Air Pollution, Susceptibility, and COVID-19 Learnings.
by Sarah Aliko, PhD Computational Neuroscience UCL, BSc Hons Molecular Biology
July 2020
Most epidemiology studies are based on population-level or cohort statistics, drawing conclusions from measures of central tendency such as mean and median calculations.
The results of such studies are often used to provide guidelines and thresholds for clinical applications and health advice. For instance, the WHO provides guidance for global/governmental health bodies, such as Public Health England and the Environmental Agency, on acceptable levels of air pollution based on epidemiological evidence, which are meant to preserve and safeguard global health. The WHO guidelines are obtained through analysis of global trends and averages, and are often applied as absolute thresholds and set as goals of air pollution levels.
However, the reality is much different from the one presented by health organisations. In any given area, there will be people who are suffering greatly from the consequences of air pollution whilst others may not see any consequences. This phenomenon is worth understanding, rather than dismissing it as not statistically significant. When this happens three things can occur;
We fail to understand the relationship between a person’s health and air pollution.
Could turn a manageable health challenge into a health crisis as time progresses.
Current Thresholds
The health risk thresholds, delineated by the WHO and other bodies, are averages across normative and immuno-compromised populations: since the latter has a smaller sample size, the normative population significantly skews the results in favour of less restrictive measures and higher thresholds. Vulnerable populations are masked and not represented by the central tendency data. This leads to a severing of the symptoms and conditions of immuno-compromised and vulnerable communities - such as those living in high pollution and high inequality areas - which we are currently experiencing in urban and suburban areas.
It is thus imperative for health organisations to provide guidelines based on the specific population under investigation: what is the susceptibility to health hazards of the individuals in the community?
Susceptibility refers to the effects of biological inequality on the human immune response, specifically the fact that persistent environmental stressors on a community place these individuals at a heightened risk of developing severe symptoms and chronic illnesses compared to a normative population, not exposed to significant stressors.
Saez and Lopez-Casasnovas (2019) presented a comparative study on the susceptibility of a deprived vs wealthy neighbourhood in Barcelona to pollution. The results indicated that the deprived neighbourhood was significantly more at risk of dying (30% higher probability) from environmental health hazards, such as air and noise pollution, than the wealthy neighbourhood.
This increased susceptibility was due to a mixture of other psychosocial stressors that the deprived neighbourhood was already experiencing: low income and poverty, low quality of housing and low access to services. Another study (Rosa et al., 2019) showed compelling evidence of a relationship between prenatal stress and onset of asthma in children.
Children born to mothers who experienced high psychosocial stress (eg. divorce, health concerns etc), were significantly more at risk of developing asthma later on in life; the risk further increased when the mothers were also exposed to higher air pollution levels (Rosa et al., 2018).
Expanding on this, Landeo-Gutierrez et al. (2019) argue that certain types of psychosocial stressors, such as exposure to violence and crime, increased the risk of onset of asthma, especially when in co-presence with air pollution. In particular, they found a strong correlation between post-traumatic stress disorder (PTSD) and asthma onset.
Populations exposed to high levels of crime and poverty often develop PTSD, and they are also more likely to be in low-quality living environments and high air pollution, thus further increasing their risk of developing comorbidities such as asthma. Finally, a study (van der Valk et al., 2018) on obesity and chronic stress identified a feedback loop that takes place during onset of disease. Individuals who have disrupted sleep or shift work, who have a poor diet due to low income and poverty, and individuals suffering from mental health disorders, are more at risk of developing obesity because of their high stress levels.
However, onset of obesity further increases stress levels (in particular cortisol), leading to an even higher susceptibility to comorbidities such as cardiovascular diseases. This vicious cycle is a constant interplay between a poor environment and a burdened biological system.
The Biological Mechanisms of Susceptibility
At the crux of this theory is that when the body is faced with stressors it adapts through a process called allostasis, which means “achieving stability/homeostasis through change”. Chronic HPA-axis activation due to constant psychosocial (financial insecurity) and environmental stressors (air pollution), combined with maladaptive behaviours such as poor sleep due to shift work, can impede the allostatic process.
This creates ‘allostatic load’, which is “wear and tear” on biological systems that communicate with the stress response, resulting in increased systemic inflammation, and compromised immune and metabolic systems. In the long run, this can leave people more susceptible to the health risks of environmental toxins like air pollution.
It is also important to note that related to susceptibility is biological degradation. Through the stress response pathways long term exposure to air pollution can have far reaching microbiological effects, which in turn make people more physiologically susceptible to the effects of air pollution.
There is growing evidence suggesting that stress may alter permeability of bodily membranes to chemical exposures, such that stress may “alter systemic transport and chemical uptake into organs including the brain, facilitating combined and synergistic effects of stressors and air pollution on many bodily”.
Variability
It must be noted, however, that not all people living in a deprived environment and continuously exposed to stressors, will develop disorders and severe symptoms: even within a vulnerable population there is variability in the outcome of exposure to stressors. This variability is often overlooked by health authorities, leading to average measures being reported within a vulnerable community, and inappropriate guidelines being detailed.
In Practice
We can use the understanding of susceptibility to identify the communities and areas of cities that are the most vulnerable to metabolic and mental disorders.
In terms of the most recent pandemic it would have helped understand which areas would be worse hit by COVID-19. This is especially relevant as COVID-19 can cause respiratory failure, therefore identifying which areas have both a larger population with underlying health conditions and high air pollution could have helped reduce cases by protecting those communities. The protection should have been in the form of setting emergency air pollution guidelines specifically for that area, community level education about COVID-19, special quarantine regulation for them, and economic bailouts so they could adhere to said regulations.
In the near future this can be used to help with the COVID-19 recovery effort. It will be areas identified as susceptible that will have a longer recovery process due to being exposed to higher levels of air pollution. Especially if nothing is done to bring down air pollution once we have the green light to resume normal life.
Health organisations should, therefore, investigate in-depth the environmental and psychosocial stressors that a native population is experiencing in order to appropriately determine health hazards and thresholds of the most susceptible individuals within that community. A rule of thumb is that there is no “safe” amount of air pollution, and health authorities should strive to represent the most vulnerable individuals in society first, rather than assume a normative population across all reports or even average across a whole community.
We propose that guidelines on environmental pollutants be revised in terms of the susceptibility of the community to which they are applied to.