Daily, seasonal, and interannual variation in the abundance of many aeroallergens, particularly pollen, is associated with meteorological factors (Emberlin, 1994, 1997; Spieksma et al., 1995; Celenza et al., 1996). The start of the grass pollen season can vary between years by several weeks according to the weather in the spring and early summer. Pollen abundance, however, is more strongly associated with land-use change and farming practices than with weather (Emberlin, 1994). Pollen counts from birch trees (the main cause of seasonal allergies in northern Europe) have been shown to increase with increasing seasonal temperatures (Emberlin, 1997; Ahlholm et al., 1998). In a study of Japanese cedar pollen, there also was a significant increase in total pollen count in years in which summer temperatures had risen (Takahashi et al., 1996). However, the relationship between meteorological variables and specific pollen counts can vary from year to year (Glassheim et al., 1995). Climate change may affect the length of the allergy season. In addition, the effect of higher ambient levels of CO₂ may affect pollen production. Experimental research has shown that a doubling in CO₂ levels, from about 300 to 600 ppm, induces an approximately four-fold increase in the production of ragweed pollen (Ziska and Caulfield, 2000a,b).

High pollen levels have been associated with acute asthma epidemics, often in combination with thunderstorms (Hajat et al., 1997; Newson et al., 1998). Studies show that the effects of weather and aeroallergens on asthma symptoms are small (Epton et al., 1997). Other assessments have found no evidence that the effects of air pollutants and airborne pollens interact to exacerbate asthma (Guntzel et al., 1996; Stieb et al., 1996; Anderson et al., 1998; Hajat et al., 1999). Airborne pollen allergen can exist in subpollen sizes; therefore, specific pollen/ asthma relationships may not be the best approach to assessing the risk (Beggs, 1998). One study in Mexico suggests that altitude may affect the development of asthma (Vargas et al., 1999). Sources of indoor allergens that are climate-sensitive include the house dust mite, molds, and cockroaches (Beggs and Curson, 1995). Because the causation of initiation and exacerbation of asthma is complex, it is not clear how climate change would affect this disease. Further research into general allergies (including seasonal and geographic distribution) is required.

Table 9-1: Main vector-borne diseases: populations at risk and burden of disease (WHO data).
Disease	Vector	Population at Risk	Number of People Currently Infected or New Cases per Year	Disability- Adjusted Life Years Losta	Present Distribution
Malaria	Mosquito	2400 million (40% world population)	272,925,000	39,300,000	Tropics/subtropics
Schistosomiasis	Water Snail	500-600 million	120 million	1,700,000	Tropics/subtropics
Lymphatic filariasis	Mosquito	1,000 million	120 million	4,700,000	Tropics/subtropics
African trypanosomiasis (sleeping sickness)	Tsetse Fly	55 million	300,000-500,000 cases yr-1	1,200,000	Tropical Africa
Leishmaniasis	Sandfly	350 million	1.5-2 million new cases yr-1	1,700,000	Asia/Africa/ southern Europe/ Americas
Onchocerciasis (river blindness)	Black Fly	120 million	18 million	1,100,000	Africa/Latin America/ Yemen
American trypanosomiasis (Chagas'disease)	Triatomine Bug	100 million	16-18 million	600,000	Central and South America
Dengue	Mosquito	3,000 million	Tens of millions cases yr-1	1,800,000b	All tropical countries
Yellow fever	Mosquito	468 million in Africa	200,000 cases yr-1	Not available	Tropical South America and Africa
Japanese encephalitis	Mosquito	300 million	50,000 cases yr-1	500,000	Asia
a Disability-Adjusted Life Year (DALY) = a measurement of population health deficit that combines chronic illness or disability and premature death (see Murray, 1994; Murray and Lopez, 1996). Numbers are rounded to nearest 100,000. b Data from Gubler and Metzer (1999).

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