When people think of asthma or hay fever, they may think about the house dust mite or pollen, but rarely amount moulds. Moulds belong to a group of living things called fungi, and they reproduce by means of spores. Mushrooms and toadstools are also fungi, but moulds are much smaller and colonize rotting vegetation, damp walls and dead wood.

They are the green bits in stale bread and cheeses. Moulds begin to grow on a slice of bread, for example, because an airborne spore has landed on it. Moulds cover everything and there are tens of millions of spores in each cubic meter of air, far more than pollen or house dust mite droppings. Most spores are harmless and cause no trouble at all, but in the cold, damp countries of northern Europe there are about 20 moulds that can trigger allergies and asthma.

No matter how clean your home, the mite will always be there, as they will be in any building where there is upholstered furniture, including cinemas. They huddle in the seams of chairs, around buttons and under cushions. The dust mite has become more important as an asthma trigger because recent search shows that people are spending more time indoors than ever before, watching television or playing computer games.

Among the many common triggers of allergies are moulds and spores. They thrive in damp house, in potting compost and in the moist atmosphere of a greenhouse. The color scanning electron shows the fruiting body of the common bread mould. The spores form into a cap and then disperse into the air. When the spores land on a piece of bread, they germinate and grow into a mould.