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.