Environmental factors that explain the increase in the prevalence of atopic eczema in the population.

Atopiceczema, also known as atopic dermatitis, is a chronic inflammatory skin disease that progresses in flare-ups and is characterised by extremely dry skin and itchy red patches. This dermatosis usually develops in early childhood and persists into adulthood in 10 to 15% of cases. Eczema affects 2 million adults in France, which represents nearly 4% of the population. It is the second most common skin disease, just after acne. In recent years, there has been an increase in the prevalence of atopic eczema, estimated at 1% every 10 years.

Atopic eczema occurs in individuals with an atopic genetic predisposition, meaning they have allergic reactions to common environmental elements, such as animal hair, dust, or pollen. This atopy is caused by a dysfunction of the skin barrier, due to a lack of sebum, lipids, and cell adhesion molecule production. It is also observed that individuals suffering from atopic eczema often have a mutation in the gene coding for filaggrin, a protein essential for the proper functioning of the stratum corneum.

Although it is closely linked to genetics, atopic dermatitis is influenced by several environmental factors. These play a significant role in triggering eczema flare-ups. Therefore, changes in our lifestyle leading to an increase in these environmental factors also cause a rise in the prevalence of atopic eczema. The following elements can be particularly considered:

• Enhanced hygiene.

  Formulated in 1989 by David STRACHAN, an academic researcher, the hygiene hypothesis is based on the premise that a reduction in exposure to infections and microbial components in early childhood in industrialised countries leads to a decrease in the maturation of the immune system and, consequently, an increase in the prevalence of allergic, autoimmune and inflammatory diseases, such as eczema. However, while evidence has indeed been established in epidemiological studies within industrialised countries, the hygiene hypothesis has its limitations and the hygiene factor does not account for the increase in the incidence of atopic eczema in less developed countries.

• An increasingly urban lifestyle.

  According to the United Nations (UN), the global urban population is significantly increasing, while the growth of the global rural population is slowing down and is expected to decline from 2030 onwards. In 2018, the UN estimated that 55% of the world's population lived in urban environments, compared to about 40% in 2000. However, a study has shown that individuals living in urbanised environments are more prone to atopic eczema than others. Pollution, a factor that exacerbates outbreaks of eczema, is notably responsible for this.

• Still high levels of smoking.

  Smoking is a risk factor for the development of atopic eczema due to the irritating and polluting substances contained in a cigarette. These compounds easily penetrate the delicate skin barrier of atopic skin and trigger immune responses that lead to eczema flare-ups. However, despite an increasing number of prevention campaigns, the number of smokers worldwide is not decreasing, which contributes to the rising prevalence of atopic eczema.

• The increasing stress.

  Several scientific studies have demonstrated that there is a link between eczema and stress. Various mechanisms are at work, among which is the reduction of lipid synthesis in the stratum corneum mediated by cortisol, the stress hormone. The skin barrier, already fragile in atopic skin, becomes even more diminished.

  Indeed, stress is becoming increasingly prevalent in the world and is part of a constant state of dissatisfaction. Sociologists observe that individuals today seem to expect more from life than previous generations, which drives them to set ambitious goals. The desires for self-realisation and personal achievement are growing but are sometimes accompanied by an unequal time management between personal and professional lives and a (too?) heavy work commitment, leading to significant stress. Bereavement, separations, worries about global economic, political, health and climate crises are other stress-inducing factors.

• Climate change.

  Previously mentioned as a cause of stress, climate change is also responsible for a general increase in the average temperature. According to the Ministry of Ecological Transition, the annual global temperature has risen by almost 1°C since the end of the 19th century. Individuals suffering from atopic eczema are sensitive to heat, which is a trigger for itching. Therefore, it is possible that the general increase in temperature has contributed to the rise in the global prevalence of atopic eczema.

• ROGUEDAS A. M. et MISERY L. Atopie et stress. Annales de Dermatologie et de Vénéréologie (2004).

• SPULS P. & al. Is there a rural/urban gradient in the prevalence of eczema? A systematic review. The British Journal of Dermatology (2010).

• GOLDENBERG G. & al. Eczema. The Mount Sinai Journal of Medicine (2011).

• BURKEMPER N. & al. The association of smoking with contact dermatitis and hand eczema - a review. International Journal of Dermatology (2018).