Rabbits were exposed once a day for 1 to 10 consecutive days to aerosols containing spores of Aspergillus fumigatus. The number of spores used at each 5-minute exposure during an experiment was constant, but the level of exposure was different for each of the 4 experiments. Precipitating antibodies were detected only in serums from rabbits that had inhaled at least 1.6 X 10-6 spores/g of lung tissue at each of 6 exposures or 1.8 X 10-6 spores at 2 exposures. A single exposure to 3.6 X 10-5 spores was sufficient to stimulate antibody detectable in passive hemagglutination (HA) tests. None of the rabbits died from aspergillosis, and signs of infection were transient.
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Thurston et al. (1975) studied this question.