Allergenic pollen boosted by climate change?

Very present birch pollen, hornbeam, ash, willow, poplar and plane tree pollens still there… 68 French departments are now on red alert announced this Friday, April 14 the National Aerobiological Surveillance Network (RNSA). The pollen season is well established, to the chagrin of some…

The emission of pollen into the atmosphere is the (plant) life that seeks to reproduce: hence these spectacular yellow clouds. But the fact is that pollen transported by the wind contains proteins recognized by the immune system as being allergens. As such, they can trigger an allergic reaction in sensitized subjects.

This profusion of pollen in the atmosphere then becomes a test for people with allergies, hence the use in certain situations such as in Durham, North Carolina (USA) in April 2019 of the term « pollenpocalypse » – echoing the strong episodes of air pollution baptized « airpocalypse ».

Through its symptoms, pollen allergy actually causes real daily difficulty for those affected, going so far as to cause asthma and so-called food allergies. “crossed” (a person allergic to birch pollen can, for example, develop a food allergy to apples).

More than 10% of the French population is allergic to pollen, and the number of people affected has almost doubled in recent decades.

Responsible climate change?

Can we say today that climate change is an aggravating factor for pollen allergies? Are these pollen clouds the first signs of plant adaptation to changing atmospheric and climatic conditions? Will pollen allergies continue to grow in the population?

Answering these questions involves calling on several scientific disciplines:

● aerobiology (to study the presence of allergens in the atmosphere),

● plant biology (to determine the effects of climate change on plants and their reproduction),

● climate sciences (to make hypotheses about the climate of the coming decades),

● atmospheric chemistry (to analyze the effects of pollutants on pollen),

● medical research (to clarify the links between exposure and sensitization to pollen on the one hand, and allergic symptoms on the other).

The complexity of the subject requires a resolutely transdisciplinary approach, as no one can claim to be a specialist in all these fields and have a general understanding of interconnected systems.

In the light of current knowledge in these various disciplines, and even if it is still difficult to explain the reasons for the explosion of allergies – and that, in this context, any forecast of a future situation remains uncertain -, a certain pessimism is in order: it seems quite possible that pollen allergies will continue to grow over the course of the century.

Indeed, one of the consequences of the increase in average temperatures is to stimulate certain plants, and in particularincrease the amounts of pollen produced as well as the amounts of allergens present in it. This increase in the quantities of pollen and allergens can facilitate sensitization and aggravate the symptoms, even if the links between the quantities of allergens and the severity of the symptoms are however not yet fully elucidated.

Longer allergenic periods

Also, the start of the pollen season is determined by weather conditions. However, due to the rise in average temperatures, the date of appearance of the first pollen tends to be more and more precocious for some plant species.

This change in the date of the start of pollination would have little health consequences if it only caused a shift in the pollination period; but the problem is that the date of the end of pollination remains globally the same, leading in fact to an increase in the duration of the pollination period and therefore an increase in the duration of exposure to allergenic pollens.

It should be noted, however, that this trend is not true for all species, and that for some of them pollen emissions even tend to decrease. for the last decades

Climatic conditions also directly influence the geographical distribution of plants. Regarding ragweed, an invasive plant whose pollen is particularly allergenic, studies thus predict a future extension of areas favorable to its presence, which could contribute to the fourfold increase in atmospheric concentrations of ragweed pollen. by 2050 in Europe.

Moreover, climate change, because it leads to greater quantities of pollen in the atmosphere as well as the occurrence of more intense storms, could also lead to an increased frequency of episodes ofstorm asthma. These events, favoring a strong dispersion of pollen allergens, are characterized by an influx of consultations in emergency services for asthma attacks in the hours preceding a storm.

In November 2016 in Melbourne, the most violent episode of thunderstorm asthma ever recorded occurred: thousands of people who consulted urgently for asthma, nine of whom died.

Links with CO levels₂ and pollutants

In addition to the direct effects of climate change, increasing atmospheric concentrations of carbon dioxide (CO₂) is likely to amplify the trend towards increased pollen production. A study has for example shown that the concentrations of CO₂ expected at the end of the century (720 ppm against 410 ppm today) could lead to a thirteen-fold increase in oak pollen production. A other study was able to show that with regard to ragweed, the quantity of allergens per pollen grain is correlated with the atmospheric concentration of CO₂.

Finally, atmospheric pollutants can, in addition to acting directly on the respiratory system, modulate the allergenic character pollen. Allergenic proteins are, for example, modified in quality and quantity by certain pollutants such as nitrogen dioxide and ozone.

In addition, polluted, weakened pollen is better able to deliver its allergens, which can then penetrate to the heart of the respiratory system and promote asthma attacks.

As we can see, a body of data leads us to predict a probable increase in the quantities of pollen and allergens in the atmosphere, at least for certain plant species. The effects on the proportion of allergy sufferers in the future population or on the severity of symptoms are uncertain.

As a precaution, it nevertheless seems important for the public health take into account possible future changes in exposure to allergenic pollens. Thus, the fight against invasive plants, in particular ragweed, must be a clear priority.

And, if we need more vegetation in our cities, for our well-being, biodiversity and to reduce urban heat islands, taking into account the risk of allergy must become a reflex in the management of urban green spaces.