Allergen alert Factors behind our increasingly deadly hay fever seasons

Higher temperatures, pollution and invasive species are being linked to more severe pollen seasons around the world-though the effects are complex and the causes not yet fully understood

-  Patrick Greenfield

The first time it happened, László Makra thought he had flu. The symptoms appeared at the end of summer in 1989: his eyes started streaming, his throat was tight and he could not stop sneezing. Makra was 37 and otherwise fit and healthy, a climate scientist in Szeged, Hungary. Winter eventually came and he thought little of it. Then, it happened the next year. And the next.

"I had never had these symptoms before. It was high summer: it was impossible to have the flu three consecutive years in a row," he said.

The following year, a doctor finally tracked down the culprit: common ragweed. Transported to Europe from North America in the 1800s, the invasive species has become widespread in parts of central and eastern Europe. The weed is highly allergenic: a single plant produces millions of tiny grains of airborne pollen and for some asthma sufferers, exposure can be life-threatening. In the US, almost 50 million people are affected by common ragweed each year, extending the allergy season into early November.

After the diagnosis, Makra switched the focus of his research to how rising temperatures impact pollen. Now 73, the University of Szeged professor has become a leading international expert on the subject, co-authoring studies that show the pollen season is becoming longer and more severe in many places around the world as temperatures increase.

While there is a clear rise in the number of people reporting hay fever symptoms each year, the reasons behind it are complicated and not yet fully understood. Pollution, rising temperatures, increasing thunderstorms and the spread of invasive species are all transforming the world of pollen, with consequences that vary from place to place and year to year. These changes are having complex, varied effects on the human body - with sometimes deadly or debilitating consequences.

Thunderstorm asthma