Fungi and toxic metals in slum household dust pose health risks

In the crowded, poorly ventilated rooms of tropical slum settlements, the dust settling on floors, walls, and household surfaces is far more than a nuisance. According to a new study published in the journal Air Quality, Atmosphere & Health, this dust acts as a shared reservoir for two distinct classes of hazards, toxic heavy metals and a diverse community of fungi, that have largely been assessed in isolation from one another. By bringing chemical and microbiological analysis together within a single health risk assessment framework, researchers from the CSIR-National Environmental Engineering Research Institute in Kolkata have produced one of the most complete pictures to date of what residents of informal settlements are actually exposed to inside their own homes, and the results point to a hidden burden that falls disproportionately on children.

The research team, led by Rachna Jain together with Dipanjali Majumdar and Subhendu Chakraborty, collected house dust samples from multiple sites within non-compartmentalized, poorly ventilated dwellings, the kind of single-room living spaces where cooking, sleeping, and daily life all occur in the same air volume. The sampling strategy reflected the reality of slum housing, where there are no separate kitchens or bedrooms and where indoor conditions are governed almost entirely by outdoor climate, cooking fuel use, and household hygiene practices. Dust, in such environments, is not simply tracked in from outside; it accumulates continuously from a combination of road-side sources, indoor combustion, building materials, and biological growth encouraged by persistent humidity.

To characterize the fungal component of this dust, the team employed the spread plate method, a classical microbiological technique in which diluted dust suspensions are distributed across the surface of nutrient agar so that individual fungal colonies can be counted and isolated. Each isolate was then examined microscopically following lactophenol-cotton blue staining, a standard preparation that renders fungal structures such as conidiophores, phialides, and spore chains visible under the light microscope. Where morphology alone was inconclusive, the researchers used slide culture techniques, growing fungi directly on a thin agar block sandwiched between a slide and coverslip so that the delicate architecture of spore-bearing structures could be observed intact.

Five dominant fungal taxa emerged from this survey and were taken forward for molecular characterization, in which portions of the fungal genome were sequenced and the resulting sequences deposited in the NCBI GenBank database. The identified species included Aspergillus sydowii, Trichoderma longibrachiatum, Cladosporium sp., Neurospora sp., and Aspergillus nomiae. This roster is significant for several reasons. Aspergillus species are among the most clinically important indoor molds worldwide, capable of triggering allergic responses, asthma exacerbation, and, in immunocompromised individuals, invasive infection. Trichoderma longibrachiatum has been associated with opportunistic infections and is a frequent colonizer of damp building materials, while Cladosporium is one of the most ubiquitous allergenic genera in indoor environments globally. The presence of Neurospora, a rapidly growing mold often associated with burned or heat-treated organic material, hints at the influence of indoor cooking practices on fungal community composition.

Statistical analysis of the fungal data revealed a striking spatial uniformity. There was no significant difference in total fungal load between the sampled sites, suggesting that within these settlements the indoor environment functions as a relatively homogeneous exposure landscape. However, fungal diversity correlated positively with environmental variables including temperature, humidity, ventilation, indoor cooking, and hygiene practices. The diversity indices told a consistent story: both the Simpson index and Shannon Equitability exceeded 0.9, and the Shannon Index reached approximately 1.7, indicating an evenly distributed fungal community rather than one dominated by a single opportunistic species. This evenness matters from a health perspective, because residents are not merely exposed to one type of mold in high quantities but to a broad portfolio of biologically active species, each with its own allergenic, toxigenic, or infectious potential.

Alongside the biological characterization, the team quantified toxic metals in the same dust samples and translated measured concentrations into human exposure estimates using established risk assessment methodology. The exposure pathway analysis produced a clear hierarchy: ingestion of dust particles, driven by hand-to-mouth contact and the consumption of contaminated food and water handled in dusty environments, dominates over dermal absorption, which in turn exceeds inhalation of resuspended particles. This ordering is consistent with the standard EPA-style exposure model for house dust, but it takes on particular weight in slum households, where floor-level living, limited handwashing facilities, and the constant presence of young children on dust-contaminated surfaces amplify ingestion rates dramatically compared with formal housing.

The health risk calculations that followed delivered the study’s most alarming findings. The non-cancer hazard index, which aggregates the contributions of individual toxic elements across exposure routes, remained below the threshold value of 1 for adults, indicating that hazardous effects from chronic non-cancer endpoints are not expected for the adult population. For children, however, the hazard index exceeded 1, signaling a significant non-cancer health hazard even in circumstances where adults might remain below the danger line. The reasons for this discrepancy are physiological as much as environmental: children breathe more air, ingest more dust, and absorb more contaminant per unit of body weight than adults, and their developing organ systems are more vulnerable to the neurotoxic and nephrotoxic effects of metals such as lead, cadmium, and chromium.

Cancer risk estimates painted an even starker picture. For three toxic elements, cadmium, chromium, and nickel, the calculated lifetime cancer risk from household dust exposure was estimated to be significantly higher than the widely accepted regulatory limit of one in a million. Cadmium is a known human carcinogen associated with renal dysfunction and lung cancer, hexavalent chromium is one of the most potent occupational carcinogens recognized in toxicology, and nickel compounds are similarly classified as carcinogenic. That these three metals co-occur in household dust at levels capable of producing cumulative cancer risks above the acceptable threshold underscores the degree to which slum homes act as sinks for urban industrial and traffic-related pollution. Metals deposited on streets and roofs by vehicular emissions and industrial activity are resuspended, tracked indoors on footwear and clothing, and then concentrated in the settled dust that residents contact continuously.

The integration of fungal and chemical data within a single assessment framework is what distinguishes this work from much of the existing literature. Previous studies have typically addressed indoor mycobiota and indoor metal contamination as separate research questions, using different sampling campaigns, different analytical pipelines, and different risk models. Yet residents experience these hazards simultaneously, and there are plausible interactions between them: fungal spores and hyphal fragments can bind metal ions on their surfaces, metals can alter fungal community composition by exerting selective pressure, and the damp conditions that favor fungal proliferation may also influence metal speciation and bioavailability. By modeling both hazard classes together, the researchers have moved toward a cumulative exposure paradigm that more faithfully represents the real conditions under which slum dwellers live.

The authors argue that their findings carry direct implications for urban health policy. Indoor dust fungi, they suggest, should be recognized as overlooked emerging contaminants alongside the toxic elements that co-inhabit the same dust matrix, and both warrant regulatory attention and targeted interventions. Such interventions could include improved ventilation design in low-cost housing, promotion of hygienic cooking practices, dampness control, regular safe cleaning of floors and surfaces, and reduction of outdoor metal sources at the neighborhood level. Given that roughly a billion people worldwide live in informal settlements, and that climate change is expected to increase humidity and heat stress across the tropics, the conditions documented in this study are likely to become more, not less, common in the decades ahead.

The study also contributes to a growing scientific conversation about the indoor microbiome and its health consequences. Earlier research has shown that fungal diversity in house dust can influence childhood asthma development, with low diversity in some contexts associated with higher asthma risk, and that indoor fungal communities are strongly shaped by outdoor air and by dispersal limitation at short distances. The new findings extend this literature into an understudied setting, the tropical slum household, where the combination of near-permanent warmth, high humidity, dense occupation, and indoor biomass or solid fuel cooking creates an environment fundamentally different from the temperate, mechanically ventilated homes in which most indoor air research has historically been conducted.

Ultimately, the message of this research is that the air and surfaces of the world’s poorest homes harbor a compound burden of chemical and biological hazards that current regulatory frameworks barely capture. A hazard index above 1 for children and cancer risks exceeding one in a million from cadmium, chromium, and nickel in household dust are not abstract statistical outcomes; they represent quantifiable contributions to the excess burden of respiratory disease, developmental harm, and cancer borne by informal settlement residents. Making household dust a monitored, managed, and mitigated exposure medium, the study concludes, is a necessary step toward safeguarding the vulnerable populations who, quite literally, live on top of the problem.