In the first trial, we did not find significantly stronger effects in homes with animals than in homes without animals. Only a fourth of the homes had animals through the whole trial period. For this reason, we will only include homes without pets this time, in order to get a more homogenous population. In the first trial, we had a lower age limit of 7. This time, the lower age limit is 8, to make it easier for patients to co-operate, avoiding frustration for seven-year-olds who may be excluded due to co-operation difficulties. The reason that we require dog allergy this time, not dog or cat allergy as in the first trial, is that one trial indicates that air cleaners only reduce the amount of airborne cat allergens marginally if no cat is in the room. This trial was performed in homes with cats (5). On the other hand, a trial of effects of air cleaners in homes with dogs showed a significant reduction of dog allergens even in rooms where dogs were not present (1). The studies are difficult to compare, however, because different dust sampling methods were applied.