Urbanisation shapes bacterial communities in ways that can be reflected in human health and the immune system. Juulia Manninen’s doctoral thesis explores this theme, specifically the effects of urban features on both the environment and human microbiota.
The study found that rubber mats commonly used in playgrounds and daycare centres hosted considerably less diverse and stable bacterial communities than equally bare and nutrient-poor natural rock surfaces. The finding suggests that the use of artificial materials may have a greater impact on children’s microbial exposure than previously thought. In children, early microbial exposure is particularly important for the development of the immune system.
The practices for maintaining urban green spaces also seem to make a difference. The less intensively managed parks studied, where grass is allowed to grow longer, harboured more diverse microbiota starting from the park entrances. Most park visitors pass through these sites, potentially exposing themselves to beneficial microbes.
Urban pollution was associated with children’s microbiota
Manninen also investigated the effect of pollutants. A link was observed between polycyclic aromatic hydrocarbons (PAHs) transported to the homes of children living in urban areas, and the children’s skin and gut microbiota. PAH compounds are among the most common pollutants hazardous to human health, up to and including carcinogenicity. PAH concentrations particularly correlate with the land use categories typical of urban areas, including built environments.
“The links between PAH concentrations at home and children’s microbiota were worrying. It appears that even fairly low pollution levels can have an effect,” Manninen says.
The study is linked to a biodiversity hypothesis originating with Finnish researchers, according to which human activity and urbanisation shape microbial communities important to health, and reduced exposure to diverse environmental microbiota alongside increased exposure to pollutants may be associated with higher incidence of immune-mediated diseases.
“The results indicate that fairly simple choices, such as natural materials and less intense green space maintenance, can support beneficial microbial exposure in urban areas. We would also need effective measures to reduce the pollution load in urban environments,” Manninen notes.
The results can be applied directly to urban planning, green space maintenance and, for example, the development of children’s everyday environments.
Information about the doctoral thesis:
Juulia Manninen, MSc, defended her doctoral thesis entitled ‘Environmental Microbiota and Pollution in Urban Environments – Implications for Health-Relevant Exposure’ on 5 June 2026 at 12.00 at the Faculty of Biological and Environmental Sciences, University of Helsinki.
Docent Anne Karvonen from the Finnish Institute for Health and Welfare served as the opponent and Sarah Butcher as the custos.
The thesis is also available in electronic form through the
PAH compounds result from incomplete combustion of organic matter. They are composed of two or more fused aromatic (benzene) rings.
Sources: Typical sources include car exhaust fumes, chimney smoke, barbecues, fires and tobacco smoke. Asphalt and coal tar also contain PAHs.
Occurrence: PAHs in the environment are found in soil, sediment, water and air. They accumulate easily in organisms due to their fat solubility.
Health effects: Long-term exposure increases cancer risk. Benzo(a)pyrene is a known carcinogen. Some PAHs are also teratogenic (hazardous to reproductive health).
Environmental effects: PAHs degrade slowly in the environment, and they can disrupt hormonal activity in fish and other animals.