Investigating inequalities in NO2 air pollution concentrations on novel indicators relating to small spatial areas | International Journal for Equity in Health

Air pollution is a global health risk, with the World Health Organization (WHO) identifying it as the biggest environmental health risk in Europe [1]. While air pollution emissions have declined in the last two decades, resulting in better air quality, nitrogen dioxide (NO₂), exposure remains a key concern. NO₂ is a gaseous pollutant from burning petrol or diesel in vehicles [2]. In 2021, the WHO significantly lowered its annual average NO₂ guideline from 40 µg/m³ to 10 µg/m³ [3, 4]. This aligns with the EUs 2024 Ambient Air Quality Directive, which mandates Member States to achieve a maximum annual average of 20 µg/m³ by 2030 [5].

Fairburn et al. [6] comments that there are relatively few national ecological studies using small area level data to examine the social inequalities of air quality, citing challenges in obtaining small area statistics with good socio-economic, socio-demographic or index data. This paper addresses this gap by conducting the first study of NO₂ at a small geographical area in Ireland. Ireland serves as a valuable case study for international research on NO₂ air pollution due to a unique juxtaposition of its generally clean atmospheric conditions and the presence of concentrated, localised urban pollution. This favourable baseline air quality, attributed to a lack of heavy industry and its geographically exposed island location with high wind levels [7], creates a distinct environment for studying the dynamics of pollution in “hotspots” that are not overwhelmed by regional or industrial emissions. This setting allows researchers to isolate and investigate the specific impacts of NO₂ sources, predominantly traffic, in a way that is often challenging in more heavily industrialised nations. Furthermore, the country’s adherence to European legislation and its implementation of significant policy measures, such as the phased smoky coal ban [8]^{Footnote 1}, provide a robust framework for assessing environmental policies on a national scale.

In the context of the extant international literature in this field, the research presented in this paper also overcomes temporal mismatches by using high-resolution NO₂ data (2010–2020) with recent 2022 census information. It innovatively examines NO₂ concentrations alongside indicators of socioeconomic position and health, such as housing tenancy and smoking status, providing new insights into air quality disparities [9].

Health consequences of NO₂

Breathing NO₂ can irritate the airways, aggravating respiratory diseases leading to coughing, wheezing, and difficulty breathing [10,11,12]. A meta-study concluded that the magnitude of the effect of long-term exposure to NO₂ on mortality is at least as important as that of PM_2.5^{Footnote 2} [13]. The mortality burden of disease attributable to NO₂ includes diabetes mellitus, stroke and asthma [14], and a meta-analysis found a positive relationship between NO₂ and all-cause mortality, mortality from respiratory conditions, including chronic obstructive pulmonary disease (COPD) [15]. In 2021, exposure to NO₂ levels exceeding WHO recommendations was estimated to cause 52,000 premature deaths in the EU-27 [4, 14]. Furthermore, NO₂ can also react with sunlight to produce Ozone, a secondary pollutant with significant health effects [16].

Additionally, traffic-related air pollution has been found to be associated with poorer birth outcomes, such as, low birth weight and premature birth [17,18,19], as well as neurological disorders such as autism [20]. Economically, air pollution more broadly, reduces life expectancy, increases healthcare costs, and lowers productivity through absenteeism [21].

Social gradients in health are well established [22,23,24], where socially and economically marginalised groups may experience increased susceptibility to negative air pollution related health effects because of higher baseline rates of disease [25]. The potential for unequal exposure to NO₂ is a major policy concern, as socioeconomically disadvantaged or vulnerable populations (e.g., lower-income households) may live in areas with higher NO₂ concentrations. For instance, lower property values near busy transport routes make these areas more affordable [26, 27], and studies have shown that schools in disadvantaged areas are exposed to higher NO₂ levels [28, 29].

Moreover, research has suggested that spatial inequalities in environmental exposures often place disproportionate health risks on people who are already the most vulnerable or susceptible to those risks, for example, those with underlying prior health impairments or poorer access to health care [25, 30].

Extant international and Irish-based studies on NO₂ and health outcomes

International research, particularly from the US, typically finds that more vulnerable communities, particularly those comprised of racial minorities and lower income groups, are more likely to be exposed to higher NO₂ pollution levels [31,32,33,34,35]. In the Massachusetts area, Rosofsky et al. [35] found that inequalities in NO₂ exposure were more significant in urban compared to rural areas, and more pronounced along racial/ethnic lines than by income or education. However, the study also noted that poorer rural areas had higher NO₂ exposure than wealthier ones.

Crouse et al. [36], identified a significant association between elevated outdoor NO₂ concentrations and neighbourhood deprivation in Montreal, Canada. This deprivation encompassed both material factors (e.g., lower income) and social factors (e.g., a higher prevalence of individuals living alone), highlighting areas with a dual burden of high NO₂ exposure and considerable socio-economic disadvantage. However, the study also revealed that this pattern was not universally consistent. Some deprived areas exhibited low pollution, while certain affluent city-centre districts recorded high NO₂ levels.

A majority of UK-based studies that investigate air pollution associated with NO₂ find evidence of inequalities on racial and income lines [37,38,39,40,41,42]. However, the correlation between air pollution from NO₂ and socio-economic inequalities across studies arising from the European context is more inconsistent. For instance, Temam et al. [43] examined associations between indicators of socio-economic position and NO₂ for sixteen European cities, finding high levels of heterogeneity across the cities studied. In general, they found that lower neighbourhood socio-economic position (as measured by unemployment levels) was generally associated with higher NO₂ exposure. However, the opposite pattern, i.e. higher NO₂ air pollution concentrations in areas of higher social economic status, were revealed in studies originating from Denmark [44], Paris [45], and the Czech Republic [46]. In commenting on a cross-country analysis between the Netherlands and England, Fecht et al. [39] remark that relationships between the geographical distribution of vulnerable communities and air pollution (including NO₂) levels are more complex and less universal than assumed.

In Australia, research shows an inverse relationship between NO₂ concentrations and socioeconomic disadvantage [47]. Lower NO₂ concentrations were observed for areas with a higher proportion of non-Indigenous individuals. A separate study by Cooper et al. [48], found that in Australia, higher exposure to NO₂ was experienced by ethnic minority groups, socioeconomically disadvantaged residents in urban areas, and the elderly in rural areas.

Turning to the study setting for which the present research is conducted, Domhnaill et al. [49] uncovered no association between NO₂ levels and respiratory medicine prescriptions in Ireland. However, a separate study by Carthy et al. [11], found that among people over 50 in Ireland, NO₂ concentrations were associated with a higher likelihood of asthma and the use of respiratory medications.

Research by Quintyne et al. [50] connected Dublin’s hospital admissions to the Air Quality Index for Health (AQIH), finding an increased likelihood of hospitalisation for asthma, chronic obstructive airways disease (COAD), and heart failure when air quality worsened. A survey revealed that while most Irish people know NO₂ is a major pollutant, they’re unaware it increases the risk of medical problems like asthma and heart disease [51]. Hoy et al. [52] estimated the annual cost of treating stroke and asthma cases attributable to NO₂ pollution in Ireland to be nearly €2.7 million and €0.4 million, respectively.

Finally, Hoy et al. [53] found that nearly all small areas in Ireland exceeded the WHOs annual guideline for PM_2.5 in 2016. While there was no strong evidence of widespread socioeconomic inequality in PM_2.5 concentrations, more disadvantaged rural areas did exhibit higher levels than their more affluent counterparts. The present study distinguishes itself from the work of Hoy et al., (ibid) primarily through its focus on NO₂ concentrations. This difference is crucial as NO₂ is a more localised pollutant, with concentrations peaking near emission sources like roadways [54], necessitating a significantly finer analytical resolution. Consequently, the current work employs contemporary NO₂ concentration data with an enhanced spatial resolution of approximately 0.0025 km², contrasting sharply with the 1 km × 1 km PM_2.5 data used in Hoy et al., (ibid). Furthermore, the socioeconomic context is updated through the utilisation of 2022 Census data, offering a more contemporary perspective than Hoy et al., (ibid)‘s reliance on 2016 Census information.

Contribution of the present research

This study investigates the distribution of NO₂ concentrations in Ireland, a topic central to understanding environmental inequalities. We note that the concept of environmental equity posits that all individuals and communities should experience a proportionate share of environmental benefits and burdens, including air pollution and its associated health risks, alongside equitable access to environmental amenities [55]. In contrast, environmental justice seeks to achieve this equity by creating policies and processes that ensure fair distribution and recognise the diverse experiences of affected communities [56]. According to Walker and Bulkeley [57], an unequal distribution of environmental burdens is not necessarily an injustice, but rather the fairness of the processes leading to that distribution is what matters. This is especially relevant for air pollution, as socioeconomic factors can increase vulnerability. Walker [58. 215] describes this as a “triple jeopardy,” where those least able to cope with poor health are simultaneously exposed to higher levels of harmful pollutants.

In response to these considerations, this study leverages the most contemporary available data on NO₂ concentrations in the Republic of Ireland, measured at the smallest administrative spatial unit for the decade spanning 2010 to 2020. For this research, NO₂ recordings have been methodically linked to detailed demographic, socio-economic, and health profiles of small areas, as well as a composite index of deprivation derived from the most recent 2022 census. This enables a high-resolution, national-level analysis of NO₂ exposure across all small areas, alongside the ability to stratify by urban and rural regions. Given the strong correlation between NO₂ and local traffic, this stratification is vital and promises new insights into NO₂-related inequalities. This research also offers novel contributions by analysing NO₂ concentrations against housing tenancy, self-rated health, and smoking rates, offering contemporary quantitative data on environmental inequality^{Footnote 3}. This information can help environmental and governmental bodies create more effective policies to reduce air pollution.

Where patterns of NO₂ inequalities are uncovered in this work, these call into question the fairness of current environmental distributions, and the societal mechanisms that contribute to such inequalities. However, this research cannot make direct claims about environmental justice since it is limited to a descriptive analysis of exposure patterns and does not examine procedural fairness, the socio-historical development of these disparities, or the involvement of affected communities in decision-making [58]. As such, the scope of the study is confined to the documentation of environmental inequality [29].