Air pollution - health impacts: summary research findings

2 Results summary

A total of 333 studies were found using Scopus. The titles and abstracts were screened for relevance; 54 potentially relevant papers were identified, and full copies were obtained for detailed assessment. Of these, 46 papers were assessed as relevant and 8 were rejected due to reasons outlined in the main report. In addition to the Dominski review, supplementary searches identified three studies from the Health Effects Institute (HEI) report ‘Assessing Health Effects of Long-Term Exposure to Low Levels of Ambient Air Pollution’, which looked at all-cause mortality, cause-specific mortality and morbidity endpoints (Health Effects Institute, 2014), and three papers (published before 1 January 2020) in Scotland (Yap et al., 2012; Willocks et al., 2012; Lee et al., 2019).

Half of the 46 included papers (24) were from European countries, most commonly the UK (or England), Sweden, Denmark and the Netherlands. Around two-thirds of papers (31) assessed health outcome in adults, with one third on health outcomes in children. The most evaluated health outcomes were mental health and well-being (11), dementia (6) and cognition (5), with most papers (39) focussing on long-term outcomes.

The prevalence of papers on mental health and well-being is reflective of the search strategy timescale. The evidence on the physiological health impacts of air pollution has been established for several years. Therefore most of these papers would have been published prior to 1 December 2020, and were captured by the mapping review by Dominski et al. (2020) and the three HEI reports. Of all pollutants PM_2.5 (41) was the most evaluated, followed by NO₂ (23) and PM₁₀ (19) (N.B. Studies evaluated one or more pollutants).

Within the HEI reports, the ELAPSE study found significant associations between PM_2.5, Black Carbon (BC), and NO₂ exposure and natural-cause, cardiovascular, respiratory, and lung cancer mortality, as well as stroke, asthma, and COPD hospital admissions, at concentrations below the European Union limit values for PM_2.5 (25 μg/m³) and NO₂ (40 μg/m³)^[2]. The study also reported significant associations between NO₂ and acute coronary heart disease and between PM_2.5 and lung cancer incidence. The shape of the associations between exposure and natural-cause mortality showed steeper slopes at lower exposures, indicating increased risks for mortality at even the lowest observed concentrations; furthermore there were no concentration levels where associations were not found for PM_2.5, BC, and NO₂ (Brunekreef et al., 2021).

The MAPLE study found long-term outdoor PM_2.5 exposures as low as 2.5 μg/m³ were linked to an increased risk of death in a large representative sample of Canadian adults, with variation across different geographical regions and with smaller effects when adjusted for O₃ concentrations (Brauer et al., 2022).

Dominici et al. (2019) found that PM_2.5 was associated with an increased risk of all-cause mortality of 6% to 8% per 10 µg/m³ in 68.5 million older Americans (aged 65 and over). The effect estimates were larger in a low-exposure sub-cohort. The consistency of the associations across the methods suggests that long-term exposure to PM_2.5 is likely to have a causal effect on mortality, providing stronger evidence than previous studies.

From the Dominski review (Dominski et al., 2021), 75% of reviews (180/240) showed a positive association, 53/240 (22%) studies were classified as ambiguous and only seven (3%) showed a negative association or no harmful effect(s). Of these seven the health outcomes they analysed were: asthma, childhood cancer, congenital heart defects, pneumonia, stroke, telomere length, and venous thrombosis. Overall, their review found that even low levels of PM exposure can increase the risk of respiratory and cardiovascular disease (CVD), cancer, and premature death. Exposure to PM_2.5 was the most widely studied pollutant and an association was found with 8/10 health outcomes evaluated. The most frequently studied health outcome was CVD (32 reviews).