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].