Lesson 2: Stress, Pollution, and Inequity

There are 4 invisibilised enemies of urbanisation; air pollution, noise pollution, light pollution and thermal pollution. They are pollutants as they are man-made additions to an environment that have a harmful effect.

Air pollution is particularly important because as we breathe in the oxygen necessary to sustain life, we also breathe in harmful pollutants that enter our lungs and bloodstream to then travel through the whole of our system where they reach, virtually, all our cells (source). Pollutants directly damage our lungs and other organs, cause systemic inflammation (which can reduce the immune response), and initiate a stress response which, if chronic, can lead to subsequent damage to our bodies. Acute and chronic exposure to air pollution, therefore, weakens our whole system and depletes our body of resources that are essential to combat additional stressors, such as SARS-CoV-2, the virus that caused the Covid-19 global pandemic.

Noise pollution is a pollutant that is manageable in moderation when in context. People specifically experience the negative health outcomes of noise when it disrupts their sleep and ability to recover or focus and causes acute sense of panic when coming from an unknown source, such as a loud bang. The research around noise pollution shows that noise can trigger both the endocrine and autonomic nervous system responses, with sustained activation becoming a potential risk factor for diseases. The infrequent plane flying overhead will not be a likely contributor to this risk, but exposure to high traffic, construction sites, noisy machinery, and proximity to active train stations will add to risk.

Light pollution refers to artificially produced light, particularly in urban environments at night. This phenomenon is often referred to as light at night (LAN) or artificial light at night (ALAN) and is particularly important to people such as shift-workers who may not be able to access enough natural daylight due to working hour restrictions. Artificial light disrupts the circadian rhythms of humans and wildlife alike as well as lowering melatonin production, which results in sleep deprivation, fatigue, headaches, stress, anxiety, and other health problems. Furthermore, LAN has been associated with increased breast cancer incidents in shift-working women (source).

Thermal pollution is going to be a factor of increased importance due to climate change. Factors such as building materiality, ground surface materiality, machine based activity and the relation to the natural environment influence what’s called Urban Heat Island effect. This means that urban areas are artificially hotter than non-urban areas and cannot regulate heat by themselves. For example, concrete retains heat over night, meaning increased exposure to heat for people without an ability to mitigate their exposure. Heat stress is associated with elevated heart rates which can lead to heat stroke, cardiovascular issues, and a loss of sodium and potassium which is all of particular risk for those with weakened hearts, often the elderly. Urban heat islands generally have reduced air and water quality due to the industrialisation, transport, and density, so it is normal for heat pollution to rise with air pollution.

The 5th and arguably as important element to the 4 environmental stressors is the impact of psychosocial stress. It is a widely recognised and researched field of investigation. Two North American scientists act as the base for our use of it.

As a neuroendocrinologist, Robert Sapolsky has focused his research on issues of stress and neuronal degeneration. Sapolsky’s work looks into factors such as non-life-threatening stressors, such as constantly worrying about money or pleasing your boss, and how they trigger the release of adrenalin and other stress hormones, which, over time, can have devastating consequences to your health; "If you turn on the stress response chronically for purely psychological reasons, you increase your risk of adult onset diabetes and high blood pressure. If you're chronically shutting down the digestive system, there's a bunch of gastrointestinal disorders you're more at risk for as well."

In the early 1990s Dr. Arline T. Geronimus identified that there were disparities between her fellow white people of North America and African-American women. In 1992 she coined the term “weathering hypothesis”, which was proposed to account for early health deterioration as a result of cumulative exposure to experiences of social, economic and political adversity. It is well documented that minoritised groups and marginalised communities suffer from poorer health outcomes. This may be due to a multitude of stressors including prejudice, social alienation, institutional bias, political oppression, economic exclusion and racial discrimination. The weathering hypothesis proposes that the cumulative burden of these stressors as individuals age is "weathering," and the increased weathering experienced by minoritised groups compared to others can account for differences in health outcomes.

A stressor is defined as a novel threatening environmental agent that alters the baseline human biological system in either of two ways: bringing the system to an unstable biological state, or slowing down the system’s internal response so that it cannot reach equilibrium (balance) [source]. For clarity, environmental stressors can be both internal and external, such as infections and emotional stressors, or extreme weather conditions and trauma respectively [source, source]

In order to provide an adequate response to stressors, the body engages the stress response, a cascade of physiological reactions that act at two different time frames.

• Firstly, a short-term response, also identified as ‘fight or flight’, allows the body to respond immediately to life-threatening events. This is achieved via a complex communication between the brain and endocrine, cardiovascular, immune and digestive systems [source]. 

  • This system has helped humans survive millions of years allowing our ancestors the ability to quickly evade a predator like a tiger. From a modern perspective, it allows us to quickly jump out of the way of moving vehicles and other related phenomena.

• Secondly, a long-term response occurs when the stressor persists for a significant amount of time, often preventing the body from returning to baseline biological parameters, thus posing a serious risk to health [source].

In this frame, we define two types of stressors: an acute stressor is one that lasts for minutes or hours and results in the short-term stress response; a chronic stressor is instead one that persists for days, weeks, months or years, leading to the long-term stress response [source].

The biological pathways involved in the stress response fall into two categories, namely the hypothalamic-pituitary-adrenal (HPA) axis and the autonomic nervous system (ANS). The malfunctioning of the two pathways is a leading cause of many pathologies, such as autoimmune diseases [source, source, source, source].

First, let’s start with defining allostasis: “achieving stability/homeostasis through change”. 

Chronic activation of the HPA-axis due to constant psychosocial (e.g. financial insecurity) and environmental stressors (e.g. air pollution), combined with maladaptive behaviours such as poor sleep due to shift work, can impede the allostatic process - achieving a biological balance. 

This chronic activation of the HPA-axis creates a process called ‘allostatic load’, which is a type of “wear and tear” on the biological systems that communicate with the stress response. This results in increased systemic inflammation, and compromised immune and metabolic systems. 

The body enters an allostatic state when allostasis has been active for a long period of time. Being in an allostatic state comes at a cost to the body. The physical repercussions of allostatic load manifest in a general sense of being ‘stressed out’, resulting in fatigue, anger and frustration among others [source, source, source].

A prolonged dysregulated condition of allostasis often results in an extreme state for the body, called allostatic overload [source].

Allostatic load and allostatic overload are states in which the system enters a dysregulated stress response. Non-communicable diseases have been shown to be associated with allostatic overload [source]. For instance, the following disorders are linked:

• Cardiovascular diseases, such as coronary or ischemic heart disease and peripheral arterial disease

• Type II diabetes

• Musculoskeletal disorders, such as chronic fatigue syndrome and fibromyalgia

• Cancers, such as breast and ovarian cancer

• Depression, anxiety and PTSD

Exposure to stressful environments and events have been shown to dysregulate the stress response, often leading to disease onset. For instance, exposure to chronic stress was shown to be linked to higher risk of dementia as well as autoimmune diseases and mental health disorders [source, source]. Risk of disease also increases with higher numbers of stressors, which act in a synergistic manner to further and more powerfully disrupt the proper functioning of the stress response [source].

Thus, in order to better understand why certain communities are being disproportionately affected by the incidence of non-communicable diseases, it is important to consider the environmental and psychosocial stressors that the community experiences daily, and the structural disadvantage this puts them in compared to others. It is from these considerations that we propose a new term to describe the unequal distribution of stressors and their effects on health in our society: biological inequity.

We introduce a new term called biological inequity to describe the fact that different communities experience very different levels of stress - and as a result very different health outcomes - due to historical and contemporary structural forces. 

Chronic exposure of some communities to poverty, pollution, trauma and other stressors are drivers for biological inequity [source, source]. In this framework, it is not the individual’s genetic makeup that is the indicator of health, but the environment and daily experiences they are constrained to live in. That is not to say that genetics play no role in determining health. Rather, it means that biological inequity is a more encompassing explanation for differences in health outcomes. For instance, two women carrying the same gene for breast cancer but living in different environments will have different health outcomes: it is more likely that one living in polluted areas and experiencing daily stress or trauma will experience cancer onset, while the other living in a clean environment might not.

Moreover, the mechanisms of biological inequity are more unbiased than traditional epidemiological explanations of health. Whilst traditional epidemiological research has often proposed that genetic distinctions between ethnicities are the driver of poor health outcomes for certain minoritised communities [source, source, source], biological inequity proposes that the systemic factors that force certain communities to live in environments with higher stressor levels than normal, thus resulting in poor health outcomes. It is not a matter of being Black, or Brown, or of a racial identifier that predisposes someone to poorer health outcomes, it is the lived experience from their habitat that is the driving factor.

From this perspective, when it comes to the conversation of ‘genetics’ it is more accurate to talk about the epigenetic differences between ethnicities. Previous generations who experience allostatic load/allostatic overload - a permanently dysregulated stress response due to the environment - can cause epigenetic changes that are transmitted from parent to offspring, becoming hereditary [source, source, source].