Rising global temperatures are driving longer and more intense pollen seasons, worsening symptoms for millions of seasonal allergy sufferers and contributing to rare but severe health events, according to scientists.

Experts say climate change is not only increasing the volume of pollen in the air but also contributing to extreme incidents such as “thunderstorm asthma,” where storms break pollen into microscopic fragments that can be easily inhaled.

On Nov. 21, 2016, such an event turned deadly in Melbourne, Australia. Around 6 p.m., as a storm developed, pollen particles were swept into clouds and shattered by rain, lightning and humidity into tiny pieces before falling back to the ground.

Emergency calls surged as people struggled to breathe. Hospitals were overwhelmed, with emergency rooms seeing eight times the usual number of patients with respiratory issues and nearly 10 times as many asthma admissions. Ambulances were unable to reach all those in need.

Ten people died, including a 20-year-old law student who collapsed on her lawn while waiting for help as her family attempted to revive her. One survivor described how he had been breathing normally before suddenly gasping for air within half an hour. "It was insane," he told reporters from his hospital bed.

Paul Beggs, an environmental health scientist and professor at Macquarie University in Sydney, said the scale of the incident was unlike anything previously recorded. "It was an absolutely massive event. Unprecedented. Catastrophic," he says. "The people in Melbourne, the doctors and the nurses and the people in pharmacies – they all didn't know what was happening."

Researchers later identified the ঘটনা as a major case of thunderstorm asthma, which occurs when storms rupture pollen grains and release allergenic proteins into the air. These particles can trigger severe reactions, even in people without a history of asthma.

Although such events remain rare, scientists warn they may become more likely as climate change extends pollen seasons and increases the frequency of extreme weather.

Across many regions, including the United States, Europe and Australia, allergy seasons are starting earlier, lasting longer and affecting more people. In the U.S., pollen levels are forecast to exceed historical averages across most states this year, with experts warning the trend will intensify.

Pollen plays a vital role in plant reproduction, but wind-dispersed varieties from trees, grasses and weeds are a primary cause of hay fever. The condition occurs when the immune system misidentifies pollen as a threat, triggering symptoms such as sneezing, itchy eyes and runny noses. In more severe cases, inflammation can restrict airflow and cause breathing difficulties.

Beggs said climate change is already reshaping pollen exposure. "We know that climate change is leading to greater amounts of pollen in the atmosphere," he says. "It's changing the seasonality of the pollen. It's changing the types of pollen that we're exposed to."

Warmer temperatures are lengthening growing seasons, allowing plants to release pollen earlier in spring and continue later into autumn. Elaine Fuertes, a public health scientist at Imperial College London, said this means sufferers face prolonged exposure. "You're going to get people who experience symptoms earlier in the year, for a longer period of time," she says.

Ragweed, a highly allergenic plant, is a major contributor. A single ragweed plant can release up to one billion pollen grains, and allergies to it affect roughly 50 million people in the United States alone. Studies show its pollen season has lengthened significantly across North America over recent decades.

"The winter warms, the springs are starting earlier, and the falls are being delayed, and so the time that you spend outdoors in contact with allergic pollen is definitely going up," says Lewis Ziska, an environmental health scientist at Columbia University.

Rising carbon dioxide levels are also fueling plant growth and pollen production. Experiments show that higher CO2 concentrations can significantly increase pollen output in grasses and trees. Ziska’s own research on ragweed found similar results. "Every time we cranked up the carbon dioxide, the ragweed plants responded. They grew more. They produced more pollen," he says. "And there was some evidence that they were producing a more allergenic form of the pollen, one that could induce your immune system to respond even to a greater extent than had been in the past."

The spread of ragweed into new regions is exposing more populations to its effects. Originally native to North America, it is now widespread in Europe and has reached parts of Asia, Australia and South America. Sensitization rates are rising, with significant portions of populations in countries such as Hungary, Denmark and the Netherlands affected.

Researchers estimate that ragweed pollen concentrations could quadruple by 2050, driven partly by plant spread and largely by climate change. This could introduce substantial pollen levels even in areas where the plant is currently rare.

Not all regions will see the same trends. Some areas, such as parts of Southern California, may experience shorter but earlier pollen seasons due to reduced rainfall. However, scientists caution that other climate-related factors, including wildfires, could further aggravate respiratory conditions.

Despite year-to-year variability in pollen levels, experts say individuals who are already sensitized will continue to experience symptoms even in milder seasons.

Efforts to address the problem range from reducing greenhouse gas emissions to targeted interventions. Historical examples include large-scale ragweed removal campaigns in U.S. cities during the early 20th century, which significantly reduced pollen levels.

More recently, European cities have taken action. Berlin has deployed teams to remove ragweed, while Switzerland banned its sale and import in 2024 and organized volunteer efforts to eliminate the plant.

Urban planning also plays a role. While expanding green spaces can improve city environments, experts warn that planting certain species or favoring male trees that produce more pollen can worsen allergies.

Improved monitoring is another key strategy. Scientists say better tracking of airborne allergens—not just pollen counts—would help people manage exposure. Currently, such data remains limited.

Fuertes noted that allergen levels, which vary by weather and plant type, may provide a more accurate picture of health risks. "Nobody is measuring allergen levels on a routine basis," she says. "We should be moving towards that."

Researchers say the evidence is clear: without coordinated global action, climate change will continue to intensify allergy seasons and increase health risks.

"We have the studies now to show that it's really having an impact on human health," says Beggs. "And there's more to come."