Depression as a Brain-Body Disease and its Links to Air Pollution

Depression is more than a mental disorder—it is a physical disorder too.

Depression is typically understood as a mental disorder characterised by feeling sad, irritable, and empty, and by a loss of pleasure and interest in activities—a ‘mood disorder’ (source). Physical symptoms, such as pain, weakness, and appetite loss, are often mentioned as ‘other’ symptoms, giving them a less prominent role. This view, however, is misleading, as physical symptoms often are a major part of the symptomatology of depression (source). In fact, many patients who meet clinical criteria for a diagnosis of depression only report physical symptoms (not emotional symptoms) as the reasons for seeking treatment at primary care centres (source). This suggests that physical symptoms can be the dominant reason for suffering in individuals with depression. Some scientists would even refer to chronic pain as a ‘variant of depressive disease’ (source).

Common physical symptoms of depression are pain, weakness, appetite loss, and gastrointestinal symptoms, all of which can have a great negative impact on the individual’s well-being, quality of life, and capacity to manage their everyday life. The experience of physical symptoms—pain in particular—is a predictor of the severity of depression (source). However, as physical symptoms are often not perceived as key symptoms of depression and because they are associated with physical illnesses too, a person suffering from depression may not receive the correct diagnosis (source). Similarly, when a person is rightly diagnosed with depression, they may not be treated for their physical symptoms, which means they may not achieve full remission (source). It is therefore crucial that we understand that depression is more than just a mood disorder; it has a complex symptomatology that includes a range of mental and physical symptoms, and every individual experiences a different combination and severity of these symptoms.

From a neurobiological perspective, the link between mood and physical symptoms, such as pain, makes sense. Both mood and pain are regulated in the brain by serotonin and norepinephrine. An imbalance of these two neurotransmitters is linked to both depressed mood and pain (source/source). The neurobiological pathways are complex and include several regions in the brain, several parts of the central nervous system, and several neurotransmitters; however, the neurobiological link between depression and pain has been reported repeatedly (source). One of the main questions that remains is about causality: ‘Although increasing evidence indicates that the relationship between both clinical conditions might be bidirectional, the question remains whether the specific neurobiological changes cause either pain or depression or are merely a consequence of each condition.’ (source)

The take-away message from this section is that mental and physical symptoms in depression, and any other disorder, cannot simply be separated. Mental and physical pathways of disease are intertwined—they always work together.

Air Pollution and Depression: A Possible Relationship

As depression is a global issue, air pollution too is a global problem: ‘WHO data shows that almost all of the global population (99%) breathe air that exceeds WHO guideline limits containing high levels of pollutants, with low- and middle-income countries suffering from the highest exposures.’ (source) At Centric, we have highlighted repeatedly that air pollution poses a great threat to the health of people and the planet. We have also highlighted that ‘safe’ thresholds, such as the ones used by the WHO, are misleading. Ultimately, any pollution is too much pollution, and we need to move towards a zero tolerance for air pollution.

For a long time, air pollution has been linked to respiratory and cardiovascular diseases, such as asthma and stroke. However, it is now understood that the effects of air pollution are not limited to respiratory and cardiovascular systems, but that pollutants affect every organ and virtually every cell of the human body (source). There is accumulating evidence from epidemiological studies, suggesting a relationship between exposure to air pollution and mental disorders, including depression. It should be noted, however, that these studies have limitations and that the evidence is not yet conclusive (source/source).

Although it is sometimes difficult to identify a robust link in epidemiological studies, we can take a neurobiological perspective: How does air pollution affect the brain? First, it is important to note that different pollutants (i.e., gases and particulate matter) affect the human system differently. Second, we must acknowledge that biological pathways are very complex and include a multitude of steps. We will not describe these steps in detail, but we want to highlight that there are several ways in which air pollution can affect the brain. In a 2009 paper (source), scientists distinguished four pathways:

1) air pollution causes systemic inflammation (in turn, affecting the brain);

2) particulate matter enters the brain;

3) particulate matter carries additional toxic compounds on its surface into the brain; and

4) ozone causes oxidative stress (in turn, affecting the brain). It is important to note that the specific mechanisms are still not well understood.

Nonetheless, there is agreement that air pollution does affect the brain, not only the adult brain but, crucially, the developing brain too (source).

Air Pollution and Depression: The Ethical Problem

Depression is a condition that means great suffering for the individual, both mentally and physically.

It is of great interest for the individual and society to find causes of and effective treatments for depression. Often, people suffering from depression are asked to sleep more, eat healthily, and engage in physical exercise. In other words, they are told to make lifestyle changes, to be healthier. At the same time, they are forced to breathe polluted air that makes them sick and can exacerbate their symptoms.

This is a great ethical problem, especially considering that those who contribute the least to (global) pollution typically suffer the most from pollution. People do not have control over a factor that they are exposed to constantly and that takes a toll on their health..