Household cats can trigger allergic reactions in more than a quarter of the population, suggesting the pets have a far greater impact on human health than doctors had previously believed, scientists warned yesterday. A study of nearly 2,000 volunteers across Europe found that cats could cause breathing difficulties in people with some of the most common allergies.

Researchers think even a small exposure to cats is enough to aggravate symptoms similar to the early stages of asthma if people are already sensitised to common allergens such as house dust mites, timothy grass and a mould called Cladosporidium. "People need to be aware that cats are a problem for more people than we realised," said Susan Chinn, lead author of the study at Imperial College London. "If they're thinking of getting a pet and a cat is just one of their options, they might want to pause before choosing."

In the study, Dr Chinn's team collected house dust samples from the homes of 1,884 people from 20 different areas throughout Europe and analysed them for levels of cat allergen, grass allergen, mould and dust mites.

The tests revealed that more than one in four of the volunteers, who were picked to be representative of the general population, had at least one allergy, and the higher the levels of cat allergen found in their house dust, the worse was their lung function. "This was an unexpected finding," said Dr Chinn. "Our study suggests that all allergenic individuals have signs of asthmatic responses if exposed to cat allergen, even if blood tests show they are not allergic to cats."

Writing in the American Journal of Respiratory and Critical Care Medicine yesterday, the authors claim that avoiding exposure to the cat allergen, which is found in the fur of the animals and is shed as they move around, would benefit a much wider population than previously expected.

"It appears that many individuals could benefit from reduced cat ownership and exposure," said Dr Chinn. "However, because our findings were unexpected, it is important that results are replicated in other studies before firm recommendations are made."