Pollens and allergies: it all depends on the weather
About 20% of French people are affected by allergic reactions due to pollens. Weather plays a decisive role: it plays a role in triggering pollination, the quantity of pollen produced and the transport of grains in the air we breathe.
Hay fever or pollinosis
The pollen grain is the microscopic reproductive element produced by the male organs of plants. When it enters the body through the respiratory tract, it can cause an immune system reaction. The pollen allergy from trees, plants, herbs and grasses is called pollinosis or hay fever. During pollination, pollen grains are emitted in very large quantities because the probability of reaching the female flower is very low. One ragweed, for example, can produce 2.5 billion grains in one season.
Pollens cause apparently benign affections, sometimes severe, always annoying or even disabling:
- rhinitis, sneezing attacks, often profuse discharge and nasal obstruction
- conjunctivitis with watery eyes, itching,
- cough, chest tightness or wheezing, asthma, with decreased breath
- fatigue, headache, lack of concentration or attention linked to sleep disturbed by rhinitis
- skin manifestations with worsening of certain eczemas, more rarely edema and hives.
Allergenic pollens
Pollens are not all allergenic: to cause allergy symptoms, pollen grains must have substances recognized as immunologically harmful to a given individual. In addition, they must reach the respiratory mucosa. The most allergenic pollens are therefore those carried by the wind. Cypress, birch, oak, ash, plane tree, hornbeam and the olive tree are the main trees emitting allergenic pollens.
On the initiative of the National Aerobiological Surveillance Network (RNSA), species are classified in France according to an allergenic potential ranging from 0 to 5 (0 being a zero potential and 5 a very strong potential). This classification was established using pollen sensors and the intensity of the symptoms observed in patients with pollinosis.
Pollination seasons
Pollination seasons vary according to plant species, regions, years and weather conditions. In France, winter pollination generally takes place from mid-January to April in the south and from late January to May in the north: it concerns trees and shrubs.
The second period of pollination occurs from mid-spring to summer with a gap of 3 to 5 weeks between the North and the South of the country. It is the season of grasses, plantains, primaries on the Mediterranean rim and sorrel in the North. Finally, certain plant species, such as chestnut, linden, herbaceous or ragweed, release their pollen from summer to late fall
The influence of meteorology
The most favourable weather situation for the release and dispersal of pollens is a very sunny day, without precipitation, with high temperatures and moderate wind.
The wind. During pollination, the wind plays a decisive role in the transport of pollen grains and their quantity in the air we breathe.
In low winds, pollen is deposited quickly, often near the plant.
A moderate wind keeps the grains in suspension in the air and promotes their concentration.
Precipitation and humidity. The rain prevents the release of pollens and their dispersion by the wind: the pollen, weighed down by the water, falls a short distance from its source. When it rains for several days during the pollen season, the plant retains its pollen to release it under more favourable conditions. The pollen rate is therefore low in rainy weather or when the air is very humid. (fog, mist, morning dew)
The temperature. A mild winter accelerates plant development and triggers early pollination.
On the other hand, cold winter with frost episodes delays the growth of plants and the start of pollination.
A high thermal amplitude during a day also contributes to the release of pollen grains.
The. Good sunlight favours an early start of pollination and the abundant emission of pollens.
Prevention of allergic risks
The National Aerobiological Surveillance Network (RNSA) is responsible for studying the content of the air in biological particles and measuring their consequences on health. Among these biological particles, pollens and moulds are the main factors of respiratory allergies. The RNSA measures pollen levels in France using around fifty specific sensors and centralizes the associated clinical information. Météo-France provides the RNSA with weather forecasts as well as statistical forecasts of pollination start dates for birch and grasses throughout the season in a dozen municipalities in France.
Thanks to this set of observed data and forecasts, the RNSA analysts thus determine for a given area the main taxon (s) (pollens corresponding to a botanical family) and the risk of allergic exposure to pollens calculated from 0 (zero) to 5 (very strong). These weekly allergo-pollen bulletins are drawn up by region and are intended for allergists and sensitive people.
Prevention tips for sensitive people
- Consult your doctor
- Avoid opening windows at work or in the car
- Avoid mowing the lawn especially in case of allergy to grasses
- Avoid cypress hedges if you are allergic to it
- Do not make intense efforts in the pollination period
- When you go on vacation, find out about the pollens present
- Enjoy the mountains, especially in the off
- season – Rinse your hair in the evening during the pollination period