A laboratory setup was used to challenge a glass fiber media filter, a polymer fiber media filter, and a two-stage electrostatic precipitator (electronic air cleaner) with 100% ambient outside air for one year. Over the year, fractional efficiency measurements were made as well as microbial sampling to determine filter effectiveness on ambient viable bioaerosols. Both of the media filters demonstrated greater than 90% effectiveness on fungi and bacteria bioaerosols. The electrostatic precipitator demonstrated initial efficiency of 95% or greater, which decreased over time as the device was overloaded. Normal maintenance of the electrostatic precipitator was minimized in an effort to create conditions favorable to microbial growth. Over the span of the year, no observable microbial growth took place on any of the filters. Subsequent tests on the loaded filters, however, showed that observable microbial growth occurred within a month of continuous exposure to 90% relative humidity.