Assessment of the atmospheric fungal prevalence through field ergosterol measurement II: Establishing the conversion factor

A general empirical conversion factor for estimating ambient fungal prevalence from filter ergosterol concentration data is proposed. The derivation was based on 28 high-volume filter samples, each with six viable samples collected simultaneously, over a period of 6 months, and specific ergosterol content data previously determined for 12 common airborne fungal species. A number of simplifying assumptions were made to arrive at the weighted-average ergosterol content based on the specific ergosterol content and the corresponding fraction in the viable sample. The filter ergosterol concentrations and the viable counts varied by one order of magnitude, but the general empirical conversion factor derived from the weighted-average ergosterol content data was relatively constant at 0.191±0.040 pg spore−1 (mean±s.d., n=28). Using this factor to calculate the total spore number concentrations for the 28 filter samples gave a mean value of 1211 spores m−3, which was not significantly different from the mean calculated from the individual weighted-average ergosterol values (1236 spores m−3). Comparison with literature reports confirmed that such conversion factors are experiment and environment specific.