Introduction

Eczema, also known as atopic dermatitis, is a chronic inflammatory skin condition, characterized by defective skin barrier function and it affects around 5% to 10% of adults and up to 20% of children.1 Eczema has a heterogeneous presentation, which varies in terms of severity, age of onset, and response to treatment.2 This variation in presentation is determined by an interplay of genetics, immunity and environmental factors, including skin microbiome and indoor and outdoor air pollution.2,3
The World Allergy Organization has recognized at least two types of eczema.4 An atopic type (AE) with skin inflammation driven by T-cell responses and Th2 cytokines in the initial phase, which is usually associated with IgE-mediated sensitisation to environmental allergens and high levels of both total and allergen-specific IgE. This form of eczema is strongly associated with increased tendency of developing other allergic conditions.5 The second form, non-atopic eczema (NAE), is characterized by normal levels of total IgE and lack of sensitisation to environmental allergens.5 The pathophysiology of NAE is poorly understood4, and, particularly in older patients and those with chronic eczema, other non-atopic inflammatory mechanisms might be involved.5
There have been relatively few studies of the risk factors for eczema in adults.6 In paediatric studies, higher levels of ambient air pollutants have been associated with increased eczema prevalence.7 It has been proposed that air pollutants may generate reactive oxygen species which damage the outer-most layer of the skin through oxidative stress.8 This process may drive the inflammation and pruritus that are associated with eczema, and this may subsequently downregulate filaggrin expression, further compromising the structural integrity of the epidermal barrier.9 The effect of ambient air pollution on the prevalence and incidence of eczema in adults has received less attention.
A recent longitudinal analysis of middle aged German women, which investigated the influence of traffic related air pollution on lung function, inflammation and Aging (SALIA) found that baseline concentrations of traffic-related air pollution (TRAP) markers (NO2, NOx, PM2.5 and PM10) were significantly associated with increased odds of incident eczema over a 19-year follow-up period, these associations being stronger for NAE.6 Therefore, environmental factors, including air pollution, might be important for development of eczema in middle age, particularly NAE. These findings need to be replicated using similar longitudinal data to draw firmer conclusions. Better understanding of the potential effects of ambient air pollution on adult eczema may lead to targeted interventions to prevent eczema. Using data from a large established longitudinal health study, we investigated whether exposure to ambient air pollution was associated with the incidence and prevalence of AE or NAE in middle-aged adults of both sexes.