What is it?

The house dust mite is a very small, wingless, eight-legged creature which can be found in every home in the UK. It is particularly comfortable in places which are warm, relatively humid and without draughts - as are most modern British homes - and favours especially carpets, mattresses and pillows, soft furnishings and soft toys. Officially, the mite is known as Dermataphagoides pteronyssinus, a name which describes its fondness for human skin scales and for feathers.

For most people the dust mite is a harmless companion. About two thirds of British children with asthma, however, are allergic to it, or rather to proteins found in its faecal pellets. These proteins are very small and easily inhaled. When this happens, sensitive asthmatics develop wheezing.

Why does it matter?

This much is known: the house dust mite is everywhere, many asthmatic children have an allergy to it and when it is inhaled their asthma may deteriorate. What is less clear is whether the house dust mite is itself a cause of children developing asthma in the first place. This is one of the questions which we are hoping to answer with the Asthma in Ashford study.

What did we do?

Around two months after each child was born, we used a modified household hoover to collect samples of 'dust' from their living rooms and mattresses. We also collected samples from the beds of about half the parents. These samples were sent to our laboratory in London where they were tested for the quantity of house dust mite protein (allergen) they contained. The allergen we examined is called Der p1. In this way we have been able to make a rough estimate of the level of Der p1 each child encountered in their first months of life. Eventually we will be able to examine whether those children who met high levels of house dust mite were more likely to develop asthma.

What did we find?

We collected samples from 620 homes. On average the level of Der p1 in living room carpets was 1.4 micrograms per gram of dust. This is what we would expect from UK homes but there was a wide range of values between homes. The lowest value was 0.02 µg/g and the highest 385µg/g!

Homes with;
· fitted carpets
· double glazing
· visible damp patches or
· more people
had higher levels of Der p1

House dust mite levels in the living room carpets were a little higher where there were more children in the home house dust mite levels (ug per gram of dust) number of children in the home

Homes with:
· one or more smokers
had lower levels of Der p1.

Unsurprisingly, the levels of Der p1 in the babies' beds were much lower; and in one third we were unable to find any at all. On average the level was 0.5µg/g but again there was a huge range of values (from 0.002 to 3168µg/g).

Levels in beds tended to be lower:
· in beds with new mattresses or
· where there were few children in the home.

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