Operation of a mechanical ventilation system has been found to have both positive and negative effects on asthma and allergy symptoms. Use of heat recovery ventilation, compared with a placebo unit, was associated with a progressive fall in the odds ratio for the reported wheeze . Xu et al.  reported that the installation of ventilation units effectively reduced asthma symptoms and indicators of airway inflammation, although the effect could not be separated from the effect caused by the parallel operation of an air cleaner, which was a part of this unit.
Domestic mechanical heat recovery ventilation systems that were expected to increase ventilation rate by 0.5 h−1 in addition to allergen avoidance measures, reduced the relative humidity and improved peak expiratory flow indicating relieved asthma symptoms, but there were no effects on HDMs . Howieson et al.  reported that the installation of such a system in homes reduced asthmatic symptoms and improved lung function in children, presumably because it reduced house dust mites by reducing the relative humidity. Emenius et al.  found no relationship between the type of ventilation system in operation and the risk of recurrent wheeze. The presence of a ventilation system was associated with increased levels of allergic symptoms in studies reported by Ezratty et al.  and Takahashi et al. . The large amounts of pollen that could enter dwellings through the air ducts and other factors not related to mechanical ventilation system probably account for these unexpected observations.
Read more in the scientific paper “What does the scientific literature tell us about the ventilation–health relationship in public and residential buildings?” by Pawel Wargocki and co-authors published at the Journal of Building and Environment.