Air pollution - health impacts: summary research findings

3 Discussion

This section discusses the evidence identified by the search strategy as well as the by the preliminary and supplementary searches.

3.1.1 Cardiovascular disease (CVD)

The three HEI reports, the Dominski review, and the single study identified by this review that focused on CVD (So et al., 2023), all provide significant evidence for the association and impact of air pollution on cardiovascular health globally. One of the Scottish studies (Yap et al., 2012) also found significant associations with all-cause mortality, cardiovascular mortality, ischaemic heart disease and respiratory mortality. Conversely, Willocks et al. (2012) and Lee et al. (2019) found no association between air pollution and CVD in Scotland.

The absence of an association found by Willocks et al. (2012) does not prove that there is no association, and may be due to several limitations in their methodology. Firstly, they only collected data on PM_10, rather than PM_2.5, BC, NO₂, or O₃, which have all been strongly associated with adverse health outcomes including CVD. The authors also discuss the possibility that the study design did not provide enough statistical power to detect a pollution-health relationship as the number of CVD admissions was relatively low. The study also only analysed short-term exposure to particulate matter (PM₁₀) and hospital admissions due to CVD and not the long-term effects of air quality on CVD (Willocks et al., 2012).

Lee et al. (2019) found no evidence that CVD or total non-accidental mortality are associated with any of the four pollutants (PM₁₀, PM_2.5, NO₂, and NOx), however, it is possible that the adjustment for confounders may be oversensitive due to the historic data used to quantify the association between deprivation and smoking.

In conclusion, whilst Willocks et al. (2012) and Lee et al. (2019) did not find an association between air pollution and CVD, this is likely an artefact of the study design and data. These limitations explain in part why the results are different from the earlier research by Yap et al. (2012) and the vast consensus of global research and data.

3.1.2 Mental health and well-being

The majority of studies reviewed focused on mental health and well-being, which is likely to be due to the recent focus in this area relative to more established links between air pollution and other health effects including CVD, respiratory and cancers. While causality cannot be assumed, the evidence clearly indicates an association between air pollution and negative effects on mental health and well-being in a wide variety of populations and settings.

3.1.3 Dementia

Multiple studies identified in this review provide consistent evidence linking air pollution, particularly PM_2.5, to an increased risk of dementia and potentially the exacerbation of Parkinson's disease symptoms.

3.1.4 Cognition

The papers identified support the harmful effects of air pollution on cognition across various populations, including adults, children, and individuals exposed prenatally. Studies consistently demonstrate associations between air pollution exposure and cognitive impairments, including attention, memory, language skills, and academic performance.

3.1.5 Neurological

This review provides further evidence of the link between air pollution and Parkinson's disease, dementia and stroke, and evidence of an association with other neurological outcomes including multiple sclerosis, emergency department visits for nervous system disorders, autism spectrum disorder and attention-deficit/hyperactivity disorder in children. These findings highlight the detrimental effects of air pollution on neurological health, both in terms of mortality and the incidence and severity of neurodevelopmental and neurological disorders.

3.1.6 Respiratory

The Dominski review provided a strong evidence base for the harmful effects of air pollution on respiratory health. The studies reviewed here are consistent with previous findings.

3.1.7 Development

The effects of prenatal exposure to air pollution on prenatal and childhood development presented in these studies is inconclusive and more comprehensive studies are needed to better understand the complex relationship.

3.1.8 Mortality

Three studies provide evidence for the association between air pollution and mortality. Bai et al. (2022) found a significant relationship between PM_2.5 exposure and mortality, with a substantial proportion of deaths attributable to diabetes and major cardiovascular events. The Danish study by So et al. (2022) revealed that long-term exposure to PM_2.5, NO₂, and BC was associated with increased mortality from various causes, while O₃ showed a generally negative association. Furthermore, So et al. (2023) identified significant positive associations between mortality and exposure to specific PM_2.5 elemental components. These findings highlight the adverse health effects of air pollution on mortality, in addition to the specific health outcomes discussed in this report.

3.1.9 Cancer

The association between air pollution and cancers is evidenced in the literature, however the strength of the association is not as well established as it is for other health outcomes. Furthermore, the relationship more specifically with childhood cancers has limited evidence and needs further assessment.

3.1.10 Neonatal

Recent studies have shed light on the concerning impact of air pollution, particularly PM_2.5, on neonatal health, however further evidence is needed to understand the extent of this impact and the underlying biological mechanisms.

3.1.11 Type-2 Diabetes

The Dominski review, as well as the evidence discussed, supports a significant association between air pollution, specifically PM_2.5, and the incidence and prevalence of type-2 diabetes.

3.1.12 Ocular

Only one study from the search analysed the effects of air pollution on ocular outcomes such as visual impairment and age-related eye disease and observed associations between PM_2.5 and ocular outcomes, suggesting the need for further studies to confirm these associations and explore potential mechanisms (Grant et al., 2021).

3.1.13 Primary care healthcare service use

One study analysed the effects of air pollution on short term primary and pharmaceutical care usage which highlighted the potential health effects of PM_2.5 exposure during critical developmental periods and the importance of considering sex differences in susceptibility (Ziou et al., 2023).

3.1.14 Contradictory evidence

This report identified two studies (Cortes et al. 2023; Kusters et al. 2022) which did not report any positive association between air pollution and the studies’ health outcome. These studies do not negate the level of evidence for the negative health impacts of air pollution; however, they do highlight the need for further research using more robust methodologies and comprehensive confounding adjustments to better understand the complex relationship between air pollution and health outcomes and importantly the best way of evaluating the associations and potential causality.