Rhinitis, conjunctivitis, even asthma attacks… The pollen of ragweed with sagebrush leaves – whose release peak is usually in mid-August – is highly allergenic, and often causes acute symptoms. According to the National Agency for Food, Environmental and Occupational Health and Safety (Anses), currently 3% of the French population is aware of it.
But it is above all the projections of the number of allergy sufferers by 2050 that are alarming: according to some scientific workup to 15% of the French population could become sensitized!
Originally from North America, ragweed is an invasive plant in our country. It was accidentally introduced in France in the middle of the 19th century, imported from the United States together with red clover seed.
How could such a dazzling increase over a period of only a few decades be explained? Is air pollution involved? What role will climate change play? Will the increase in the atmospheric concentration of carbon dioxide (CO2) have an impact? And above all, how to fight against this progression?
Allergies more difficult to manage
Cough, stuffy, runny and itchy nose… Ragweed allergy causes symptoms that are more severe and harder to manage than those of allergies to other pollens, according to health professionals.
In France, the ragweed pollen season lasts nearly two months, making it one of the longest. The consequences in the event of allergies being particularly debilitating, the quality of life of the persons concerned is particularly degraded.
Historically, the main areas of ambrosia implantation were the Rhone and Loire valleys. But since 2005, an acceleration of the expansion is observed in other regions : New-Aquitaine, Occitanie, North of PACA, Centre-Val de Loire (FREDON). This expansion occurs naturally through the dispersal of seeds which can be favored by agricultural activities.
However, to explain the future spread of ambrosia in France, we must look to other factors.
Air pollution, suspicious but not guilty?
Air pollution is the first potential culprit that comes to mind when it comes to allergies. However, in the case of ambrosia, it is probably not the cause.
Of course, air pollutants do have a proven effect on worsening the severity of symptoms. The period of pollination of ragweed sometimes coincides with peaks in ozone or fine particle pollution (particularly from forest fires). Air pollutants are in fact responsible both for the irritation of the respiratory system and for a direct alteration of the pollen. These two phenomena can lead to an exacerbation of inflammation and symptoms of respiratory allergy.
Ragweed pollen grains can cause severe allergies.
cellimagelibrary.org / Louisa Howard – Dartmouth College EM Facility
However, the role that air pollution could play on sensitization, in other words the possibility of developing symptoms, has not yet been formally demonstrated.
Beyond pollution, the roles of increased atmospheric CO2 concentration and climate change are among the main suspects.
The impact of increasing CO2 concentration
The concentration of carbon dioxide in the atmosphere has been steadily increasing since the 19th century: it has gone from 0.028% in 1850 to 0.042% today. By the end of the century, it should reach between 0.06% and 0.1%, depending on the decarbonization efforts that will be made in our societies.
However, when the concentration of atmospheric CO2 increases, photosynthesis also increases, which means plant growth is greater. Note, however, that the latter is also influenced by other parameters, such as temperature and episodes of droughtand that all plants do not respond in the same way to these changes.
Nevertheless, the impact of this increase in atmospheric CO2 on plant growth and reproduction is already visible in some species. What about ambrosia?
To find out, scientists are using greenhouses to simulate the conditions corresponding to the atmospheres of the future and try to predict their effects on pollen production. Ambrosia plants are thus grown either under atmospheres whose CO2 concentrations correspond to the current era (i.e. 0.042% CO2), or under atmospheres enriched in CO2 (for example at a percentage of 0.06%) .
We might as well be warned right away: the projections are not good…
More allergenic ragweed pollen, in larger quantities
These experiments indicate that increased atmospheric CO2 will most likely lead to us breathing in more ragweed pollen in the future.
In fact, various studies have shown a multiplication by a factor of 1.5 to 3 pollen production due to an increase in atmospheric CO2. Work has also revealed faster ragweed growth and longer pollination period.
Furthermore, it has been demonstrated that ragweed cultures growing in an atmosphere enriched with CO2 produce more pollen. Worse: under these conditions, released pollen contains increased amounts of allergens (the proteins responsible for the allergy). Recent work has confirmed that pollen from plants doped with CO2 is more allergenic.
As a result, we can fear that in the future, the levels of allergens with which we will come into contact will be higher, and the allergic symptoms will be exacerbated.
The role of climate change
Carbon dioxide will have a direct effect on the growth of ragweed and the quantity and quality of its pollen. It will also have an indirect effect on its pollination, by inducing (along with other greenhouse gases) climate change.
Warmer temperatures favor the production of pollen grains. In 2022, the quantities of ragweed pollen detected in New Aquitaine, on the pollen sensors located in Mareuil and Angoulême, have never been so high.
However, a positive correlation has been established between the amount of pollen in the air and the number of sick people. In other words, more pollen production means more people allergic to that pollen.
Ragweed could benefit from climate change.
Shutterstock / Lipatova Maryna
Climate change could also lengthen the length of the pollen season, which would increase the length of time during which allergy sufferers manifest symptoms. However, this lengthening effect of the season is not sufficiently documented in the current state of knowledge to be certain about this trend in the decades to come.
One thing is certain: the future climate will be more conducive to the spread of ragweed. A greater number of French regions will be infested by this invasive plant, and a larger population will therefore be exposed to its pollen. The consequence, again, is an increase in the probability of being sensitized to pollen and, in finean increase in the number of people with allergies.
How many additional allergy sufferers?
Modeling work has recently made it possible to estimate the evolution of ragweed allergy in France over the next few decades.
The progression of the geographical establishment of ragweed in France between now and 2041-2060 was calculated by taking into account both the influence of climate change and that of the increase in atmospheric CO2 concentration. In short, the increase in ragweed allergies will directly depend on the degree of reduction in global CO2 emissions. The higher our emissions, the higher the number of allergy sufferers…
Beyond the level of atmospheric CO2, the progression of ambrosia will also depend on the speed of propagation of the plant, and therefore on the means allocated to slow it down. For France, the prevalence of ragweed allergies is likely to increase from 3% currently to more than 15% in 2050, which would represent more than 10 million people…
In order to avoid reaching this point, the resistance to ambrosia is organized. Several structures or associations have already entered into the fight against this plant with the support of the public authorities. THE National Aerobiological Surveillance Network (RNSA) takes care of measuring the quantities of pollen grains present in the air. These analyzes make it possible to follow in real time the areas affected by ragweed and to inform health professionals and allergy sufferers, who can subscribe to a free “pollen alert” newsletter.
To reduce the spread of ragweed, and therefore to relieve the symptoms, it is crucial to fight plant by plant against its proliferation. The watchword is the uprooting of young plants before they mature and release pollen. Everyone can get involved in this fight, in particular by reporting the areas of infestations that must be cleared via the site signalement-ambroisie.fr. The report of the plants is then transmitted to the ambrosia referent of the municipality concerned, who organizes the destruction of the reported plants.
The number of people with allergies in the years to come will directly stem from our ability to identify and eradicate this invasive plant, as well as drastically reduce our CO2 emissions. An additional argument to convince those who still doubt the need to improve our carbon footprint!
To know more :
– Additional information is available on the Ambrosia Observatory website risque.info/piloted by the FREDON France network;
– The reporting of ragweed plants can be done via the site signalement-ambroisie.fras well as by e-mail at the following address [email protected] or by telephone at 0972 376 888.
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