PAHs are a family of organic compounds built from two or more fused benzene rings. Their high thermal stability, low water solubility, and lipophilic chemistry allow them to persist in the environment and accumulate in food webs. In the atmosphere, they split between the gas phase and airborne particles depending on molecular weight: lighter two- and three-ring compounds such as phenanthrene and anthracene stay largely in the gas phase, four-ring species partition between both, and heavier molecules with five or more rings ride almost exclusively on soot particles. The United States Environmental Protection Agency lists sixteen PAHs as priority pollutants, and the International Agency for Research on Cancer classifies benzo[a]pyrene as a Group 1 human carcinogen, directly linked to lung, skin, and bladder cancers. Chronic exposure is associated with asthma, chronic obstructive pulmonary disease, cardiovascular harm, reproductive toxicity, and neurodevelopmental deficits in children.

Urban PAHs arrive through two fundamental pathways. Petrogenic inputs involve uncombusted petroleum products released by fuel evaporation, storage leaks, solvents, and the wear of asphalt and lubricating oils. Pyrogenic sources, which dominate the urban burden, arise when organic matter burns incompletely or thermally cracks under oxygen-starved, high-temperature conditions, whether in industrial furnaces, residential stoves, or vehicle engines. Vehicle exhaust carries a distinctive fingerprint: gasoline engines, running hot, tend to emit lighter PAHs, while diesel engines, through heterogeneous compression-ignition, release heavier four- to six-ring compounds bound to fine soot. Congested stop-and-go driving and aging fleets without modern catalytic converters or particulate filters amplify these emissions, a pattern especially pronounced in Latin American cities where fleet turnover lags behind that of high-income regions.

For decades, researchers have traced these fingerprints using diagnostic ratios, simple quotients of the concentrations of specific PAH pairs. Ratios such as fluoranthene divided by fluoranthene plus pyrene, or indeno[1,2,3-cd]pyrene divided by itself plus benzo[ghi]perylene, are compared against threshold values drawn from the literature to label a sample as petrogenic, liquid fossil fuel combustion, or biomass burning. The trouble is that these thresholds overlap. Fuel composition, engine technology, air-to-fuel ratios, atmospheric reactivity, and photochemical degradation all blur the boundaries, and different studies assign the same ratio interval to different sources. Interpreted one at a time, diagnostic ratios can yield contradictory verdicts about the very same air sample.

The research team, led by Carla Sofía Stadler of the Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires and colleagues at CONICET institutes in Argentina, set out to close this gap. Their decision-support matrix takes five established diagnostic ratios, harmonizes their classification thresholds from published literature, and converts each ratio into a discrete score of zero, one, or two points for each of three source categories. A ratio earns two points when it points unambiguously to a single source, one point when it falls in an overlapping transitional zone, and zero when it provides no attribution. The scores are then normalized so that every sample receives relative contributions from all three categories rather than a single verdict. Where the literature disagreed, for example on the upper boundary of the indeno-pyrene ratio, the matrix defined a transitional interval in which both combustion sources share the evidence equally, and a sensitivity analysis shifting those boundaries by five and ten percent confirmed the framework’s robustness.

To test the tool, the researchers monitored PM2.5 and PM10 at three sites in Tandil, a rapidly growing city of roughly 145,000 inhabitants in Buenos Aires province, from June 2025 to May 2026. The sites spanned a traffic gradient: a high-traffic location in the commercial center frequented by passenger cars, buses, and delivery trucks; a medium-traffic site dominated by passenger vehicles; and a low-traffic residential neighborhood. Particulate matter was collected on glass fiber filters over 24-hour periods using low-volume samplers, weighed gravimetrically, and analyzed for eighteen priority PAHs by gas chromatography coupled with mass spectrometry, with calibration linearity exceeding 0.994 and detection limits in the sub-nanogram range.

The measurements revealed a clear combustion-dominated atmosphere. High-molecular-weight PAHs made up 98 percent of the total PAH burden in both particle fractions. Principal component analysis found that a single combustion-related component, loaded heavily with benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[a]pyrene, indeno-pyrene, and their heavy-weight relatives, explained more than 76 percent of the variance in PM2.5 and 82 percent in PM10. A second, much weaker component carried the lighter phenanthrene and anthracene, hinting at lower-temperature combustion and petrogenic traces. Total PAH concentrations peaked at the medium-traffic site and were lowest in the residential area, and they rose during the colder months, a seasonal pattern consistent with heating activity.

When the decision matrix was applied, it assigned biomass combustion the highest relative diagnostic score at every site, ranging from 54.3 to 73.9 percent of the attribution. Liquid fossil fuel combustion followed at 20.0 to 38.3 percent, while petrogenic inputs remained minor, below 12.3 percent. Crucially, even the high-traffic city center showed a biomass-dominated signature, suggesting that a widespread urban combustion background, driven by wood stoves and open fireplaces, blankets the entire city and dilutes the contrast between neighborhoods of different traffic intensity. The finding echoes earlier studies across Latin America, from Lima and Medellín to São Paulo and Mexican cities, which likewise identified biomass burning alongside vehicular activity as a leading PAH source.

The matrix and the multivariate statistics told a mutually reinforcing story. Spearman correlations, adjusted for multiple comparisons using the Benjamini-Hochberg procedure, showed that the indeno-pyrene ratio was the only diagnostic indicator consistently aligned with the principal component structure in both particle fractions, with a strong negative correlation of minus 0.81 for PM2.5. Ratios built from lighter PAHs, by contrast, lost their statistical footing, likely because those compounds are more vulnerable to gas-particle partitioning and photochemical aging during atmospheric transport. This divergence among individual ratios is precisely the weakness the matrix was designed to absorb: by pooling evidence, it reduces the uncertainty of any single molecular indicator.

The authors are careful about what the tool is and is not. The matrix yields relative diagnostic evidence, not absolute mass fractions, and its thresholds, drawn from literature values, may need refinement in regions with different fuels or combustion technologies. The 24-hour sampling window also smooths over short traffic peaks. Still, its virtues are practical: it requires only routinely measured PAH concentrations, simple statistics, and no specialized modeling expertise, making it a realistic screening instrument for medium-sized cities that lack the resources to run full receptor models such as Positive Matrix Factorization continuously. The researchers envision the framework eventually feeding into smart-city platforms, digital twins, and AI-driven transport models, giving urban planners a fast, reproducible way to decide which emission sources deserve regulatory attention first. For Tandil, the immediate message is clear: cleaning the air will take more than electrifying the bus fleet, because the smoke rising from the city’s chimneys matters just as much as what pours from its tailpipes.

Subject of Research: Source apportionment of particle-bound polycyclic aromatic hydrocarbons in urban air using a semi-quantitative diagnostic ratio decision matrix

Article Title: A semi-quantitative decision matrix for Polycyclic aromatic hydrocarbons source apportionment to support sustainable urban transport and air quality management

Article References: A semi-quantitative decision matrix for Polycyclic aromatic hydrocarbons source apportionment to support sustainable urban transport and air quality management. (n.d.). https://doi.org/10.3389/ffutr.2026.1964066

Image Credits: AI Generated

DOI: 10.3389/ffutr.2026.1964066

Keywords: polycyclic aromatic hydrocarbons, air quality, source apportionment, diagnostic ratios, biomass combustion, vehicular emissions, PM2.5, PM10, principal component analysis, urban transport, Argentina, decision-support matrix

PM2.5 refers to particles with a diameter of 2.5 micrometers or less, roughly one-thirtieth the width of a human hair. Because of their tiny size, these particles evade the respiratory system’s natural defenses, penetrating deep into the alveoli of the lungs and even crossing into the bloodstream. Once lodged in lung tissue, they trigger chronic inflammation, generate reactive oxygen species that damage DNA, and interfere with cellular repair mechanisms, a cascade of biological insults that epidemiological studies worldwide have linked to elevated rates of lung cancer, cardiovascular disease, and premature death. The International Agency for Research on Cancer classified outdoor air pollution as a Group 1 human carcinogen in 2013, and subsequent cohort studies across Europe, North America, and Asia have consistently confirmed that long-term exposure to even modest PM2.5 concentrations raises lung cancer risk in a roughly linear fashion, with no clearly identified safe threshold.

To translate this global evidence into local numbers, the Moroccan team applied the AirQ+ software tool developed by the WHO Regional Office for Europe. The model works by combining three ingredients: measured or estimated concentrations of a pollutant, the size of the exposed population, and a concentration-response function derived from large epidemiological cohorts. From these inputs it computes the relative risk, the attributable proportion, and the absolute number of health outcomes that can be ascribed to the pollutant, complete with 95 percent confidence intervals that reflect the statistical uncertainty of the underlying risk functions. The researchers fed the model with PM2.5 concentration data for 2019 covering 46 urban and rural districts spanning Morocco’s remarkably diverse geography, from humid Atlantic coastal plains to arid pre-Saharan valleys, and paired each concentration estimate with district-level population figures and lung cancer baseline incidence.

The results exposed a fourfold spread in exposure across the country. At the clean end of the spectrum sat El Jadida, a coastal city southwest of Casablanca, where PM2.5 averaged just 7.1 micrograms per cubic meter, well within reach of the WHO guideline of 5 micrograms per cubic meter for annual exposure. There, the calculated relative risk of lung cancer was a modest 1.02, with an attributable proportion of 2.35 percent. At the opposite extreme lay Errachidia, a southern oasis city on the edge of the Sahara, where concentrations reached 28.4 micrograms per cubic meter. The relative risk there climbed to 1.30, and the model attributed a striking 23.29 percent of lung cancer cases to particulate pollution, meaning nearly one in four diagnoses in that district could be linked to the air. Between these poles, the team identified other hotspots, including Ouarzazate at 20.2 micrograms per cubic meter with an attributable proportion of 15.82 percent, and Taourirt at 19.0 micrograms per cubic meter with 14.67 percent.

Perhaps the most counterintuitive finding concerns Morocco’s largest metropolises. Casablanca, the country’s economic engine and home to millions, recorded only a moderate relative risk of 1.06, with a 95 percent confidence interval of 1.04 to 1.08. Yet because of the sheer size of its population, the city accounted for a substantial number of attributable lung cancer cases, with the model estimating 24 cases linked to PM2.5 exposure. This distinction between relative risk and absolute burden is a cornerstone of environmental health statistics: a small percentage increase applied to a very large population can produce more disease than a large percentage increase applied to a small one. For policymakers, it means that intervention priorities cannot be set by concentration levels alone; both the toxicity of the local airshed and the number of people breathing it must enter the calculus.

Equally revealing is the divergence in pollution sources the study identified between urban and rural Morocco. In major urban centers, the dominant contributors are vehicular traffic and industrial emissions, the familiar cocktail of diesel exhaust, brake and tire wear, construction dust, and factory plumes that characterizes rapidly growing cities across the developing world. In contrast, several of the highest-exposure districts were rural or small-town areas in the interior and south, where the signature points instead to residential biomass combustion, the burning of wood, charcoal, and other solid fuels for heating and cooking, compounded in some regions by dust from arid landscapes and unpaved roads. This urban-rural split matters because it demands different remedies: traffic restrictions, fuel standards, and industrial emission controls in cities, versus cleaner household energy, improved stoves, and land management in the countryside. A single national air quality policy, the findings suggest, would miss the mark in both settings.

The broader scientific context reinforces the urgency of these numbers. Meta-analyses pooling cohort studies from multiple continents have estimated that each 10 microgram per cubic meter increment in long-term PM2.5 exposure raises lung cancer risk by roughly 8 to 10 percent, and recent large-scale projects such as the European ELAPSE consortium have shown that these risks persist and may even steepen at concentrations well below current European Union limits. Mechanistic work published in recent years has fleshed out the biological plausibility, documenting how fine particles promote tumorigenesis through oxidative stress, epigenetic alterations, and chronic activation of inflammatory pathways in lung epithelial cells. In the Middle East and North Africa region, where lung cancer remains a leading cause of cancer death and tobacco smoking compounds the pollution burden, locally grounded risk estimates have been conspicuously scarce, making the Moroccan assessment a template for neighboring countries facing similar data gaps.

The study’s authors frame their results as direct input for Morocco’s National Cancer Prevention Plan, and the policy implications are concrete. The southern regions and specific rural districts flagged as priority zones, including Errachidia, Ouarzazate, and Taourirt, would benefit first from strengthened air quality monitoring networks, which remain sparse outside the largest cities, and from source-targeted mitigation strategies addressing the dominant local emitters. Aligning national ambient air standards with WHO guidelines, phasing out the most polluting fuels, and expanding the monitoring infrastructure to capture the spatial variability the study uncovered are among the recommended steps. The researchers also acknowledge the limitations inherent in their approach: the analysis relied on district-average concentrations for a single year rather than individual exposure measurements, and it could not disentangle the independent contribution of PM2.5 from that of tobacco smoking, which remains the leading cause of lung cancer globally. Future work incorporating personal exposure monitoring, satellite-derived concentration surfaces, and multi-year averaging would sharpen the estimates considerably.

What makes the findings resonate beyond Morocco’s borders is their demonstration that air pollution is not a uniform hazard and that the communities suffering the highest proportional cancer burden are often not the ones that attract the most attention. The dusty southern towns and rural interior districts that topped the risk ranking receive far less regulatory scrutiny than the smoggy megacities of the coast, yet their residents face attributable fractions approaching 23 percent. As climate change intensifies aridity and dust generation across the Maghreb, and as urbanization accelerates vehicle fleets and energy demand, the exposure gradient documented in this study is likely to shift. The Moroccan analysis offers both a warning and a method: a warning that fine particle pollution is already carving measurable cancer burdens into communities far from any smokestack skyline, and a method, transparent, replicable, and grounded in WHO tools, by which any nation can hold its own air to account.

Subject of Research: Spatial assessment of lung cancer burden attributable to long-term PM2.5 exposure across Moroccan districts using the WHO AirQ+ model

Article Title: Spatial analysis of the burden of lung cancer attributable to long-term PM2.5 exposure in Morocco: a national assessment using the WHO AirQ+ model

Article References: Bouchriti, Y., Ait Haddou, M., Achbani, A., Rida, J., Sine, H., Lkoul, A., Gougueni, H., Acim, R., Amiha, R., & Kabbachi, B. (2026). Spatial analysis of the burden of lung cancer attributable to long-term PM2.5 exposure in Morocco: a national assessment using the WHO AirQ+ model. Environmental Monitoring and Assessment, 198(10), Article 1107. https://doi.org/10.1007/s10661-026-15942-6

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15942-6

Keywords: PM2.5, lung cancer, air pollution, Morocco, AirQ+ model, attributable risk, environmental health, spatial analysis, WHO guidelines, biomass combustion, cancer epidemiology, public health