Allergic & Respiratory Disease and Climate Change
Climate change is not something that might happen in the future. It is happening now – since the middle of the 20th century, Australian average temperatures have risen about 1°C. This has been accompanied by an increase in the frequency of heat-waves, a decrease in the numbers of frosts and cold days, and a redistribution of rainfall from eastern Australia and southwest WA to the northwest (www.bom.gov.au/climate/change).
Last year (2009) will be remembered for extreme bushfires, dust-storms, lingering rainfall deficiencies, areas of flooding and record-breaking heatwaves (BOM). It was Australia’s second warmest year since high-quality records began in 1910. Last year’s annual mean temperature was almost 1°C above the 1961-90 average, with Victoria, South Australia and NSW all recording their warmest July-December periods on record. The decade just ended (2000-2009) was our warmest on record, continuing a trend where each decade since the 1940s has been warmer than that preceding it (BOM 2010). The likelihood that this warming trend is due to anthropogenic causes is over 90% (IPCC 2007).
Anthropogenic climate change is predicted to have a range of effects on respiratory and allergic diseases over the coming decades, most of them negative.
The possible effects are wide-ranging, and predicted to occur in the following areas:
- Extreme temperature events
- Worsening air pollution
- Altered aeroallergens
- Extreme weather events
Extreme temperature events
The globe is warming and will continue to do so over the coming century. This process will continue, even if rapid and marked reductions in carbon dioxide emissions occur soon, due to the inertia of the global climate system. Mean temperatures will continue to increase, as will climate variability (the range of expected maxima and minima). The effect of this two-fold change will be to significantly increase the number of very hot days for many areas (IPCC 2007).
Each 1°C increase above long-term city averages increases all-cause mortality by up to 3%
The health effects of extreme heat are well-known. The record-breaking 2003 European heat-wave, for example, caused tens-of-thousands of premature deaths across several countries, especially France (McMichael et al 2006). Most affected were the elderly, disadvantaged or chronically ill (especially those with respiratory or cardiovascular disease). There is also evidence from a European study that each 1°C increase above long-term city averages increases all-cause mortality by up to 3% (and respiratory mortality up to 6% – Stafoggia et al 2008).
As heat-waves are expected to become more severe, frequent and prolonged as global warming progresses, strategies to reduce the associated morbidity and mortality must become a public health priority (Ayres et al 2009).
A paradoxical effect of increased global temperatures is that winters should become warmer, leading to a modest decrease in cold-related morbidity and mortality .
Worsening air pollution
The relationship between air pollution, climate change and respiratory illness is complex, and more research is needed to allow better predictions to be made. Despite this, enough is known about ground-level ozone, nitrous oxides and particulates to inform future research and mitigation / adaptation strategies.
Ground-level ozone
Ozone is a potent oxidising substance known to have several adverse respiratory effects (Ayres et al 2009):
- Increased new onset asthma
- Decreased lung function
- Exacerbation of COAD
- Allergen sensitisation
- Increased hospital admissions
- Increased all-cause mortality
Ozone (O3) is a component of photochemical smog, produced by the action of sunlight on fossil-fuel combustion products (primarily vehicle exhaust in cities). Its production is increased by warmer temperatures (especially due to the urban heat-island effect), and it is this property that will probably lead to increased ozone formation during the coming century.
Produced by the action of sunlight on exhaust fumes (especially on hot days), high concentrations of ozone found in the Earth's lower atmosphere (troposphere) are hazardous to life.
Although urban areas are most affected, O3 crosses national and even continental boundaries (there is increasing evidence, for example, that Asia is “exporting” its ozone to the USA – Zhang et al 2008). The future effects of this issue are difficult to predict due to warming-induced changes in air circulation and wind patterns.
As hydrocarbon combustion is the main source of ground-level ozone, patterns and levels of future O3 concentrations also become difficult to predict with certainty if carbon-free fuels become dominant.
Other forms of air pollution
Nitrous oxides are produced by high-temperature hydrocarbon combustion (especially in vehicles). They are important, in part, because they produce an enhanced response to inhaled allergens, probably due to bronchial irritation and inflammation (Shea et al 2008).
Small ambient particles are also produced by fossil-fuel combustion. Short-term inhalation of these particles increases cardiopulmonary hospital admissions and mortality, while long-term exposure worsens paediatric asthma and causes higher mortality in adults (Ayres et al 2009). It is difficult to predict future patterns of particle production and exposure, as mitigation strategies (eg electric- or hydrogen-powered vehicles) may significantly reduce their production.
Conversely, expected increases in wildfires, droughts and desertification due to climate change may produce more particles, and they may spread over wider areas (Ayres et al 2009). Wildfires in particular pose many health risks. Apart from the direct threat to life, their smoke may contain plastic and toxic (ie herbicide & pesticide) residues (Shea et al 2008).
Altered aeroallergens
Global warming is already having demonstrable effects on plant behaviour and distribution (IPCC 2007). There has been a general shift polewards and upwards (altitude) of plant habitability zones in many regions, related to warmer conditions. There have also been changes due to altered precipitation patterns and land-use variations.
Flowering times are also changing, with a general trend towards earlier spring flowering (although paradoxically, plants that rely on a longer duration of winter chill to trigger budding are flowering later due to warmer winters).
Additionally, pollen appears to be more allergenic, with higher levels of some core proteins (the molecules mainly responsible for triggering allergy – Rom et al 2008). As respiratory allergies may follow dose-response curves, more pollen + more allergenicity = more severe allergies.
Changes in the timing of the pollen season (longer and earlier) may produce worsening allergies if there is overlapping with peak ground-level ozone production in summer.
Other changes due to global-warming include (Reid & Gamble 2009):
Changed wind patterns may disperse pollens in new directions or further than before.
Changed local conditions may alter dust-mite and mould distributions.
Changed agricultural practices may alter the level of pollens related to farming activity (eg rye grass is a component of farm pasture; the mould Alternaria is related to agricultural production).
Changed species makeup of forests and grasslands (whereby one species is replaced by another due to altered microclimate or species invasion) may also change pollen type and load.
All of these phenomena are having the effect of changing pollen distributions. This exposes new populations to novel allergens that will trigger new-onset allergies, as well as worsening existing ones.
Climate change is happening now, and some of the changes described (such as changes in flowering times and geographical range) have already been documented by the IPCC. We know from paleoclimate research that plants react quickly to environmental changes – ice-core and ocean sediment data show that significant vegetation changes (in response to previous climate disruptions) have taken just a decade to occur.
Extreme weather events
Climate change is expected to magnify the hydrological cycle, causing more frequent drought but also more hurricanes, storms and extreme precipitation events (IPCC 2007). It has been known for some time that thunderstorms may cause asthma exacerbations (Ayres 2009). It appears likely that this is due to the disruption of pollen particles, thereby exposing their allergenic cores. Heavier rainfall has the potential to cause flooding and increased mould growth. Both occurred after Hurricane Katrina inundated much of New Orleans. Following such extreme events, it is likely that population displacement, crowding, drinking water contamination and malnutrition will increase respiratory infections, especially pneumonia, and possibly increase TB transmission (Ayres et al 2009).
More frequent flooding, expected as the hydrological cycle intensifies due to global warming, will have adverse health consequences.
Conclusions
The expected impacts on respiratory morbidity and mortality discussed in this article are just a small part of the adverse heath effects predicted to occur over the coming decades due to climate change. The barriers to accepting and acting on anthropogenic global warming are largely psychological rather than scientific, as climate change science is now very robust (IPCC 2007). As health professionals we are well-equipped to deal with risk, uncertainty and levels of evidence, as we do so on a daily basis in clinical practice. We have a unique opportunity to educate and inform our patients, communities, colleagues and politicians about these issues. I believe we have an ethical duty to do so, in order to limit, as far as possible, the expected increased burden of illness that is on the horizon.
References
Ayres JG et al. Climate change and respiratory disease: European Respiratory Society position statement. Eur Resp J 2009; 34: 295-302.
BOM. http://www.bom.gov.au/announcements/media_releases/climate/change/20100105.shtml. Accessed 17.01.2010.
IPCC 2007. Summary for policy-makers (WGI). In Climate Change 2007: The Physical Science Basis. Cambridge University Press NY. 2007.
McMichael AJ et al. Climate change and human health: Present and future risks. Lancet 2006; 367: 859-869.
Reid CE and Gamble JL. Aeroallergens, allergic disease, and climate change: Impacts and adaptation. Ecohealth 2009; published online 12 Nov 2009; accessed 10.01.2010: http://www.springerlink.com/content/mqu4540p63284360
Rom WN et al. Global warming: A challenge to all American Thoracic Society Members (editorial). Am J Respir Crit Care Med 2008; 177: 1053-1057.
Shea KM et al. Climate change and allergic disease. J Allergy Clin Immunol 2008; 122: 443-453.
Stafoggia M et al. Factors affecting in-hospital heat-related mortality: a multi-city case-crossover analysis. J Epidemiol Community Health 2008; 62: 209-215.
Zhang L et al. Transpacific transport of ozone pollution and the effect of recent Asian emission increases on air quality in North America: an integrated analysis using satellite, aircraft, ozonesonde, and surface observations. Atmos Chem Phys 2008; 8: 6117-6136.
A version of this article appeared on the Medical Observer blog on 18.01.2010.
