| Ultrasonic humidification is very popular in heating rooms to ease air dryness but may cause serious microbes-originated health issues such as"humidifier fever"and"hypersensitivity pneumonia"because the bioaerosol concentration and community in the humidified space may alter quickly.However,due to the lack of real-time microbes monitoring equipment adopted in humidified spaces,the indoor air microbial environment may deteriorate before occupants’awareness via odor or respiratory symptoms,etc.that are too late to alert.Therefore,clarifying the temporal changes of microbial concentration and community in ultrasonic humidified is of great value for reducing the risk of microbial exposure and protecting the health of occupants.To this end,a thorough experimental investigation on the temporal impacts of ultrasonic humidifiers on the concentration,size distribution,and community variations of bacterial and fungal aerosols was carried out.4 experimental chambers（three as testing chambers for different humidification levels vs.one as parallel）were built to simulate the actual office room.During continuous humidification in 4 chambers,bacteria and fungi in air,water and dust were sampled and analyzed with different ultrasonic humidification levels（no humidification,RH=40%,55%,70%）to clarify:1)Microbial proliferation of 3 typical humidifier water（distilled water,tap water and cool boiled water）under the ultrasound.2)Temporal change of indoor microbial aerosol concentration,size-distribution and microbial community during continuous humidification with different humidity levels.3)Change of concentration and spatial and temporal distribution of dust-borne microorganism on the indoor floor and wall in chambers.The results showed that:1)Ultrasonic promoted the bacterial reproduction in the humidifier water.After 150 minutes of operation,the bacterial concentration in the three types of water has increased significantly.Cool boiled water possessed the fastest proliferation due to its rich nutrients without free chlorine residual.After days,the concentration of microorganisms in the humidifier water exceeded the standard limit of GB 5749-2006.2)During ultrasonic humidification,the indoor fungal aerosol concentration kept a low level（≤177 CFU/m~3）.The bacterial aerosol concentration maintained a low level in the first 2 days,but increased exponentially from the third day of humidification,exceeding the limit of 1000CFU/m~3 within a week（RH=70%）or on the 9th day（RH=55%）,and particle size distribution was concentrated in the sub-micron level（<1.1μm）,which can cause a serious risk of inhalation.The risks were also proportional to the humidification level.The air bacterial ecologies deteriorated significantly,with the increased abundance of potentially pathogenic Pseudomonas spp.,Brevundimonas spp.,Acinetobacter spp.and Legionella spp.,and the total relative abundance reach 45%.Statistical analysis indicated that the bacterial aerosol after humidification had a high similarity with the bacterial community in the water.3)The dust-borne bacteria on the surface of the humidification room increased rapidly,and the concentration was as high as 8.8×10~7 CFU/mg after 10 days of humidification,which was 13times that before humidification.Although the growth of fungi was lagging,the concentration also increased by 5 times,reaching 6.0×10~7 CFU/mg,far exceeding the limit proposed by WS394-2012;the increase in microbial concentration is mostly concentrated on the ground and is closely related to the level of humidification,satisfying:ΔCRH55%>ΔCRH70%>ΔCRH40%.In summary,long-term use of ultrasonic humidification will increase the concentration of indoor microorganism exposure and cause deterioration of indoor air quality.It is recommended that the user set the humidity not too high when humidifying,alter the humidifier water in 1～3days,and ventilate and clean in time after humidification.This study can provide a positive reference for creating a healthy living environment. |
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