A wave of research in the past few years shows a positive correlation between the ongoing climate change and increases in prevalence and severity of asthma and other related respiratory allergic diseases. Increasing temperatures disrupt normal pollen production, worsen ground-level ozone pollution, increase ambient air pollution and alter climate patterns, resulting in storms and wildfires, all of which contribute to asthma. The links between climate and the above conditions are supported by numerous studies.
The WHO estimates that each year, 300 million people worldwide suffer from asthma (1). In the United States alone, 20 million people have active asthma, including 6.2 million children under 18 (2). It is hoped that stronger measures will be taken to reduce greenhouse gasses and other factors that lead to global warming.
Mechanism of Allergic Asthma
Lithographic plate from Gray’s Anatomy of the bronchi and bronchioles. During an asthma attack, the bronchioles constrict, and breathing can become extremely difficult. Severe attacks can be life-threatening.
Asthma is a chronic respiratory disease that inflames and narrows the airways, leading to symptoms including wheezing (a whistling sound with each breath), chest tightness, breathlessness, and coughing (3). There are two main categories of asthma, nonatopic and atopic, with the latter comprising the majority of asthma cases (4). Atopic asthma involves T helper 2 cells (TH2), which drive hypersensitivity to innocuous antigens (also known as allergens). It is therefore commonly referred to as allergic asthma (5). While genes like IL13, IL4RA and filaggrin have been found to be positively associated with asthma, there is much evidence that environmental factors play a substantial modifying role (6).
The mechanism behind asthma is complex, involving airway inflammation, intermittent airflow obstruction and bronchial hyperresponsiveness (7). This article focuses on the basic mechanism behind allergic asthma, but from what is known, the mechanism of nonatopic asthma is very similar (8).
The initiation mechanism for allergic asthma requires that immunoglobin E (IgE) antibodies react with allergens such as dust particles and pollen grains. This sensitizes respiratory mast cells, found in connective tissue, which release substances like histamine in response to inflammation of body tissues (9). Later inhalation of allergens leads to interaction of epitopes with cell-bound IgE and activation of secretory pathways that release histamine, leukotrienes, prostaglandin, platelet activating factor (PAF), and a range of cytokines and chemokines that all mediate the inflammatory response (10). There is an immediate “early-phase” allergic response of one to twenty minutes with vasodilatation and bronchoconstriction, followed by a slower “late-phase” response of four to eight hours with cellular infiltrate, including TH2 lymphocytes, eosinophils, monocytes, and basophils, and a greater sustained increase in bronchospasm, even if the allergen is no longer present (11).
Climate Change Alters Vegetation
Vegetation, particularly pollen grains, has long been linked to allergic reactions, particularly asthma. Pollen proteins like Amb a1 are well-recognized causes of TH2 immune responses. Recent studies have identified lipid components in pollen that might modulate TH2 immune responses as well (12). In fact, Jan Gutermuth of Division of Environmental Dermatology and Allergy at Technische Universität München in Munich showed that aqueous extracts of white birch pollen, instilled intranasally into mice increased TH2 immune responses (13).
How does climate change affect vegetation, and what role does this play in worsening asthma?
Vegetation changes have proven to be very sensitive indicators of climate change. In transitional zones, vegetational responses can occur within a decade of climate change (14). For many herbaceous and woody plants, flowering and pollination are intimately linked to temperature. Flowering speeds up with global warming, due to both higher temperatures and higher carbon dioxide (CO2) levels. A study of 385 British plant species conducted in 2002 found that during the past decade, the average first flowering had advanced by 4.5 days (15). Furthermore, in Switzerland and Denmark, there has been a distinct rise in the annual quantity of hazel, birch, and grass pollen over the past 30 years (16, 17). Increasing pollen production and longer pollen seasons due to earlier blooming increase the burden of asthma and other allergic diseases.
An increased global temperature means an increase in greenhouse gases, predominantly CO2, which increases the allergenicity of plants. One study showed that high CO2 levels on poison ivy increased photosynthesis and biomass and that these CO2-enriched plants produced a greater percentage of unsaturated urushiol, which is one of the antigenic products that tend to worsen or induce asthma (18). Another study of ragweed pollen showed that increased pollen production implies an increase in airborne allergenic load. Four sites, namely urban, suburban, semirural and rural, were studied, using the urban environment as a surrogate for climate change. The urban area was around 2°C warmer with a 30 percent higher CO2 level than the rural site, and as expected, urban ragweed grew faster with a greater above-ground biomass, flowered earlier and produced more pollen than the rural site (19). There was over a 7-fold increase in pollen production from the urban sites, indicating an increased airborne allergic burden in the urban model that represents a warmer climate change.
The evidence points to the fact that climate change has had and will have further impact on a variety of allergenic plants (20). Increased temperature stimulates earlier flowering and longer pollen seasons for some plants and increased CO2 increases plant biomass and pollen production and may cause plant products to be more allergenic.
Satellite image of Hurricane Ivan making landfall near Alabama in 2004. Climate change will likely increase the frequency and severity of storms, leading to increased levels of allergy-laden pollen.
Climate Change Increases Ground Ozone
Tropospheric (ground-level) ozone is formed by a heat dependent photochemical oxidation of volatile organic compounds (VOCs), nitrogen oxides (NOx), and atmospheric hydroxyl radicals. Even without these precursor molecules, higher temperatures increase ozone production (21, 22). In urban areas, anthropogenic nonmethane VOC compounds from combustion of fossil fuels including vehicle exhaust and industrial emissions are key contributors to ozone production.
Acute ozone exposure is known to decrease pulmonary function, increase airway hyperresponsiveness (AHR), and induce airway inflammation (23, 24). Exposure of mice to both ozone and carbon particles for four hours had a synergistic effect, significantly decreasing alveolar macrophage phagocytosis and increasing lung neutrophilia (a condition marked by a abnormally large number of neutrophils, a type of white phagocytotic blood cell). A mechanism for the enhanced effect may be that the carbon particles act as carriers for the ozone, bringing it into areas of lung not easily accessible to ozone in the gaseous phase. Alternatively, the ozone may change the composition of the carbon particles from an innocuous to a harmful form (25).
Michelle Bell of Yale University modeled temperature-dependent ozone pollution up to the year 2050 for fifty US cities, assuming constant anthropogenic emissions, and found a 2.1 percent increase in asthma hospitalizations across all cities (26). The increase of urban development worldwide and the continued use of fossils fuels will lead to greater ozone exposure in the future, increasing the number of asthma cases.
Climate Change Worsens Air Pollution (and Vice Versa)
The 1970 Clean Air Act (CAA) in the United States identified six criteria air pollutants (CAPs) and the 1990 amendments to the CAA defined 188 hazardous air pollutants, and set standards to protect human health and the environment. As the climate warms, pollutants increase, and as pollutants increase, climate warms even more, thereby escalating the disease burden related to allergy and asthma worldwide.
During high temperature combustion, oxygen reacts with nitrogen to generate nitric oxide (NO) and to a lesser extent, nitrogen dioxide (NO2) and other nitrogen oxides (27). Nearly half of all NOx emissions come from motor vehicles in the United States. Even though NOx emissions are relatively short-lived (only hours to days), even short-term exposure is associated with chronic and acute changes in lung function, including bronchial neutrophilic infiltration, increased proinflammatory cytokine production, and enhanced response to inhaled allergens (28, 29). Continuing the status quo of NOx emissions will lead to continued increases in ground level ozone, and increased allergen sensitivity.
Sulfur dioxide (SO2) is a known respiratory irritant. Over 65 percent of SO2 emissions in the United States comes from coal-burning electric utilities (30). Because SO2 is fifty times more soluble than CO2 in water, it is likely to be absorbed in the upper airways in subjects at rest and increasing ventilation results in deposition in deeper parts of the lung. Shortly after exposure, inhalation of SO2 causes significant bronchospastic effects with rapid onset of symptoms. Most individuals with asthma experience bronchospasm at levels of 0.5 ppm (28). Since the burning of coal is currently the second largest global fuel source of SO2 emissions and is predicted to be the first by 2010, coal burning will contribute substantial amounts of atmospheric sulfur oxides in the future, exacerbating asthma and other respiratory diseases and promoting additional climate change.
Another form of air pollution is atmospheric particulate matter (PM), from both natural and human sources, the latter particularly in suburban and urban areas where diesel fuel-burning vehicles are mainly used (31). Many studies have shown that increased exposure to PM worsens asthma and is linked with decreased lung function in both children and adults (33, 34).
The impact of air pollution on asthma and allergies was examined in a cross-sectional study analyzing the long-term exposure to background air pollution related to respiratory and allergic health in schoolchildren (34). The study involved 6,672 children from ages nine to eleven who underwent a clinical examination including a skin prick test (SPT) to common allergens, exercise-induced bronchial reactivity (EIB), and skin examination for flexural dermatitis. The prevalence of asthma, allergic rhinitis (AR) and atopic dermatitis was assessed by a standardized health questionnaire completed by the parents. Using measurements from background monitoring stations, three-year-averaged concentrations of air pollutants (NO2, SO2, PM and O3) were calculated at the 108 different schools. The results demonstrated that a moderate increase in long-term exposure to background air pollution was associated with a significant increase of respiratory diseases in the children (34).
Thus, pollution contributes to climate change, which in turn leads to more pollution and more asthma cases worldwide, propelling the continuation of a vicious cycle.
Climate Change Induces Wildfires and Storms
In addition to the alteration of pollen production and worsening of air pollution, the changing climate will likely lead to increased drought, heat waves, and wildfires in some areas, and increased storms and extreme precipitation events in other areas (35). These changing regional patterns may further exacerbate allergic disease and asthma. Wildfires emit smoke that contains a concoction of carcinogenic and respiratory irritant substances, including CO, CO2, NOx, ozone, PM, and VOCs (36, 37). Epidemiologic studies have shown modest short-term increases in cardiorespiratory hospitalizations resulting from acute exposure to wildfire smoke (38, 39).
The Intergovernmental Panel on Climate Change (IPCC) considers it very likely that there will be an increase in heavy precipitation and tropical cyclones events globally (40). Places where climate change causes heavy precipitation during pollen season are expected to worsen asthma by an increased airborne burden of respirable allergen-laden particles released from fragmented pollen grains (41). Therefore, climate change induces wildfires and storms, which further exacerbate asthma cases worldwide.
Conclusion
The changing global climate is negatively affecting human health. In this review, the adverse effects of a warmer Earth are discussed with a focus on asthma and allergic respiratory diseases in both children and adults. Increased antigenic pollen grains, ground-level ozone pollution, and air pollution as well as dynamically changing climate patterns are all factors that contribute to asthma. The rate of climate change in the future will depend on how rapidly and successfully global mitigation and adaptation strategies are implemented. Hopefully, a global effort will develop to work towards reduction of the causes of anthropogenic climate change as part of a long-term commitment to protect public health.
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