pairing FAAS quantification with SEM-EDS particle analysis, a methodological lesson that other biomonitoring teams in data-poor regions may wish to emulate. In an era when air quality data increasingly shape public health policy, the image of an unassuming moss quietly archiving the metals drifting through Bosnian skies is both scientifically sound and quietly compelling, a reminder that some of the most effective environmental sentinels cost nothing at all and have been growing at our feet all along.

The researchers emphasize that their results confirm the suitability of Hypnum cupressiforme as a robust biomonitor for atmospheric metal deposition in Southeastern Europe and provide an evidence base for its continued application. For a country navigating the intersection of post-industrial legacy, urban growth and European environmental integration, the humble moss may prove to be one of its most valuable scientific instruments.