Ragweed in France: an invasive plant and its allergenic pollen.

Background: Ragweed is an annual, herbaceous, wind-pollinated plant that is responsible for strong allergies. In France, it is mainly present in the region of Lyon, where it threatens the health of the population.

Objective: To analyze annual, daily, and bihourly pollen concentrations in the air to determine the characteristics of ragweed pollination and in particular its diurnal rhythm.

Methods: Ragweed pollens were sampled from 1987 to 2001 by a Hirst volumetric trap calibrated to handle a flow of 10 L/min of air, which roughly corresponds to a human breathing rhythm. Pollen counts were performed on a daily and bihourly basis, which is of particular importance in allergologic practice.

Results: Ragweed pollination occurs from the beginning of August to the end of September. Annual and daily levels of pollen have increased significantly since 1987. The pronounced diurnal periodicity shows a peak from 9 to 11 AM. Pollen counts increased from the early morning with temperature increase and relative humidity decrease.

Conclusions: Daily pollen counts of this strongly allergic plant are above the allergic thresholds commonly defined, limiting the interest of eradication campaigns. Thus, prevention of ragweed allergy depends on informing and educating the public through reports and updates. Diurnal variations seem to be the most crucial part of such a report, allowing allergy patients to adapt their daily outdoor activities in respect to the pollen peaks and off-peaks.