Introduction
Trends in the incidence of childhood asthma worldwide have paralleled the sharp increase in carbon dioxide (CO2) emissions, over at least the last two decades. The prevalence of asthma in the United States has quadrupled over the last 20 years in part due to climate-related factors. In a report released by Harvard Medical School and the Center for Health and the Global Environment, it was noted that there was an increase in asthma incidence of 160% from 1980–1994 among preschool children. This observation was linked to the global rise in CO2 emissions, which in turn affects respiratory exposure to a variety of atmospheric pollens, mold, and fungi.Citation1,Citation2 While asthma is associated with genetic predisposition, the changing environment and air pollution are major contributory factors in the pathogenesis of the disease, and may help explain the rapid change in the incidence of asthma over the last few decades.Citation3 Even though the actual amount of CO2 in the atmosphere is minute, greenhouse gases are very effective in forming a blanket that prevents heat from escaping the earth’s atmosphere.Citation4
Dr Charles Keeling’s research laboratory at Mauna Loa on the Big Island of Hawaii records the amount of CO2 in the atmosphere, adjusted annually for seasonal variations. Analysis of ancient air bubbles trapped by glaciers reveals that the amount of CO2 consistently varied between 200 to 300 parts per million (ppm) for over 80,000 years.Citation4
Since 1960, for the first time in the known history of the earth, CO2 emissions exceeded 300 ppm. In 1980, the levels approached 350 ppm, and have been increasing relatively rapidly ever since, according to the Keeling curve. In turn, global temperatures fluctuate in a pattern that is closely associated with the amount of CO2 in the atmosphere.Citation4
Fossil fuels and deforestation: the major anthropogenic sources of CO2
Carbon dioxide emissions derive primarily from burning fossil fuel. Approximately 75% of all CO2 emissions during the last 20 years resulted from the burning of fossil fuels and the rest from deforestation.Citation5 CO2 is described as the most important greenhouse gas by some authors and has high inertia, and long residence in the atmosphere.Citation5 Two of the largest sources of CO2 emissions have been China and Brazil. China, from 1990–2009, tripled the use of fossil fuel used in its economic development from approximately 10 to 30 tons burned per year.Citation4 The use of solid biomass fuel as an energy source is especially prevalent in rural and non-Westernized societies. The use of wood and coal fuel releases a number of respiratory toxins, including CO2, which may provoke bronchospasm.Citation2,Citation3
Childhood asthma incidence
The parallel trends in the global incidence of asthma and the rise in CO2 emissions are remarkable. As atmospheric CO2 levels have risen and global temperature fluctuations have increased, so has the incidence of childhood asthma. According to one CDC-based survey, the number of children under 17 years of age with asthma increased from almost 40 to 60 per 1000 from 1980 to 1993.Citation6 Globally, data collected using the international study of allergy and asthma in childhood (ISAAC) questionnaire showed that the recent incidence of childhood asthma in China had increased in 2008–2009 (n = 24,290), in three selected cities in China (Beijing, Chongqing, and Guangzhou) compared to prior reports from the same cities in the 1990s.Citation7 The study also noted that the incidence of allergic rhinitis was increasing in those areas. In Phase 2 of the ISAAC study, at Spanish-speaking study sites, the prevalence of current wheezing in Brazil (25.6%) was the highest.Citation8,Citation9 There are regional variations of asthma prevalence in Latin America, which may be related to use of fossil fuels. The World Health Organization (WHO) estimates that 300 million people worldwide currently have asthma, including 6.2 million children in the US. While air pollution and CO2 emissions alone cannot account for the increase in asthma observed globally, factors cited by proponents of the hygiene hypothesis, such as cleaner indoor environments, reduction in family size, early use of antibiotics and fewer infections during infancy, also cannot fully account for the atopy and asthma incidence trends. Regional variation in climate change is impacted by rainfall, urbanization, and transportation patterns and the association between climate change and health is therefore complex.Citation10–Citation13
Pollen exposure and CO2 levels
Higher CO2 levels hasten blooming of certain plants. In a 2002 study of 365 British plants, it was reported that the average first bloom had advanced by 4.5 days. One-sixth of the plants studied demonstrated advanced growth by an average of 15 days.Citation14 This observation was also made by studies of the European olive trees in Spain. Based on projected estimates, pollen release is expected to occur earlier over the next century. As CO2 levels hypothetically double, the pollen season for oaks will start earlier and concentrations will be 50% higher.Citation15 Similar findings have been reported for other allergens such as mugwort and ragweed.Citation16,Citation17 The observations and predictions are not uniformly applicable though to all plants, as indicated by a study of Japanese cedar tree pollen, in which the authors investigated the levels of airborne pollen at eleven sites in Japan from 1987 to 1998 using a gravity sampler.Citation18 They did not observe a trend of increasing pollen levels nor earlier pollen seasons. There are several studies that support the cause and effect relationship between increased atmospheric CO2, and the increase in biomass and pollen release. Regional atmospheric conditions can be related to the amount reaching the airway. Airborne pollen concentrations depend on the degree of urbanization, air temperature, and wind conditions that spread pollen. In one study, urban sites demonstrated a 7-fold increase in ragweed growth, an average of 2°C increase in temperatures, and 30% higher CO2 levels, compared to other sites. Based on this study, urban residents may be more affected by higher pollen concentrations.Citation19,Citation20
In a study by Wayne and colleagues, ragweed was grown under regulated conditions in a greenhouse. The authors reported that stand level pollen production was 61% higher in elevated vs ambient CO2 environments (P < 0.005), but that the size of the pollen grains was not altered. Most studies support the concept of longer pollen exposure and an earlier start to the pollen season.Citation21 This has been confirmed in studies from diverse geographic regions.Citation14,Citation19,Citation20 One study from Switzerland, however, described a shorter pollen season in Basel, and an earlier onset of high pollen levels.Citation22 Higher latitudes are warming at a faster rate than mid-latitudes, and the pollen season length has lengthened in proportion to the rate of warming.Citation23 Spore counts of molds have also mirrored this trend and the spore counts for Alternaria have increased since 1992.Citation24 The expansion of grass growth in northern areas (eg, Denmark) is also related to CO2-enriched environmental conditions.Citation25
Seasonality and global temperature trends
The Fourth Assessment Report by the United Nations commission panel on climate change stated unequivocally that climate warming is an established trend.Citation26 Global temperatures have been rising for the last 40 years. These climate changes have altered the pollen season, increased pollen mass and led to forest fires which generate large amounts of air pollutants, compounding the adverse effect on the respiratory system.Citation27,Citation28
As the earth “breathes”, summer levels of CO2 decrease slightly with the growth of CO2-absorbing plants. In the winter, this effect is lost and levels of CO2 emissions again increase.Citation4 Seasonal variation in the number of childhood asthma exacerbations has a wide variety of causes, including outbreaks of respiratory viruses during the winter season. It is intriguing to speculate that the rise in CO2 emissions during the winter season may contribute to the increase in childhood winter asthma exacerbations.
Conclusion
Asthma is a complex disease and its pathogenesis has multiple causes and contributing factors. Among the non-atopic factors, the level of CO2 emissions and its respiratory health effects are among the most important. The global health impact of a potential decline in anthropogenic CO2 emissions, as new energy policies are enacted, may provide more evidence of the link between disease pathogenesis and CO2 emissions.
Disclosure
The author declares no conflicts of interest in this work.
References
- AultAReport blames global warming for rising asthmaLancet20043639420153215139337
- EpsteinPRClimate change and human healthN Engl Med J20053531414331436
- MantzouranisECTaking your child’s breath away- the extension of asthma’s global reachN Engl Med J20083581212111213
- GillisJSlowing the runaway train of carbon dioxide emissionsNew York TimesA12112222010
- US Energy Information AdministrationInternational Energy Outlook 2008Washington DCEIA2008 Available from: http://www.eia.gov/oiaf/archive/ieo08/index.html. Accessed September 23, 2011.
- Asthma mortality and hospitalization among children and young adults-United States: 1980–1993. Centers for Disease Control and Prevention (CDC)MMWR Morb Mortal Wkly Rep199645173503538604212
- ZhaoJBaiJShenKSelf-reported prevalence of childhood allergic diseases in three cities of China: a multicenter studyBMC Public Health20101055120836838
- PearceNDouwesJBeasleyBIs allergen exposure the major primary cause of asthma?Thorax200055542443110770825
- MallolJSoleDBalzaBRegional variation in asthma symptom prevalence in Latin American ChildrenJ Asthma201047664465020642377
- D’AmatoGCecchiLEffects of climate change on environmental factors in respiratory allergic diseasesClinical and Experimental Allergy20083881264127418537982
- LiuAHLeungDYRenaissance of the hygiene hypothesisJ Allergy Clin Immunol200611751063106616675333
- SchaubBLauenerRvon MutiusEThe many faces of the hygiene hypothesisJ Allergy Clin Immunol2006117596997716675321
- BloomfieldSFStanwell-SmithRCrevellRWPickupJToo clean or not too clean: the hygiene hypothesis and home hygieneClin Exp Allergy200636440242516630145
- FitterAHFitterRSRRapid changes in flowering time in British PlantsScience200229655731689169112040195
- Garcia-MozoHGalanCJatoVQuercus pollen season dynamics in the Iberian Peninsula: response to meterological parameters and possible consequences of climate changeAnn Agric Environ Med200613220922417195993
- StachAGarcia-MozoHPrieto-BaenaJCPrevalence of Artemisia species pollinosis in western Poland: impact of climate change on aerobiological trends, 1995–2004J Investig Allergol Clin Immunol20071713947
- WeberRWFloristic zones and aeroallergen diversityImmunol Allergy Clin North Am200323335736914524380
- KishikawaRKotoEIwanagaTLong term study of airborne pollen, C japonica and cupressaceae in JapanArerugi2001504369378 Japanese.11398333
- ZiskaLHGebhardDEFrenzDACities as harbingers of climate change: common ragweed, urbanization, and public healthJ Allergy Clin Immunol2003111229029512589347
- SheaKTrucknerRWeberRPedenDClimate change and allergic diseaseJ Allergy Clin Immunol2008122344345318774380
- WaynePFosterSConnollyJBazzazFEpsteinPProduction of allergenic pollen by ragweed is increased in CO2 enriched atmospheresAnn Allergy Asthma Immunol200288327928211926621
- FreiTThe effects of climate change in Switzerland 1969–1996 on airborne pollen quantities from hazel, birch and grassGrana199837172179
- FischerDClimate change extends allergy season in North AmericaScientific American221201149 Available at: http://www.scientificamerican.com/article.cfm?id=climate-change-extends-allergy-season.
- KimHBernsteinJAAir pollution and allergic diseaseCurr Allergy and Asthma Rep200992128133
- OlesenJEBindiMConsequences of climate change for European agricultural productivity, land use and policyEur J Agron200216239262
- ConfalonieriUMenneBAkhtarRClimate change 2007: impacts, adaptation and vulnerability Contribution of Working Group II to the fourth assessment report of the intergovernmental panel on climate changeCambridge UKCambridge University Press2007391431
- MyerSBernsteinAThe coming health crisisThe Scientist20112513237
- BernsteinASMyersSSClimate change and children’s healthCurr Opin Pediatr201123222122621412082