Composting may be a potential health risk among individuals predisposed to pulmonary infections caused by Asperigillus fumigatus (pulmonary aspergillosis), as well as immunocompetent individuals with increased exposure to A fumigatus fungal spores. These findings were published in Applied and Environmental Microbiology.

A fumigatus is a fungus which plays an important environmental role as a decomposer. Its small size (2-3 mcL) makes it easily aerosolized and transported over air currents, and its fungal spores have been associated with an increased risk for severe asthma with fungal sensitization, allergic bronchopulmonary aspergillosis, and chronic pulmonary aspergillosis.

A citizen science project was conducted in the United Kingdom in 2019 to determine whether gardening activities are associated with an increased risk for exposure to A fumigatus spores resistant to triazole antifungals. A total of 246 individuals collected 509 soil samples from their gardens, from which 5174 A fumigatus isolates were cultured and assessed for tebuconazole resistance. The researchers noted that tebuconazole is “the third most-sprayed triazole fungicide in the UK, which confers cross-resistance to the medical triazoles used to treat A fumigatus lung infections in humans.”

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Of the 509 soil samples cultured, 64% grew a total of 5174 A fumigatus isolates and 20% grew a total of 736 tebuconazole-resistant A fumigatus isolates. The researchers noted that most soil samples (89%) were collected from a single location in a residential garden.

The mean concentration of spores and mycelial fragments across all soil  samples from which cultures grew A fumigatus was 316 colony forming units (CFU)/g. Samples collected from a manure bag were found to have an increased concentration of spores (600 CFU/g). The mean concentration of spores and mycelial fragments across all soil samples from which cultures grew tebuconazole-resistant A fumigatus was 146 CFU/g, with concentrations most increased in soil samples collected from compost heaps (214 CFU/g).

The researchers found that the only significant predictor for growing A fumigatus was the location of soil sample collection (c2, 67.3; P <.01). The odds of growing A fumigatus were significantly increased among soil samples collected from compost bags (odds ratio [OR], 15.70; 95% CI, 5.50-66.19; P <.01), compost heaps (OR, 3.45; 95% CI, 1.93-6.40; P <.01), and pots and/or planters (OR, 2.42; 95% CI, 1.50-3.95; P <.01). Similar findings were observed in regard to the odds of growing tebuconazole-resistant, with odds significantly increased among samples collected from compost bags (OR, 6.79; 95% CI, 3.25-14.37; P <.01) and compost heaps (OR, 4.74; 95% CI, 2.45-9.32; P <.01).

According to the researchers, “the evidence presented here supports the recommendation for [the use of facemasks] whilst handling compost and the introduction of health warnings on bags of compost [in regard] to [the risk for] inhaling A fumigatus [spores].” They noted that “measures could also be taken by compost producers to sterilize the composting before packaging, thereby killing viable A fumigatus spores and eliminating the immediate hazard it poses to the [individual].”


Shelton JMG, Collins R, Uzzell CB, et al. Citizen-science surveillance of triazole-resistant Aspergillus fumigatus in UK residential garden soils. Appl Environ Microbiol. Published online January 5, 2022. doi:10.1128/AEM.02061-21