AUTHOR: DR MADELEINE BELL
Air Pollution: Components, Particulate Matter, and Health Impacts
Air pollution occurs in both the indoor and outdoor environments with 11.65% of deaths globally being accredited to it . It is the third greatest mortality risk factor across the world , after hypertension and smoking. The burden of disease is seen predominantly in low to middle income countries (LMICs) where indoor air pollution is a much bigger problem due to reliance on solid fuel for cooking. There have been significant reductions in the last 20 years in the use of these fuels, with a corresponding fall in the death rates associated with air pollution , as more countries make the transition towards ‘clean fuels’ (bioethanol, natural gas, electricity.)
Air pollution is made up of gaseous components (carbon monoxide, carbon dioxide, nitric oxide, sulphur dioxide, ozone), organic compounds (acetetone, benzene) and particulate matter (PM). There are three types of particulate matter, PM10 (<10μm), PM 2.5 (<2.5μm) and UFP (<100nm). The smaller the size of particle, seemingly the more pro-oxidative they are and the higher risk to health . There’s a closer positive association between high PM 2.5 levels and rates of hospitalisation as well as all cause mortality . The health impacts seen are both acute, causing pneumonia and airway irritation, and more chronic. It is thought that PM 10 particles are generally deposited in the upper airway with associated respiratory disease, while PM 2.5 is engulfed by the bronchiolar and alveolar macrophages before being absorbed into the bloodstream. From here the fine particulate matter has a wealth of systemic effects. These appear to be predominantly due to mitochondrial dysfunction, inflammation and oxidative stress, due to increased propensity for formation of free radicals which are associated with a range of chronic illnesses .
Air Pollution’s Impact on the Cardiovascular System
With regards to the cardiovascular system, studies correlate air pollution with higher incidence of ST-elevation myocardial infarction, sudden cardiac death and peripheral arterial disease . The strongest association is with heart failure as admissions increase proportionally to air pollution exposure . There’s a suggestion that PM 2.5 in particular could be detrimental to cardiac function inducing arrhythmias . This is mainly mediated by oxidative stress, coagulation dysfunction, autonomic disturbance, and inflammation. PM 10 and PM 2.5 are thought to upregulate sympathetic tone therefore leading to vasoconstriction, hypertension and endothelial dysfunction. Increased systemic oxidative stress and inflammation can then lead to atherosclerosis progression, plaque instability and a prothrombotic state .
Air Pollution’s Impact on the Gastrointestinal system
The gut is also affected as particulate matter is cleared from the lungs to the gastrointestinal tract through mucociliary clearance into the oropharynx . Here the larger PM 10 particles are swallowed. Moreover, gaseous components can cause inflammation of the gut, causing oxidation of intraintestinal lipids, alongside impacting the intestinal microbiome . The rising body of evidence regarding the importance of the microbiome in supporting the immune system  attests to the compromise this puts on human health.
Air Pollution’s Impact on pregnancy and infancy
Exposure to air pollution prenatally and in childhood is associated with worse neurological outcomes and foetal growth restriction (FGR) . Black carbon and other particles have been detected in placentas which could reduce placental function . Black carbon has been seen to alter brain development in animal models and is a possible mechanism for impaired neurological development in foetuses . This is in addition to the known increase in oxidative stress and inflammation that comes with air pollution. These are two factors well established to cause FGR due to interference with growth hormone release. MRIs of children in a study from the Netherlands have proven changes in brain morphology with air pollution exposure prenatally . Throughout life there continues to be evidence of the detrimental impact of poor air quality: for example, with increased exposure to air pollution, 9-12 year olds had an increase in amygdala and cerebellum size, and a smaller corpus callosum when compared to their less-exposed peers .
Take home messages
Air pollution is one of the more established, well-researched effects of climate change. Researchers are becoming increasingly concerned with the systemic effects of particulate matter, especially affecting people of LMICs where solid fuel use in the home is common. With increasing understanding of the risks, and as some of these countries increase their economic power, the shift towards cleaner fuels has been seen. As the maps in figure 1 illustrate, from 1990 to 2019 the number of deaths from indoor air pollution is falling, so all is not lost. However, biomass still makes up a significant proportion of fuel resources as electricity cannot be relied upon globally . Harnessing natural resources is an enormous challenge to ensure we can remove the need for traditional energy sources in every community. Now is the critical time to act.
Figure 1: deaths from indoor air pollution in 1990 and 2019.
Maps reproduced from Our World in Data (https://ourworldindata.org/indoor-air-pollution)
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