The hayfever season may trigger an increase in the numbers who die from heart attacks and respiratory disease, say experts.

Although increases in other forms of air pollution have long been associated with this, pollen has not been closely examined.

The study, in the Netherlands, looked at days in which pollen pollution was higher than normal - and looked at how many people died from heart attacks and other causes on those days.

They found that there were 5% to 10% more deaths on "high pollution" days.

The increased risk mainly affected people suffering from pneumonia or chronic obstructive pulmonary disease, a combination of bronchitis and emphysema.

Small heart attack risk

But there was still a smaller increased risk of dying from a heart attack.

The higher the pollen levels, the higher the risk of mortality from either cause.

The team reported: "Our findings, if substantiated, suggest that high airborne pollen concentrations, which nowadays are mainly seen as triggers of allergic symptoms, may have far more serious effects than previously thought."

Dr Raymond Agius has carried out his own research into the effects of air pollution on health.

He also found that high levels of air pollutions were likely to trigger deaths from heart attack and respiratory disease.

He said: "What we are trying to find out now is whether these were patients who would have died anyway within the following few days or weeks."

A report by a government committee examining the effects of air pollution suggested that more than 8,500 people a year die as a result of so-called "particulate" pollution, which includes traffic pollution and pollen.

It is thought that pollen can also trigger fatal asthma attacks in children and adults.

The research was published in The Lancet medical journal.