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Abstract: The presence, quantity and origins of potentially toxic airborne substances were searched in moisture damaged indoor environments, where building related ill health symptoms were suspected and reference sites with no health complaints. Boar spermatozoa were used as the toxicity sensor. Indoor aerosols and dusts were collected from kindergartens, schools, offices and residences (n =25) by electrostatic filtering, vacuuming, wiping from elevated surfaces and from the interior of personal computers. Toxicity was measured from the ethanol or methanol extracts of the dusts and aerosols. EC50 was expressed as the lowest concentration of the airborne substance that inhibited motility of >50% of the exposed sperm cells compared to vehicle control, within 30min, 1day or 3–4days of exposure. Remarkably toxic aerosols (EC50 ⩽6μgml−1) were found from 11 sites, all of these were sites with known or suspected for building related ill health. Toxic microbial cultures were obtained from subsamples of the toxic aerosols/dusts. From these cereulide, amylosin, valinomycin and a novel indoor toxin, stephacidin B were identified and toxicities measured. Airborn dispersal of valinomycin from Streptomyces griseus cultures was evaluated using a flow-through chamber. Significant amounts of valinomycin (LC–MS assay) and toxicity (boar sperm motility assay) were carried by air and were after 14days mainly recovered from the interior surfaces of the flow chamber. [ABSTRACT FROM AUTHOR]