Working Group II: Impacts, Adaptation and Vulnerability

Other reports in this collection

9.6.2. Aeroallergens (e.g., Pollen)

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
Life Years Losta
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/
(river blindness)
Black Fly 120 million 18 million 1,100,000 Africa/Latin America/
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
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).
height="1" vspace="12">

Other reports in this collection

IPCC Homepage