Effects of temperature, relative humidity, and air flow on microbial growth on loaded ventilation filters

Particulate air filters are used extensively in all types of buildings to capture aerosols. The prevalence of microorganisms in indoor and outdoor environments and the availability of nutrients make these air filters potential sites for microbial growth. Once microbes proliferate on air filters, contact spores or fragments and microbial volatile organic compounds can be released into the air stream and cause adverse effects on occupants.;The objective of this dissertation is to investigate the effects of dry bulb temperature, relative humidity (RH), and air flow on microbial growth on loaded filters using Cladosporium sphaerospermum as the challenge microorganism. Full size synthetic media filters, loaded with malt extract (an artificial nutrient) and fungal conidia in a wind tunnel built according to ASHRAE Standard 52.2 1999, were incubated at different combinations of constant and variable temperature, relative humidity, and air flow conditions. The fungal growth was detected using elution-culture, ergosterol assay, carbon dioxide concentration measurement, and volatile organic compounds analysis methods. The colony forming unit (CFU) accounts from the elution-culture method were found to be susceptible to desiccation stress and underestimated the initial fungal growth rate. The ergosterol content is stable in low temperature storage and dehydration and may not be used as an indicator of living cells. The carbon dioxide concentration measurement was found to be limited to strong fungal growth and is recommended as a noninvasive detection method of significant fungal growth. Ethylbenzene, styrene, 2-heptanone, 2-octanone, 2-ethyl-1-hexanol, and 2-nonanone were associated with fungal growth with malt extract as the culture media. The detection of these compounds was not consistent among the samples. None of these compounds may be used as markers of fungal growth because they may also be released by building materials, furniture, or other chemical products.;Room temperature at approximately 23 °C and relative humidity of 97% near saturation were used as the reference test condition for optimum growth of Cladosporium sphaerospermum. Fitting the data from ergosterol analysis with the modified Gompertz model using the maximum specific growth rate, lag time and the asymptotic value for characterization, a sinusoidal temperature with an amplitude of 10 °C, an average of 23 °C, and a period of 24 hours with RH of 97% was found to able to reduce the specific growth rate by about 55%. Switching between RH of 97% and a lower RH on a cycle of 12 hours per day was demonstrated to be effective at reducing or preventing fungal growth. Intermittent exposure to 75% RH reduced the growth rate to 40 to 50% of the reference value, 43% RH to nearly 20% of the growth rate, and 11% RH completely inhibited growth. The lag time was delayed from one day at constant RH of 97% to 3 days at intermittent exposure to 43%. The effect of air flow at room temperature and near saturation RH on fungal growth rate was not significant. A combination of air flow with other environmental conditions may be needed to be effective at controlling fungal growth.