Climate warming that has occurred over recent decades (about 0.6°C thus far) has dramatically advanced budburst in spring (Fitter and Fitter 2002), therefore bringing forward the aller- genic pollen season for spring-flowering taxa (Figure 2) (Rasmussen 2002; van Vliet et al. 2002). The rate of these advances (0.84–0.9 days/year) (Clot 2003; Frenguelli et al. 2002) provides some of the best evidence of the current impacts of recent climate change. The most predictable climate changes over the next century involve an approximate dou- bling of atmospheric carbon dioxide (CO2) and a rise in average global temperature within the range of 1.4–5.8°C [Intergovernmental Panel on Climate Change (IPCC) 2001]. Numerous studies on plant responses to ele- vated CO2 indicate that plants will exhibit enhanced photosynthesis, biomass production, water use efficiency, and reproductive effort (Bazzaz 1990; Drake et al. 1997; Jablonski et al. 2002; LaDeau and Clark 2001; Stiling et al. 2004). These are considered positive developments for agriculture, but for allergic individuals they could augur increased exposure to airborne pollen. Some studies have in fact shown increased pollen production under con- ditions of elevated CO2. Ragweed (Ambrosia artemisiifolia) is a weed of open disturbed ground that produces potent pollen allergens. In controlled environment experiments, plants grown at elevated CO2 had greater biomass and produced 61–90% more pollen (Wayne et al. 2002; Ziska and Caufield 2000). Temperature and CO2 can also have interac- tive effects on pollen production. In experi- ments simulating early spring release from dormancy, ragweed plants grew larger, had more inflorescences, and produced more pollen than did later cohorts. Early cohorts under high levels of CO2 produced the same amount of pollen as those under ambient CO2, but later cohorts at high CO2 differed 16 May 12 May e t a D 8 May 4 May 30 April 26 April 1978 by producing 55% more pollen than their ambient CO2 counterparts (Rogers et al. 2006). Long-term records at pollen-monitor- ing stations in Europe show increasing annual totals for other types, including hazel, birch, and grass (Frei 1998; Spieksma 1995). Finally, a likely result of climate change will be shifts in the distributions of taxa as some species will be able to take advantage of new conditions, whereas others will not (Ziska 2003). For example, droughts may create open habitat that ragweed can colonize and therefore expand the range of this invasive and highly allergenic species in Europe resulting in indi- viduals being exposed to new allergens. CO2, climate, and molds. There is little evidence of the effects of climate change on fungal growth and reproduction, although the implications for allergic disease are just as important. As it is for pollen, exposure to fun- gal spores is unequivocally associated with exac- erbations of allergy and asthma (IOM 2000). Long-term ﬁeld experiments with elevated CO2 show that some fungi in mycorrhizal associa- tions with trees have enhanced growth and sporulation (Klironomos et al. 1997; Treseder et al. 2003; Wolf et al. 2003). Although more evidence is needed to establish the certainty of these effects for a wider range of fungi over a gradual increase in CO2 (Klironomos et al. 2005), plausible arguments can be made for the likelihood of increased fungal biomass (and resulting sporulation) under climate change sce- narios that would include increases in both mycorrhizal fungi to facilitate enhanced plant growth and saprobic fungi to degrade the increased plant biomass generated. Fungi are also an important factor in indoor exposures leading to allergic and asth- matic events (Jaakkola and Jaakkola 2004). Several studies have shown that home damp- ness is a significant predictor of respiratory symptoms (Bornehag et al. 2001; Dales et al. Viborg Copenhagen Environmental exposures and asthma 1991; IOM 2004). In a warmer climate, increased and more widespread reliance on air conditioning will occur, and the inevitable mis- management of building ventilation will likely result in more cases of inappropriate moisture conditions in buildings. Changes in precipita- tion regimes are also anticipated, with heavier downpours and more widespread flooding. Increased ﬂooding in coastal areas is projected with increases in sea level. All of these scenarios indicate a higher likelihood of wet interior sur- faces that are prone to fungal growth. Inequities are likely to occur as lower-income families are less able to cope with expensive remediation or ﬂood insurance (if available). In summary, projected changes in climate over the next century will inﬂuence plant and fungal reproductive responses and alter the timing, production, and distribution of aeroallergens. Increased allergen exposures as a result of global warming, in combination with pollutant exposures such as DEPs that can act synergistically to enhance the allergic response, could mean increased respiratory difﬁculties in the years ahead. Summary of Key Issues The increasing asthma incidence worldwide is a complex issue that is not well understood. There are more cars and trucks on the road now than ever before, and half the U.S. popu- lation lives and works in areas out of compli- ance with U.S. EPA standards for ozone or PM2.5 (Figure 3). There is compelling evi- dence that exposure to ETS increases the risk of asthma in both children and adults, and although not conclusive, some prospective studies are showing small but statistically sig- niﬁcant associations between air pollution and the incidence of asthma. In addition, increased temperature and CO2 due to climate change likely will result in increased production of pollen and fungal spores that could exacerbate symptoms of allergic diseases. More climatic and population studies linking chemical expo- sures, genetic susceptibility, daily activity pat- terns, dietary factors, and preexisting disease are needed to understand and quantify the associations between environmental factors )s140 ion120 l il100 m (e80 lp60 o pe40 f .o20 oN 0 NO2 1Oh3r 8Oh3r SO2 PM10 PM2.5 CO Pb NAAAnyQS Figure 3. Number of people living in counties with air quality levels above the National Ambient Air Quality Standards (NAAQS) in 2000. Abbreviations: CO, carbon monoxide; Pb, lead. Data from U.S. EPA (2005). 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 Year Figure 2. Peak date of airborne birch (Betula) pollen concentrations in Denmark, 1978–1999. Viborg: Ym = 122.1; β = –0.67; r = –0.81; p < 0.001. Copenhagen: Ym = 123.3; β = –0.74; r = –0.92; p < 0.001. Adapted from Rasmussen (2002). Environmental Health Perspectives • VOLUME 114 | NUMBER 4 | April 2006 631