According to research by the Max-Planck-Institut and Johannes Gutenberg-University, there are between 1,000 and 10,000 fungal spores per cubic meter of air.
(The average pair of human lungs can hold about 6 liters of air and 1 cubic meter = 1000 liters, so we're talking roughly 6 to 60 spores floating around in there. I hope you've had your breakfast already.)
