It's exam time - and unfortunately for 15 per cent of children, it's also hayfever season. The pollen allergy, which has a peak age of onset in adolescence, is at its worst around now, when the grass pollen season is at its height.

As school exams don't finish until the end of June, that causes huge problems for the 15 per cent of children who are hayfever sufferers. In fact, a new study found that children who experienced hayfever symptoms while taking their GCSE exams were 40 per cent more likely to drop a grade between their mocks and final exams.

The study, by the charity Education for Health, found that the figure rose to 70 per cent if candidates were taking a sedating antihistamine treatment, which can cause drowsiness.

The study leader Dr Samantha Walker, director of education at Education for Health, says: "Parents and health professionals need to understand that having hayfever could cause their children to perform less well in their exams and that effective treatments are widely available."

And the wide availability of both over-the-counter and prescribed non-sedating medications to control symptoms means there should no longer be any excuse for hayfever affecting exam performance, says the medical charity Allergy UK.

"The impact of hayfever on school, and in particular exam performance, has been a concern for Allergy UK for a long time," says Muriel Simmons, the charity's chief executive.

"The findings of this study will hopefully prompt students to ensure that their symptoms are under control, and that they are not on a sedating medication, which clearly worsens the impact on exam performance."

If hayfever sufferers feel the condition has affected their exam performance and can produce evidence of this, it can be submitted by their head of centre or examination officer to the relevant awarding body.

A spokesman for the Joint Council for Qualifications says: "Candidates who are present for an examination but disadvantaged due to a serious case of hayfever may be eligible for special consideration where evidence is provided.

"The maximum allowance is two per cent, and all applications are treated on an individual basis."

He added that there were no plans to change the time of year that exams are held.

Symptoms of hayfever include a runny nose, sneezing, itchy eyes and poor smell, but perhaps the most troublesome symptom is nasal congestion, which 85 per cent of people with hayfever suffer from. It can often cause sleep disturbance, which in turn has an impact on exam performance.

Allergy UK points out that many people with hayfever are self-diagnosed, and have no idea which type of pollen they should try to avoid.