| As emerging pollutants,microplastics have been found in the air;human inhalation and health risk of microplastics is of great concern.Microfibers（MFs）are the main form of airborne microplastics,but their distribution and transport behavior in the indoor environment is poorly understood.Air conditioning（AC）is a common device in modern living rooms.It is an environmental health issue of great concern whether AC affects distribution of indoor airborne MFs and their associated contamination of microorganisms.Therefore,this study investigated the distribution of airborne MFs in urban common indoor space types,focusing on occurrence characteristics of microplastic fibers（MPFs）and impacts of AC filters on the distribution of indoor MFs,and further explored characteristics of microbiome loaded on MFs and their health risks.In this study,we first collected suspended MFs and MFs fallout samples from three types of spaces including living rooms,student dormitories and offices by 24-hour continuous sampling.These samples were then identified and analyzed by combination of stereo microscopy andμ-fourier transform infrared（μ-FTIR）spectroscopy.After that,we analyzed contamination characteristics of MPFs on AC filters,and further designed indoor simulation experiments to explore accumulation and release patterns of MPFs on AC filters,and estimated relative human exposure risks.Finally,biofilm morphology on indoor MFs was further observed,and the number of microorganisms loaded on MFs and bacterial diversity characteristics were analyzed by plate counting method and 16s r RNA high-throughput sequencing technology to predict the bacterial phenotypic characteristics and assess their potential health risks.The main results were as follows.（1）In the three typical indoor environments including living rooms,dormitories and offices,the abundance of suspended MFs and MFs fallout were 6.07 and 9.30×10~3items/m~2/day,respectively.MFs were predominantly transparent,blue,and purple.Among them,the proportion of MPFs was about 32.5%,with polyethylene terephthalate（PET）,rayon and cellophane fibers being the most dominant types of MPFs.The abundance and size of MFs fallout were significantly higher than that of suspended MFs in all sample sites（p<0.05）.Among the three types of indoor spaces,the highest abundance and the biggest size of MFs were found in dormitories.（2）MFs were detected in all AC filter samples,and the average density of MFs was 1.47-21.4×10~2 items/cm~2.The sizes of MFs in AC filters in living rooms,dormitories and offices were 12±430μm,670±918μm,and 526±628μm,respectively.In addition,27.7-35.0%of the MFs were MPFs,most of which were PET（45.3%）,rayon（27.8%）and cellophane（20.1%）,while most of the natural fibers were cotton.The main primary colors were transparent,blue,and purple,and other colors accounted for less than 20%of the total MFs.（3）Simulated cumulative experiments revealed that the density of MPFs on AC filters gradually increased with the increase of AC’s running time.Compared with the office and living room,the dormitory’AC filter showed the faster accumulation rate of MPFs.The simulative release experiment showed that some of these accumulated MPFs on AC filters were released into the indoor air again.Statistical analysis showed that the estimated maximum daily human exposure to MPFs caused by AC filters increased with usage of ACs,reaching a maximum value of 11.2±2.2-44.0±8.9items/kg-bw/day at day 70,exposure risks varied among different age groups and the highest risk for infants.（4）Microorganisms were attached to the surface of both indoor MFs fallout and MFs on AC filters.The number of bacteria on MFs ranged from 0.23 to 84.0×10~7 cfu/g,and bacteria attached on MFs fallout were significantly more than those loaded by MFs from AC filters.Forty-two bacterial phyla were detected on all MFs samples,but genus-level clustering analysis revealed significant differences in the microbial community composition between the fallout group and AC filters group.By linear discriminant analysis（LEf Se）,the abundance of pathogenic risk-associated bacteria such as Betaproteobacteriales,Ralstonia,and Streptococcaceae were found to be significantly higher on MFs from AC filters than those on MFs fallout.The above results indicate that:（1）There are prevalent micro（plastic）fibers in indoor environments.MFs can suspend in the air,and large size MFs is more likely settle into fallout dust.PET and cotton fibers are the most common indoor MPFs and natural fibers,indicating that textiles are an important source of indoor MFs.The distribution of indoor MFs can be influenced by application of textile,the degree of human activity,and ventilation.（2）MPFs contamination is prevalent on the filters in split-type ACs.As ACs are used for a longer period of time,MPFs can accumulate on the filters,and some of them would be released into the indoor air again.The AC filter can act as a"sink"and"source"of MPFs,and become a transit station for indoor airborne MPFs.According to our study,AC filters should be replaced or cleaned after usage of about 70 days,in order to reduce the risk of human exposure.（3）Indoor MFs can become carriers of microorganisms including bacteria and fungi,and MFs fallout and MFs on AC filters have different microbial loading and community structure characteristics,but all have high biofilm-forming and pathogenic potential.Due to the confined and humid environment,MFs on AC filters load more species of pathogenic bacteria than MFs fallout,which may cause higher health risks of human skin contact or inhalation.This work revealed systematically the prevalence of airborne MFs and MPFs contamination in indoor environments.For the first time,we discovered that AC filters could be a sink and a source for indoor air MPFs through simulation experiments,which in turn affected the distribution of indoor MPFs.We also found that indoor MFs can be loaded with bacteria of associated contamination characteristics,and may cause disease causing risks.These findings provide new ideas for indoor MPFs pollution monitoring,and offer a scientific basis for health risk assessment of this type of emerging pollutants. |
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