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Air pollution - mammoth task of modern medicine

Since the beginning of technical development, the effects on humans and the environment have not always been exclusively positive. From catastrophic water quality in the Rhine in the 1960s, caused by industrial waste water, to the ozone layer attacked by CFCs in the 1980s, the effects of air pollution have increasingly come to the fore in the last decade. Since 2008, the effects of particulate matter and nitrogen oxides, as well as other harmful substances in the air, have become an important part of European legislation, not least as a result of the EU Air Quality Directive 2008/50/EC, and are now increasingly the focus of medical research. And the results make you sit up and take notice. The list of suffering caused by air pollution is getting longer and longer. In addition to respiratory diseases, which are obvious when breathing bad air, damage to important organs such as the heart and brain, but also sensory organs, is increasingly being detected. In addition, mental and psychological problems such as depression are attributed to air pollution.

Around 55% of the world's population live in cities and are permanently exposed to poor air quality. Although the effects of air pollution in Europe have already been massively reduced in recent decades (22), the impact of particulate matter (PM) on human health is enormous. The "Global Burden of Disease" project, which quantifies disease and death, estimates that air pollution is the fifth leading cause of premature death worldwide, with around 4.2 million deaths (2). The European Environment Agency identifies air pollution as the biggest health risk caused by environmental factors, leading to around 400 000 premature deaths in Europe in 2018 (22). During this period the limit values set by the EU were frequently exceeded. In 2016-2018, 4-8% of the population lived in areas where the EU limit value for PM2.5 was exceeded. The WHO limit value up to which PM2.5 is considered not to be harmful to health is significantly higher than the EU's legal limit, so that as many as 74-78% of the EU population lived in areas where the limit value was exceeded (22).

The harmful substances are released into the air mainly through combustion processes in passenger and freight transport, but also in industry, agriculture and households. These processes produce primary particulate matter, such as metallic particles, which are released directly into the air from their source, and secondary particulate matter, which is produced in the air from gaseous precursors such as nitrogen. The size of the particles is used to divide particulate matter into categories: for example, PM10 corresponds to particulate matter with a diameter of no more than 10µm (1). When inhaled, the particulate matter enters the lungs via the airways. The smaller the particles, such as PM2.5 or PM0.1, the further the particles can travel, reaching the alveoli and lung tissue or even entering the bloodstream. From here they can directly reach and damage almost all the cells of the body.

The European Environment Agency lists lung cancer at 17%, coronary heart disease, i.e. the undersupply of oxygen to the heart, at 12%, strokes at 11% and chronic obstructive pulmonary disease (COPD) at 3% as the main causes of premature death linked to air pollution (22). PM2.5 in particular has been linked to cardiovascular disease through various mechanisms. These include dysfunction of the endothelium, the barrier between blood and tissue, vasoconstriction, high blood pressure, systemic inflammation and oxidative stress caused by free radicals (3).

Even during childhood development, particulate matter PM2.5 can impair the development of the bronchial tubes and thus restrict later lung function (2) In addition, inhaled particles in the lungs can weaken the immune system by inhibiting scavenger cells. Researchers at the University of Cambridge found that in the presence of metals in particulate matter, such as iron and copper, but also arsenic, tin, antimony and vanadium, bacteria and other foreign bodies, the scavenger cells can no longer absorb and digest them. In addition, under the influence of the particulate matter, the scavenger cells emitted increased amounts of inflammatory messenger substances that can trigger inflammatory diseases such as bronchitis and pneumonia (4). Particulate matter can also upset the balance of different types of white blood cells and thus promote asthma (2).


Researchers at the University of Versailles St-Quentin-en-Yvelines also found that rhinitis symptoms, i.e. a blocked or stuffy nose without the appearance of a cold, were aggravated by the particulate matter. PM10 in particular triggered an intensification of the symptoms, but PM2.5 and nitrogen dioxide (NO2) also led to increased symptoms. The presumed mechanism that triggers this intensification is again an increased occurrence of inflammation in the airways and oxidative stress, i.e. free radicals that can damage cells (5).

In the heart, particulate matter impairs the function of the mitochondria, according to research findings from the University of Lancaster. Mitochondria are components of human cells, also known as cell power plants. They produce the molecule ATP, which acts as an energy supplier. In the heart, this energy is used, among other things, for the contraction of the heart chambers, which pump blood and thus fresh oxygen through the body. The researchers found that particulate matter mainly damaged the membranes of the mitochondria, causing the cells to produce more stressproteins (6).

Researchers have also found considerable amounts of particulate matter in the brain and studied its effects there. For example, researchers from the University of Lancester found nanoparticles of magnetite in brain tissue. These magnetic particles form free radicals and have already been linked to Alzheimer's disease and other neurodegenerative diseases. Because of the highly rounded and polished shape of the particles, the researchers were able to attribute their origin to internal combustion engines and brake friction, because the heat generated in this process creates almost spherical particles. The extremely small size of these particles (usually less than 200 nanometres) also enabled them to pass directly through the blood-brain barrier into the brain after inhalation (7).

As in the brain, particulate matter can have neurodegenerative effects in other organs. A study by the National Institute for Health Research in London has now found that even a risk to eyesight can be attributed to air pollution. This means that people who were exposed to elevated levels of particulate matter PM2.5 were more likely to get glaucoma. This irreversible disease damages the optic nerve and is the most common cause of blindness in the world, affecting about 60 million people. Researchers were not able to link a change in the eye pressure of those who have the disease, which is often identified as the cause of glaucoma, to particulate matter. So here too, the researchers assume that neurotoxic effects such as oxidative stress and inflammation and the associated vascular changes could be the cause of glaucoma (8).

So particulate matter can cause enormous physical damage. But research goes much further and has also linked particulate matter to various mental and psychological illnesses.


Children and young adults in particular are enormously affected by air pollution during the development of the brain. Here too, researchers assume that neurotoxic processes triggered by particulate matter are the trigger and can cause, for example, learning and developmental disorders. Research in Barcelona showed that children who studied in schools with high levels of air pollution showed reduced cognitive development in various areas of memory. The working memory of these children, which is important for learning to read, write and do arithmetic, was thus impaired. Their inattention was also increased (9). Research from the University of Cincinnati showed that children who had lived on busy streets in their first year of life had a 4% lower volume of grey matter in their brains when they were 12 years old. The grey matter is associated with memory but also with language learning. Areas of the brain that perform sensomotoric tasks and the limbic system, which is partly responsible for emotions, were also significantly smaller than in children whose early childhood development took place in rural areas (10).

Other studies have even found links between air pollution and mental disorders such as schizophrenia. It has already been shown that inflammation, for example of the central nervous system, can trigger mental disorders (11). Researchers therefore also think that particulate matter, which can cause such inflammation, might be a possible cause of mental health problems. Research from Umeå University in Sweden has now shown that children and young people do indeed suffer more from mental disorders when they are exposed to increased air pollution. An increase of 10 µg/m3 NO2 led to a 9% increase in mental illness. For fine particulate matter (PM2.5 and PM10), the change was somewhat smaller at 4%. (12). Research at the children's hospice in Cincinnati also showed that mental illnesses such as anxiety, schizophrenia and depression and even suicidal thoughts were more frequent when children were exposed to increased air pollution (13). Pregnant women who were exposed to high levels of air pollution just before or during childbirth were even more likely to give birth to children with autism or autism spectrum disorder (14).

Adults have also been linked to poor air quality with concentration problems and serious diseases such as Alzheimer's disease (21) and dementia.  Researchers at Beijing Pedagogical University found that air pollution can significantly reduce the cognitive abilities of adults. Older people in particular showed impairment comparable to a year's reduction in schooling (15). An experiment at Maastricht University showed that chess players made concentration mistakes significantly more often when the concentration of particulate matter increased. The probability of making a mistake thus increased by a whole 26% for a 10 µg/m3 increase in particulate matter PM2.5 (16). Research in London showed that people living in areas with NO2 concentrations above 41.5 µg/m3 were at significantly higher risk of developing dementia than people living in areas with less than 31.9 µg/m3 of NO2 (17).


Outlook for research

The direct causal link between air pollution and disease is still difficult to establish, despite increasing research. This is mainly due to the fact that it is difficult to observe the processes in vivo, i.e. in the living affected tissue, over long periods of time and that research is usually carried out under laboratory conditions. For example, although the effects of particulate matter can be studied directly in the tissues, it remains difficult to clearly attribute conditions such as lung or cardiovascular disease to air pollution and identify it as the main cause (8, 11, 17, 20). This is mainly because genetic predispositions and environmental factors, such as lifestyle and nutrition or stress, have an influence on the suffering of those affected. Precise figures on the effects of air pollution are therefore difficult to obtain and, in terms of both illness and death, there are usually only estimates (2, 19).

Nevertheless, as described here, there is clear research evidence that particulate matter has a wide variety of mechanisms that toxify cells, destroy their membranes and encourage them to produce a wide variety of substances. It is therefore clearly linked to various diseases. In addition, the empirical research projects described here show a significantly increased incidence of various diseases in populations exposed to high levels of air pollution.

Research institutions have now even begun to investigate the effects of particulate matter on other living organisms. The results are hardly surprising: what is harmful to humans also endangers other organisms. For example, increased bee mortality has now been linked to poor air quality (18). So particulate matter could not only have a direct impact on our health, but could also indirectly influence factors in our lives, such as our diet. The extent of this influence is a question for research. In any case, it is already clear that, given the range of diseases and ailments, both physical and mental, and the enormous number of people exposed to poor air quality on a daily basis, air pollution seems to be the mammoth task of modern medicine.


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