Introduction
Dirty air is now recognized as the single biggest environmental threat
to human health [1]. The air we breathe is becoming increasingly
polluted and its impact on health is appraised to be as great as other
major global health risks such as unhealthy diet, sedentarism, or
tobacco. Nonetheless, air pollution is hard to escape. An emerging body
of evidence emphasizes the associations between poor-quality
environments and socioeconomic conditions at both local and regional
scales [2]. Whether you benefit from high-quality urban
environments, such as those rich in green and blue spaces, that may
offer benefits to allergic and respiratory health depends on where you
live and work. Environmental inequality, therefore, results from the
unequal distribution of the risks and benefits that stem from
interactions with our environment.
There are several dimensions of environmental inequality. First, the
imbalanced distribution of negative and positive impacts. The helpful
effects of infrastructure such as an industry often affect much broader
scales than its adverse influences such as pollution [3]. Secondly,
the distribution of environmental risks remains in time with future
generations facing the risks created by the dirtying activities of
today. Infants whose mothers have been exposed to higher levels of air
pollution during pregnancy are much more likely to develop asthma
[4]. Thirdly, who produces air pollution and who undergoes the
consequences follows at many stages and is a driver of inequity. For
instance, the environmental footmark for Europe is over twice the size
of its capacity to produce beneficial biological materials and to
engross waste materials [5]. Thus, much of the impact manifests not
in the region’s carbon footprint but outside and with implications for
the health of persons living in those environments. A fourth dimension
relates to public participation in decision-making and access to justice
when the topic is air pollution. This social media dimension is of major
importance nowadays. Public awareness drives political and social
measures to pressure to tackle air pollution. Finally, methods of
assessment of exposures to air pollution are complex and challenging and
not within our scope. A brief overview of both direct and indirect
approaches as well as their strengths and limitations are presented in
supplementary online table S1.
Environmental inequalities are not a novel phenomenon. Conditions of
social vulnerability in education, income, or access to health coupled
with air pollution exposure have been present for a long time. But a
different perspective is presented in this article, arguing for the case
of air pollution and asthma as one of the features of environmental
inequality. In a rapidly changing world, economic development and
industrial activities, traffic-related air pollution (TRAP),
urbanization, and indoor pollutants exposure, together with rapid
population growth, are major driving forces of air pollution and climate
change [6] that all impact asthma risk in children. Within this
perspective, this article aims to answer i) what is the evidence for an
association between air pollution caused by industrial activities,
traffic, disinfection-by-products and tobacco/e-cigarettes and asthma in
children? ii) what the proposed mechanisms behind this association are,
and finally, iii) what can be done to mitigate the burden of air
pollution on asthma.
Outdoor air pollution from industrial activities
Industrial activities are characterized by frequent emissions of air
pollutants such as carbon dioxide (CO2), sulphur (SOx) and nitrogen
oxides (NOx), particulate matter (PM), heavy metals and volatile organic
compounds (VOCs) [7]. Consequently, and although the aetiology of
allergic diseases is continuously evolving as novel information is
unveiled by epidemiological studies, an association between allergy and
industrialization has long been perceived. Back in 1996, Schäfer and
co-workers published a study where they compared the prevalence of
allergic diseases in pre-school children from different parts of Germany
and observed a significantly higher risk of atopic eczema in the East
when compared to a countryside town in the West [8]. Authors argued
this difference could potentially be explained by the characterization
of pollutants in each region, with more sulphureous air pollution in
Eastern industrial areas when compared to a more oxidizing Western air
pollution [8, 9]. Since then, several findings of associations
between industrial pollution and asthma started to emerge. Children
living in areas characterized by higher industrial pollution were found
to have a 5% increase in the risk of developing asthma [10].
Emergency department visits for asthma, and particularly pediatric
asthma, were also found to be significantly higher [11, 12]. The
more frequent exacerbations in patients with asthma living in industrial
areas may also be explained by the impact of the pollutants on lung
function. As an example, peak expiratory flow and forced vital capacity
were found to be significantly hampered by the exposure to particulate
matter and NOx [13].
However, it is still not clear when the relevant window of exposure to
industrial pollution occurs. Which period of exposure (prenatal,
perinatal, early-life and/or most recent) has the most determinant
associations? How much is gestational exposure relevant to the
industrial exposome? Although there are few longitudinal studies focused
on early-life exposure to industrial pollution, there is evidence that
children living in neighbourhoods with industrial facilities, from
birth, are at a higher risk of chronic respiratory morbidity [14].
Despite the eventual overlapping of effects from TRAP, studies have
shown that industrial pollution alone can be a significant and
independent determinant of asthma in early life [15, 16]. The
combination of both sources, however, could be even more deterministic
during childhood [14], since it has been shown that PM2.5 exposure
(largely associated with TRAP) in combination with secondary sulphate
sources (mainly related to industrial activity) was found to
significantly exacerbate respiratory symptoms in children [17].
Although it is possible to hypothesize, based on other exposure factors,
that gestation would be a determinant window of exposure to industrial
pollution, there is no strong definitive evidence of such. One
retrospective study showed that preschool children whose mothers worked
at an industrial facility were at a higher risk of wheezing [18].
However, these mothers, and consequently their children, would be more
likely to live near industrialized areas (closer to workplace), which
could potentially confound the proper estimation of the relevant window
of exposure.
The level of urbanization associated with industrialized zones may also
be an important factor behind the different magnitude of observations in
published studies. By analyzing air pollution data from air quality
stations across Europe [19], it is possible to conclude that most
pollutants associated with industrial zones are, on average, at higher
levels in urban and suburban environments when compared to rural areas
(Figure 1 ). This difference may not only be attributed to the
higher density of industrial activity in urban and suburban zones but
may also reflect the protective effect of greener rural areas against
environmental pollutants. Hence, it is possible to hypothesize that
these two factors – industrialization and geographical area – may
partially explain the increased prevalence of asthma in urban settings
and the heterogeneity in observations between different studies [20,
21].