Climate change and the spread of invasive ragweed are set to double the number of seasonal allergy sufferers across Europe, with similar impacts likely in North America, researchers said Thursday.

By mid-century, some 77 million people in Europe will be hit by hay fever misery, up from 33 million today, they reported in the journal Environmental Health Perspectives.

"Ragweed pollen allergy is likely to become a major health problem across much of Europe," said lead author Iain Lake, a researcher at the University of East Anglia in England.

Not only will more people be laid low by allergic asthma, itching and swollen eyes, the severity of the symptoms is likely to increase, the study concluded.

This greater impact will stem from higher concentrations of ragweed, and a pollen season extending into September and October across most of the continent.

Countries less affected today -- France, Germany and Poland, for example -- will probably be hit hardest over the coming decades. 

The main culprit is climate change, but the continuing spread of the invasive species Ambrosia artemisiifolia -- commonly known as ragweed -- is also a key driver.

A single ragweed plant can produce about a billion grains of pollen per year, mostly in August. 

Ragweed accounts for more than half of pollen production in many European countries. Tree pollen in the spring, and grass pollen in early summer, account for the rest.

- Surge of 2.7 C to 3.5 C -

Hay fever is a common allergic condition that affects nearly 40 percent of Europeans at some point in their life. 

More broadly, allergic disease has increased rapidly in both rich and developing nations, and is now recognised as a global epidemic.

The World Health Organization estimates that 400 million people around the world suffer from allergic rhinitis, and 300 million from pollen-related asthma. 

The economic burden of these conditions in calculated in the tens of billions of euros or dollars in both Europe and the United States.  

Previous research has predicted global warming would hasten the spread of pollens, but this is the first study to quantify those consequences, Lake said.

His team combined maps of estimated ragweed pollen counts with population projections, data on where people live, and current allergy levels.

The researchers presumed a "moderate" climate change scenario that would see an increase in global average temperature -- compared to an 1850 benchmark -- of about 2.5 degrees Celsius (4.5 degrees Fahrenheit) by the end of this century. 

The world is currently on track for a surge of 2.7 C to 3.5 C (4.9 F to 6.3 F).