Study On Aerosolization Of Aspergillus Niger Spores In The HVAC Ducts

People in modern society spend more than 80%of their time indoors,and indoor air quality has an important impact on people's health.Approximately 21%of indoor air quality problems originate from microbial contamination.Microorganisms include bacteria,viruses,fungi,small protozoa,and microalgae.Among them,fungi can cause allergies,infections and inflammations in susceptible people through inhalation and epidermal exposure.The risk of fungal exposure originates from aerosolization of fungal spores,and accurate assessment of fungal exposure risk requires comparatively accurate airborne fungal spore content.The heating,ventilating,and air conditioning(HVAC)duct has suitable temperature and humidity for the growth of fungi.The spores may detach from their colony and then transmit in the air when subjected to the blowing air in the HVAC duct.Factors that affect spore aerosolization include self-factors of the colony represented by the growth age and environmental parameters represented by the air speed.The spores of an older colony are more easily detached than those of a younger colony.Until the present time,the accurate threshold force to detach a spore from a colony remains unknown.We also cannot predict whether fungal spores can detach when subjected to a specific airflow and the aerosol concentration of spores from the colony after blown by airflow in the HVAC duct.In response to the above problems,this paper started the investigation on the measurement of detachment and the prediction of aerosolization of fungal spores,considering the two factors of growth age and air speed that affected the spore aerosolization.The Aspergillus niger(A.niger)fungus is one of the indicator organisms for mold contamination and was therefore chosen as the test microorganism.The A.niger colony was grown on the nutrient agar and the components of the nutrient agar were peptone(10 g/L),glucose(40 g/L),and agar(15 g/L).The incubation temperature of the colony was 28? and the relative humidity was close to 100%.This study first proposed an experimental scheme for measuring detachment of fungal spores from growing colonies with an atomic force microscope(AFM).The AFM-measured forces were compared with the estimated drag forces to aerosolize spores in a wind tunnel.Then,the prediction method for spore aerosolization under blowing air from the respiratory intensity was proposed.The respiratory intensity was adopted as an indicator of the growth age,and the spore release proportion was then predicted.Two predicting methods,i.e.,the global method and the partition method,were proposed to predict the release proportion.In the global prediction method,the spore release proportion was estimated from the average age of the whole colony.To determine the average age,the respiratory intensity of the whole colony was measured,and then the average age was estimated from the respiratory intensity.In the partition prediction method,the colony was divided into a number of sub-colonies,each of which had its own age.The spore release proportion for the whole colony was estimated from that for the sub-colonies.Finally,the airflow characteristics of several kinds of air duct and cooling coil components in the HVAC duct were investigated.The differences between the local air speed at the height of the colony and the mainstream air speed were analyzed.The influence of the local airflow field on the aerosolization of fungal spores in different positions of the HVAC components was estimated.The hydraulic force of the airflow acting on the fungal spores was accurately calculated to predict the aerosolization risk of the spores.The AFM measurements revealed that an average force of 3.27 ± 0.25 nN was required to detach a single spore from the four-day-old colony,which was significantly greater than that from the 10-day-old colony(1.98 ± 0.13 nN).Slightly smaller detachment forces were observed by the AFM than were determined by the aerosolization tests.The age of a colony is a significant factor in the threshold detachment force of spores and their aerosolization.Then,the respiratory intensity of the colony was closely correlated with the growth age when the age of the colony was less than 14 days.The spore release proportion increased with the blowing air speed and the growth age of the colony,while the threshold air speed for spore detachment decreased with an increase in growth age.The spore release proportion for the unknown colony as predicted by the partition method was more precise than that predicted by the global method.The relative errors of the predicted spore release proportions by the partition method as compared with the test values were less than 10%.The averaging of the growth age of the colony by the global method resulted in underestimation of the spore release proportions.Finally,the range of the high-speed zone of the bending section increased with the ? value of the ratio of the radius of curvature to the inner diameter of the bend,and then the spores were more easily detached for the larger ? value than that for the lower ? value.The quantities of collected airborne spores when the colonies growing on the circular tube were positioned tangentially to the incoming airflow were four to 12 times those when the colonies were aligned with the airflow under the threshold blowing air speeds.The local air speed at the height of the growing colony could differ greatly from the inlet mainstream air speed.The drag force required to aerosolize the fungal spores should be estimated by the local air speed.