Allergies in the Anthropocene: A Planetary Health Problem

Joyce Hu, Yale-NUS College

Context of Allergic Diseases as a Planetary Health Issue

Allergic reactions are abnormal adaptive immune responses to both biological (e.g. pollen, spores) and non-biological allergens such as fine particulate matter. A few of the most common allergic diseases worldwide include allergic rhinitis (hay fever), asthma, and atopic dermatitis (eczema).

The prevalence of these allergic diseases has been increasing globally over the last few decades. While improved diagnosis has indeed contributed to such observable increase, this can only partially explain the unprecedented rise in allergies. While genetic dispositions influence the occurrence of allergic diseases, a key factor thought to be causal to allergic reactions and diseases is various environmental exposures, such as indoor and outdoor air pollution. Recent studies have begun to demonstrate a likely link between the increasing prevalence of allergic diseases and the anthropogenic impact of humans.

The Anthropocene refers to the current era where humans exert a pervasive impact on the environment. Both industrial and everyday lifestyle factors linked to urbanisation and modernisation are associated with allergic diseases (among other impacts to human health), with air pollution identified as posing the most immediate threat to human health from anthropogenic energy production.

This article thus focuses on the effect that anthropogenic factors and lifestyles have on climate change, and how such climate change exacerbates the impact of allergens by increasing the prevalence of hay fever, asthma, and eczema as a planetary health problem.

Anthropogenic Impacts Exacerbating Allergic Diseases

Seasonal variation for allergies already exists by nature of the pollutants and factors causing these conditions. This section explicates how anthropogenic impacts exacerbate the existing nature of environmental factors on contributing to allergic diseases and the consequences of this on the prevalence, distribution, and severity of these diseases.

Urbanisation and Other Anthropogenic Factors

Urbanisation refers to the demographic shift of populations from rural to urban areas, marked by the rapid development of urban areas in the 21st Century. A variety of primary and secondary environmental processes are associated as direct and indirect impacts of urbanisation, respectively. Primary impacts include deforestation (i.e. the direct removal of vegetation), linked with the development of infrastructure (i.e. city-building), and the associated pollution generated as a result. These processes lead to secondary impacts of habitat loss and degradation (including loss in biodiversity), associations with climatic variability (e.g. seasonal and long-term changes in temperature, humidity), among many other factors. Increasing air pollution, including fine particulate matter, is linked to increasing prevalence in allergic diseases.

Further, many lifestyle factors associated with urbanisation also act as anthropogenic environmental impacts. At its core, modern human societies, particularly urban societies, rely on energy. Global energy consumption more than doubled between 1973 and 2016 and only 4% of energy comes from renewable sources. Popular and easy energy sources such as coal, oil, and gas are not only non-renewable — thus acting as irreversible environmental extractions — but are also toxic and impose both direct and indirect harms to human health such as through emissions and pollution. The synergistic effect of this large-scale energy reliance combines with the usage of and reliance on toxic pollutants in urban, modern lifestyles such as cleaning products, adhesives, paints, heaters, aerosols, and gas cooking. Domestic and industrial emissions foreground fossil fuel combustion as the foremost anthropogenic source of atmospheric carbon dioxide emissions, followed by deforestation. These two key causes have significantly contributed to the rising greenhouse gas emissions at the core of the unravelling anthropogenic climate change crisis in the present day. Deforestation also has flow-on indirect impacts on allergies due to the influence and mediating effect of tropical rainforests on pollen spread.

The combined impacts of urbanisation and modern urban lifestyles therefore play a direct role in contributing to climate change overall, and thus an indirect role in the proliferation of allergic diseases.

Overall Effects of Climate Change on Allergic Disease Profiles

Anthropogenic impacts of fossil fuel emissions, agriculture, industrial, household, and traffic pollution have rapidly increased since the 19th-20th centuries, following fast technological advancements and the Industrial Revolution. Greenhouse gas emissions, particularly carbon dioxide, have led to spatial and temporal changes in climate, notably resulting in increases in temperature and humidity.

Carbon dioxide has a fertilising effect, which combined with increasing global temperatures, has been linked to increased pollen production as well as increased allergen content in pollen grains. That is, increased atmospheric carbon dioxide concentration becomes a fertilizer and stimulant for plant growth. Higher airborne pollen concentration also increases the chance of sensitisation leading to the exacerbation of illness and potential for more serious allergies among patients. Further, changing (in this case, increasing) temperatures alter the timing and duration of the pollen season. For example, in some cases, the increasing length of spring and summer seasons associated with greater pollen output increases the duration of seasonal allergies. Additionally, changing regional climates in local contexts results in changes in atmospheric circulation which blows pollen and spore-containing dust into new areas. People living in areas previously unexposed to allergens now experience aeroallergens and allergies for the first time.

Moreover, aside from the gradual changes in long-term everyday climate, changes in global climate have also influenced increasing frequency and intensity of extreme weather events such as floods, droughts, tornadoes, and hurricanes — all of which have a range of repercussions for human health. Notably, an observable association between increased asthma attacks and extreme weather events has led to the phenomenon coined “thunderstorm asthma”, wherein the rates of asthma attacks (measured in hospital admissions, mortality, morbidity or otherwise, in various studies) in both pre-existing asthmatic patients and for people who had previously never experienced asthma (only rhinitis) were significantly higher than usual during thunderstorm events. The most serious case of thunderstorm asthma occurred in Melbourne, Australia in November 2016 where over 3500 patients were admitted to hospitals for asthma, with 10 mortalities as well a large cohort of patients who suffered attacks that were not admitted to hospital. Thunderstorm precipitation and winds are hypothesised to be associated with the conditions required for pollen grains to rupture and release aeroallergen matter at a micro-scale small enough to enter lower airways. While this topic is still not fully understood, current literature issues a warning that as extreme weather events increase, incidences of allergies are likely to also increase even in terms of extreme one-off events.

Overall, research thus far indicates that the increased prevalence, intensity and distribution of human exposure to aeroallergens in the environment exacerbated by urbanisation and anthropogenic actions is associated with the quantitative and qualitative changes in human reactions to allergies e.g. increasing severity of symptoms and number of affected individuals.

Outlook: The Need for Further Research

Planetary health is an emerging academic field of great importance. While allergic diseases have a more ‘obvious’ causal link with environmental factors by nature of environmental allergens, the exacerbation of these illnesses with the onslaught of anthropogenic impacts and factors such as urbanisation and air pollution is still under-studied. Further research is required to gain a more comprehensive, causal understanding of the impact of climate change on the increase in and spread of allergens and how this interacts with and exacerbates allergic diseases among humans.

Images sourced from Unsplash

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