Pollens and the risk of allergies

About 20% of the population suffers from allergies caused by pollen accumulating in the respiratory tract. These allergies are characterised by common colds, allergic rhinitis, headaches and asthma attacks. The increase in allergic disorders (which has doubled in ten years) has resulted in national awareness of the issue, and an increase in the number of measures taken to combat the problem. Together with the RNSA (National Aerobiological Monitoring Network), Atmo Auvergne has been measuring pollen counts since 1999.

Pollens are reproductive grains produced by male plant organs. To complete their reproductive role they have to find a way of getting to the female plant organs. They are transported by insects, animals or the wind. Wind is the most widespread form of pollen transportation. These airborne pollens are transported in great numbers to offset the risky nature of this type of pollination. The small size of these pollens (20-60 µm) means they are very effective at contaminating the respiratory tract.

Pollens are classified according to their allergenic potential which differs according to the producer plant. For instance, birches and grasses have highly allergenic pollen, while pollen from chestnut and elm trees have a low allergenic potential. The pollination period also varies among the different plant species. In springtime, most pollen is produced by trees such as the birch or the oak. Herbaceous plants come next, with grasses from May to July, followed by ragweed in August producing highly allergenic pollen which is new in Auvergne.

Pollen counts are generally taken between March and September using a volumetric meter, which is placed in Clermont-Ferrand in an area of high population density. Air is drawn in at a rate of 10 litres per minute, a flow rate comparable to that of human respiration. Pollen grains are deposited on circular strips of cellophane which are collected once a week. Analysts then cut the strip into daily sections and stain them in order to detect the pollen grains. They then count the pollen grains under the microscope, and note the various families (taxonomic categories). Pollen counts for the main taxonomic categories enable analysts to set the weekly pollen index, which represents the level of allergenic risk.

Airborne pollen concentrations are affected by a number of factors. In addition to the type of plant and the time of year, weather also plays an important role. Pollen concentrations increase with higher temperatures and wind speeds. When it rains, pollen grains are beaten to the earth where they explode, releasing starch grains loaded with allergens that directly penetrate the bronchial tracts. By sensitizing individuals, air pollution would appear to aggravate the allergenic effects resulting from pollination. Accordingly, ozone and nitrogen dioxide increase allergen-specific bronchial hyperresponsiveness by promoting the production of antibodies which activate the allergy. Particles in suspension also modify allergic sensitivity thresholds. This results in more widespread sensitivity in urban areas as opposed to rural areas.