Natamycin, a fungicide marketed as Tymasil, is claimed to reduce house dust mite numbers and would therefore be expected to improve asthma in children with mite sensitivity. We have tested this assertion by a double-blind, placebo-controlled trial. There was no significant effect on levels of Der p I in mattress dust between active and placebo groups at the end of the spraying period. Histamine inhalation challenge PC20, clinic visit symptom scores and lung function tests reflecting either large or small airways obstruction were also unchanged. Therefore this product is not a therapeutic option for mite-allergic patients using the manufacturer's recommended dose and method of administration. Other factors influencing the Der p I levels were also investigated. Of these, only month of measurement and bedroom wall humidity showed any association.

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The site and degree of deposition of particles may be further affected by reflexes such as cough and bronchoconstriction and by disease in the lower respiratory tract.3 4 The optimum particle size for the deposition of drugs seems to be about 2 to 10

Mattress dusts from the beds of 51 asthmatic children with positive skin tests to house dust mite were assayed for Der p I, Fel d I and certain viable fungi. The results of skin tests to grass, cat, dog, Alternaria, Aspergillus, Cladosporium and Penicillium, radioallergosorbent tests to cat, mite, dog and grass and the chemiluminescent assay were compared and correlations made with allergen concentrations in the dust. The sensitivity of the three tests appeared comparable (percentage agreement = 54.5–94), however, an increased level of allergen in the mattress dust was not a good indicator of the degree of sensitization of the child (p > 0.05).