8.2.7 Aeroallergens and disease

Climate change has caused an earlier onset of the spring pollen season in the Northern Hemisphere (see Chapter 1, Section 1.3.7.4; D’Amato et al., 2002; Weber, 2002; Beggs, 2004). It is reasonable to conclude that allergenic diseases caused by pollen, such as allergic rhinitis, have experienced some concomitant change in seasonality (Emberlin et al., 2002; Burr et al., 2003). There is limited evidence that the length of the pollen season has also increased for some species. Although there are suggestions that the abundance of a few species of air-borne pollens has increased due to climate change, it is unclear whether the allergenic content of these pollen types has changed (pollen content remaining the same or increasing would imply increased exposure) (Huynen and Menne, 2003; Beggs and Bambrick, 2005). Few studies show patterns of increasing exposure for allergenic mould spores or bacteria (Corden et al., 2003; Harrison et al., 2005). Changes in the spatial distribution of natural vegetation, such as the introduction of new aeroallergens into an area, increases sensitisation (Voltolini et al., 2000; Asero, 2002). The introduction of new invasive plant species with highly allergenic pollen, in particular ragweed (Ambrosia artemisiifolia), presents important health risks; ragweed is spreading in several parts of the world (Rybnicek and Jaeger, 2001; Huynen and Menne, 2003; Taramarcaz et al., 2005; Cecchi et al., 2006). Several laboratory studies show that increasing CO₂ concentrations and temperatures increase ragweed pollen production and prolong the ragweed pollen season (Wan et al., 2002; Wayne et al., 2002; Singer et al., 2005; Ziska et al., 2005; Rogers et al., 2006a) and increase some plant metabolites that can affect human health (Ziska et al., 2005; Mohan et al., 2006).