Scientists say that there are certain types of grass that irritate patients even without huge amounts

It has long been a cruel mystery for many people suffering from hay fever: why should they be badly hit on days when the pollen count is low?

But now, British scientists have discovered that the solution lies in the type of pollen present in the air.

There are types of grass that greatly irritate some people even without releasing huge amounts of particles.

The breakthrough also addresses the opposite phenomenon that some sufferers have a surprisingly mild allergic reaction on days with high pollen counts.

This is explained by species that can release large amounts of pollen, but only cause low level reactions for many people.

The team of experts from universities and research institutes in the United Kingdom hope that their flagship study, called PollerGEN, will lead to personalized pollen predictions, based on the exact species to which each hay fever victim is allergic.

Until recently, it was almost impossible to differentiate pollen from many species of British grasses simply by looking at them under the microscope, as they look very similar. All grass pollen has therefore been grouped together to give a single measure or "load".

However, thanks to a new way of identifying pollens called "barcodes" of DNA, they can now be distinguished from each other.

Academics are currently studying prescription data for antihistamines to see if prescription peaks coincide with the "spikes" of pollen release from particular species. They do the same thing with the figures on people taken to the hospital in the summer because of asthma. The data should give them clues about grass pollen, which are the most potent allergens.

Professor Simon Creer of Bangor University, who leads the research, said, "I suffer from hay fever myself and I know that on some days – despite a high pollen forecast – I can be less affected than I am. other forecasts seem lower.

"That's what led me and others to ask if it's the high pollen load alone that's causing the problem, or whether different grass pollens are causing different levels of response."

Natasha de Vere, DNA code expert at the National Botanical Garden of Wales, said: "Some grass pollens are likely to be more allergenic than others for most people with hay fever.

"But it's also likely that different people will react differently to the same species, but in the future, we could test people to find out which grass pollens they are most allergic to."

The information could then be used to create custom pollen forecasts, she said, based on knowing when different grass species release most of their pollen and the inherent sensitivity of each nobody to these.

The Met Office, which produces pollen forecasts, helps by mapping the geographical distribution of common grasses.

Dr. de Vere said that this week's milder and lighter temperatures meant that trees and grasses released their pollen earlier than usual. Typically, the season of tree pollens extends from late March to mid-May. The grass pollen season then takes over until July. This year, tree pollen has already peaked as grass pollen levels are already on the rise, she said.