Johannes Mazomeit walks into a field of gray, dried-up shrubs. The botanist would be kitted out in protective gear if it were summertime. But it's winter and still too early for these plants to be producing their potent pollen. 

This field is full of ragweed (Ambrosia artemisiifolia), an invasive species that is also one of the most allergenic plants in the world.   

"Just a few pollen grains can be enough to trigger allergic reactions, even in people who have never been allergic to anything before," Mazomeit told DW. "It can hit anybody." 

The pollen can cause those who are allergic to sneeze, get itchy eyes and a runny nose, or provoke more serious reactions, such as asthma attacks. It can also cause skin allergies.

For years, Mazomeit has been hunting ragweed in his home state of Rhineland-Palatinate in southwestern Germany. It's his job to stop it getting out of control.  

"There has been a clear spread in recent decades," he says, with clusters popping up in sandy soil in forests, along roads, in private gardens and on farms. 

Allergies on the rise 

Originally from North America, ragweed's roots and fern-like leaves have been harvested by native tribes for their medicinal properties, for example for use as an antiseptic. The plant also provides food and cover to wild animals, including rabbits and birds.  

The ragweed's seeds were brought to Europe in the 19th century, but it's only in the past few decades that the species has become a problem. Warmer temperatures provided the right conditions for it to spread, and it's now found in more than 30 countries in Europe, including Germany. 

Each plant can produce up to 3 billion pollen grains capable of flying hundreds of kilometers. These grains are extremely small, allowing them to travel deeper into the lungs than pollen from other species. 

That's what makes ragweed's spread particularly concerning, says Professor Claudia Traidl-Hoffmann, head of environmental medicine at the University of Augsburg and author of the book Overheated: The consequences of climate change for our health.  

Allergies are already a major health problem in Europe, and ragweed's foothold is only expected to complicate things further. Almost 40% of Europeans suffer from some sort of pollen allergy, according to Traidl-Hoffmann and that's predicted to rise to 50% by the middle of the century.

She also cites studies that found the number of allergic asthma cases, which can be caused by pollen allergies among other things, grew 10-20% in Germany between 2007 and 2017. 

The situation is getting worse, says Traidl-Hoffmann, "because pollen is affected by climate change."  

What role is climate change having? 

Warmer temperatures are causing some plants to speed up growth and produce pollen earlier and for longer.  

"The pollen-free season is getting shorter," Traidl-Hoffmann says, to the point where there is pollen in the air all year. 

On top of that, she says, the presence of pollution and carbon dioxide in the air is boosting pollen production and changing the protein of the pollen itself, making it more aggressive. 

One recent US study found that hotter weather and higher levels of CO2 in the atmosphere have caused pollen seasons in North America to start an average of 20 days earlier since 1990, with trees releasing around 20% more pollen.  

A study from Italy that looked at allergenic trees such as birch, olive and cypress also concluded that pollen seasons were getting longer, while the amount of pollen, and people sensitized to pollens, had increased. 

Scientists have also shown that ragweed plants exposed to carbon dioxide levels from the year 2000 (370 parts per million) produced 132% more pollen than ragweed plants exposed to CO2 concentrations from 1890 (280 ppm). The pollen count rose another 90% with projected CO2 concentration for 2050 (600 ppm).  

So what does all this mean for our health? 

Longer pollen seasons and higher quantities of the stuff in the air are inevitably bad news for allergy sufferers. More people will also develop allergies over time, says Traidl-Hoffmann, with consequences for our productivity and wellbeing.  

"In the end it's not only a threat for our health, but for our societies from an economic (perspective)," she says. 

Studies show that allergic pupils, for example, don’t perform as well at school during pollen season. Traidl-Hoffmann explains that allergies can cause a drop of up to 30% in performance in the classroom or workplace, adding that the loss of work and medical treatment for allergies cost the EU up to €150 billion ($170 billion) annually.  

Pollen may also make us more susceptible to viruses such as the coronavirus — whether we are allergic or not. Traidl-Hoffmann co-authored a recent study that recorded higher COVID infection rates in places with higher pollen counts. The authors write that pollen enters the body and disrupts its antiviral immune response. However, experts say this area needs more research.  

Preparing for the pollen season  

When pollen season gets underway, there are precautions those who are susceptible, have asthma or other inflammatory diseases can take to limit their exposure.  

It's a good idea to stay indoors and keep windows closed on high pollen count days. Wearing a mask outside on these days is also recommended.  

Meanwhile, botanist Johannes Mazomeit stresses that anyone who comes into contact with ragweed should wear gloves and cover their face. To dispose of it correctly, "pull out the plant by the roots, wrap it carefully in a plastic bag and place it in the residual waste bin. The worst thing you can do is to put it on your compost heap."

The interviewees in this article were featured in DW's environment podcast "On the Green Fence." Listen to the episode on allergies and climate change here.