All over the world, a growing proportion of the population lives in cities – by 2050, it’s projected that more than two-thirds of the world population will live in urban areas, with little green space and limited contact with nature and biodiversity .
While body site and age are major factors affecting skin microbial composition, the living environment also discriminates the skin microbiota of urban and rural populations. This has potential consequences for our skin and wider health – particularly when it comes to inflammatory diseases such as allergies and autoimmune disease, which are more prevalent and severe in urban populations.
Read on to find out more…
Urbanization of the skin microbiome
Where we live has a major impact on the skin microbiota, as our skin surface is in constant and direct contact with our surrounding environment. As well as being subject to changes due to external conditions such as weather and climate, variation in skin microbial communities has been found between people living in urban and rural environments .
Why? Urbanization reduces microbiological abundance and diversity . In urban populations – and particularly among younger age groups – those living in urban environments have been found to have a less diverse microbial community structure than people living in rural environments [2,4,5].
Urban populations have also been found exhibit increased abundances of specific bacteria such Cutibacterium (a type of bacteria that plays a key role in acne development) and decreased levels of Corynebacterium – especially on the back of the hands, forearm and forehead, all of which are areas frequently exposed to the external environment [2,4].
Urban and rural influences on the skin microbiome
Many factors contribute to the unique skin microbiomes of rural and urban residents, such as lifestyle, house architecture, number of inhabitants per house, air pollution, detergents and cosmetic use, among many others . Here we will explore a few.
Biodiversity exposure and loss. In rural environments, we have greater exposure to microbes such as Proteobacteria and Lactobacillus that are presence in high abundance in nature and the soil, water and biomass used in agriculture – many of which have a beneficial relationship with humans and contribute to the development and regulation of the human immune system [3,6]. In urban environments, this exposure is reduced, potentially decreasing the diversity of the skin microbiota .
Housing. Due to poor microbiological assemblages in cities, urbanization reduces indoor microbial diversity , and the number of chemicals within houses also increases substantially with urbanization . This has a knock-on effect for the composition of house bacterial and fungal communities – an increased number of potentially pathogenic bacterial and fungal strains have been observed within urban houses and on the skin of the residents, along with a less rich bacterial community across the hands, arms and feet .
Pollution. Urban pollutants – for example, air pollutants such as polycyclic aromatic hydrocarbons – are another factor that affects skin microbiome composition and its functional capacity . In China, comparing subjects living across two cities of varying levels of pollution revealed a reduction in ‘friendly’ bacteria in those exposed to higher pollution levels, to the benefit of pathogenic bacteria that are associated with poor skin condition .
Hygiene practices and beauty and personal care routines. People are showering more than ever before in countries such as the UK and US, and the expansion of urban areas and associated initiatives in sanitary science mean that these are associated with increased hygiene practices (such as showering and hand washing) . Urban residents are also thought to use more beauty and personal care products such as cosmetics – due to factors such as product availability and a demand for appearance-boosting as well as anti-pollution products. These factors can negatively impact the skin microbiota by altering community structure, dynamics and functioning [3,13,14].
Smoking. Smoking is another lifestyle factor that is more common in urban environments and has increased with urbanization . As well as being detrimental for our heart and lungs, cigarette smoke as an urban pollutant can worsen the damaging effects of air pollution on the skin .
Consequences of urban living for skin health
The urban environment has been shown to impact various parameters integral to skin health and functioning.
Living in an urban area, for example, has been linked to increased sebum production and reduced levels of vitamin E and squalene within the sebum, which are integral for skin moisture and structure. An increase in lactic acid has also been found in association with urbanization, which can lead to skin irritation such as a stinging or burning sensation and red and flaky skin, as well as changes to the stratum corneum (the outermost layer of our skin), which provides the first line of defence against the external environment and potentially pathogenic microbes .
The skin’s natural defences, even when the skin is healthy, cannot cope with the levels of air pollution found in cities , with clinical studies linking continued exposure to polycyclic aromatic hydrocarbons with premature skin aging, pigmentation spots and wrinkles, acne and skin cancer .
As an integral part of the skin barrier, the skin microbiome is tightly linked to urbanization-related skin alterations. The reduction in microbial diversity associated with urbanization has been linked disrupted skin structure and functioning, as well as higher rates of infection and the development of certain diseases [3,11].
This brings us to our next section, where we will explore some examples of urban-associated disease and disorders and the interplay of the skin microbiome…
Urban skin disorders and disease
The shift to urban living has been linked to increased incidence and severity for many skin disorders and diseases – such as acne, eczema, psoriasis, rosacea and dandruff [2,6,9,11,17].
Acne. Acne is not found on the skin of indigenous tribes or on that of individuals from non-industrialized societies , whereas approximately 85% of people aged 12–24 years experience at least minor acne . Only when indigenous people moved to Westernized cities does acne appear to have become a problem , and backing this up is research showing that Cutibacterium – the bacteria that play a key role in the development of acne – is more abundant on the forehead of urban residents relative to rural residents .
Atopic dermatitis. Atopic dermatitis (the most common form of eczema) is a chronic skin condition characterized by dry skin and intense itching, with typical onset in infancy. Skin barrier dysfunction and inflammation contribute to the development of atopic dermatitis, but a link to microbiome dysbiosis (disruption) has also been demonstrated – namely, a reduction in skin microbial diversity and increased presence of the bacteria Staphylococcus aureus (as well as Staphylococcus epidermidis and Malassezia bacteria). This is particularly evident in urban children [17,20].
More generally, our skin’s resident microbes are increasingly viewed as an integral part of the skin and rest of body. Locally, microbial-immune interactions in the skin are vital for optimal barrier function, pathogen defence and tissue repair, with the production of key anti-inflammatory and anti-microbial compounds to maintain healthy tissue homeostasis .
The skin microbiome is also key to the function of the rest of body, as it interacts with wider tissues and immune networks to influence our overall health and functioning. As a result, the reduced exposure to nature-based environmental microbes in Western cities is seen as one of the major reasons for the rise in a wider list of inflammatory and immune-mediated diseases, including autoimmune diseases and allergies, as those microbes that have a commensal relationship with humans contribute to the development and regulation of the human immune system .
Future outlook: mediating the impact of urbanization on out skin
With the growth of urban areas set to continue, city dwellers could benefit from interventions that help restore the microbiome imbalances associated with urban environments.
Anti-pollution protective cosmetics are increasing in number on the shelves, and microbial pro- and prebiotic products have also shown some promise in the prevention and treatment of urban-related skin health impacts .
Another interesting approach is urban greening to restore microbiological diversity in cities and influence human microbial assemblages, increasing diversity through the transfer of environmental microbes to the skin and respiratory tract of humans following urban green space exposure. For example, soil- and plant-based biodiversity interventions and air-circulating green walls have been shown to be able to induce bacterial changes in the skin microbiome [3,23].
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- Soininen, L. et al. Indoor green wall affects health-associated commensal skin microbiota and enhances immune regulation: a randomized trial among urban office workers. Sci. Rep. 12, 6518 (2022).
- Ying, S. et al. The influence of age and gender on skin-associated microbial communities in urban and rural human populations. PLoS One. 10, e0141842 (2015).
- Lehtimaki, J. et al. Patterns in the skin microbiota differ in children and teenagers between rural and urban environments. Sci. Rep. 7, 45651 (2017).
- Prescott S.L. et al. The skin microbiome: Impact of modern environments on skin ecology, barrier integrity, and systemic immune programming. World Allergy Organ. J. 2017;10:29.
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- Leung, M. H. Y. et al. Changes of the human skin microbiota upon chronic exposure to polycyclic aromatic hydrocarbon pollutants. Microbiome 8, 100 (2020).
- Wang, L. et al. Facial Skin Microbiota-Mediated Host Response to Pollution Stress Revealed by Microbiome Networks of Individual. mSystems 6, e00319-21 (2021).
- Bouslimani, A. et al. The impact of skin care products on skin chemistry and microbiome dynamics. BMC Biol. 17, 47 (2019).
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- Powers, C. E. et al. Microbiome and pediatric atopic dermatitis. J. Dermatol. 42, 1137-1142 (2015).
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- Haahtela, T. et al. Immunological resilience and biodiversity for prevention of allergic diseases and asthma. Allergy 76, 3613-3626 (2021).
- Selway, C. A. et al. Transfer of environmental microbes to the skin and respiratory tract of humans after urban green space exposure. Environ. Int. 145, 106084 (2020).