Effect of aerosol particles generated by ultrasonic humidifiers on the lung in mouse
- Masakazu Umezawa†1, 2Email author,
- Keisuke Sekita†1,
- Ken-ichiro Suzuki2,
- Miyoko Kubo-Irie2,
- Rikio Niki2,
- Tomomi Ihara3,
- Masao Sugamata3 and
- Ken Takeda1, 2
© Umezawa et al.; licensee BioMed Central Ltd. 2013
Received: 28 August 2013
Accepted: 17 December 2013
Published: 21 December 2013
Ultrasonic humidifiers silently generate water droplets as a cool fog and produce most of the dissolved minerals in the fog in the form of an aerosolized “white dust.” However, the health effect of these airborne particles is largely unknown. This study aimed to characterize the aerosol particles generated by ultrasonic humidifiers and to investigate their effect on the lung tissue of mice.
An ultrasonic humidifier was operated with tap water, high-silica water, ultrapure water, or other water types. In a chamber (0.765 m3, ventilation ratio 11.5 m3/hr), male ICR mice (10-week-old) were exposed by inhalation to an aerosol-containing vapor generated by the humidifier. After exposure for 7 or 14 days, lung tissues and bronchoalveolar lavage fluid (BALF) were collected from each mouse and examined by microarray, quantitative reverse transcription-polymerase chain reaction, and light and electron microscopy.
Particles generated from the humidifier operated with tap water had a mass concentration of 0.46 ± 0.03 mg/m3, number concentration of (5.0 ± 1.1) × 104/cm3, and peak size distribution of 183 nm. The particles were phagocytosed by alveolar macrophages in the lung of mice. Inhalation of particles caused dysregulation of genes related to mitosis, cell adhesion molecules, MHC molecules and endocytosis, but did not induce any signs of inflammation or tissue injury in the lung.
These results indicate that aerosol particles released from ultrasonic humidifiers operated with tap water initiated a cellular response but did not cause severe acute inflammation in pulmonary tissue. Additionally, high mineral content tap water is not recommended and de-mineralized water should be recommended in order to exclude any adverse effects.
The indoor air environment is important for human health because humans spend most of their time indoors . Volatile organic compounds , microorganisms , environmental allergens , and particle matter  have been reported as important factors of the indoor environment. It is necessary to conduct safety assessments of consumer products, e.g., air conditioners, humidifiers, and air purifiers, which are all prevalent in commercial facilities, hospitals, schools, and homes.
The most important factors of the indoor air environment are temperature and humidity [6, 7]. Humidifiers are used to prevent excessive drying and to maintain comfortable room humidity. There are three types of humidifier: evaporative, steam, and ultrasonic vaporizers . While ultrasonic humidifiers need little electricity to work and release cooler vapor than the steam-type, they must be cleaned to avoid bacterial contamination since the vapor contains all impurities that are present in the reservoir [9, 10]. Only ultrasonic humidifiers release most of the dissolved and suspended components of the water, including microorganisms and pathogens, into the air . Fatal pulmonary damage, i.e., rapidly progressive respiratory fibrosis, has also been reported when a water aerosol which contained biocides was released from humidifiers and was inhaled by humans .
Even if the water has no contamination by microorganisms or pathogens, ultrasonic humidifiers may exert some effects on human health. Previous studies showed that ultrasonic humidifiers may release dissolved minerals as an aerosol . Highsmith et al.  reported that the fine particle concentration exceeded 6.3 mg/m3 when an ultrasonic humidifier was operated in a closed room. A subsequent study reported that the “white dust” induced lung injury . However, the detailed components of the aerosol (or dust) and its health effect (e.g., inflammatory response in the lung) remained unknown. The aim of the present study was to characterize the aerosol particles generated by an ultrasonic humidifier and to investigate their effects on the lung in a mouse model.
Ultrasonic humidifier operation
An ultrasonic humidifier, BBH-07 (Hanwa Ltd., Osaka, Japan) was placed in each chamber with a volume of 0.765 m3 and a ventilation ratio of 11.5 m3/hr in the Center for Environmental Health Science for the Next Generation (Research Institute for Science and Technology, Tokyo University of Science, Noda, Chiba, Japan). The humidifier was operated at approximately 60 mL/hr liquid output rate with one of the following types of water: tap water obtained in Noda-city (Chiba, Japan), high-silica water purchased from Riken Mineral Kenkyusho K.K (Ebino, Miyazaki, Japan), a graded series of calcium chloride solution (4–400 mg/L of calcium), reverse osmotic membrane-filtration water (RO water) purchased from Ako Kasei Co., Ltd. (Ako, Hyogo, Japan), or an ultrapure water generated by an Automatic Sanitization Module (Merck Millipore, Billerica, MA, USA). Tap water and high-silica water were examples of drinking water containing minerals, while the calcium chloride solutions were used as a model of mineral-dissolved water with known concentrations. The concentration of Na, Ca, Mg, and Si in each type of water was measured using inductively coupled plasma mass spectrometry (ICP-MS) by Murata Keisokuki Service Co., Ltd. (Kanagawa, Japan). The humidifier was operated at approximately 80% power.
Characterization of particles from the humidifier
The mass concentration of particles in the chamber was measured by a Piezobalance dust monitor Model 3521 (Kanomax Japan Inc., Osaka, Japan). The number concentration of particles was measured by a portable particle counter CPC 3007 (Tokyo Dylec Co., Tokyo, Japan), which can count particles with 10–1000 nm diameter. The size distribution of particles (10–410 nm) was measured by a switch-mode power supplies system Model 3936 (TSI Inc., Shoreview, MN, USA) composed of a classifier DMA3081 (TSI Inc.) and a condensation particle counter CPC 3785 (TSI Inc.) at a flow rate of 0.6 L/min.
Airborne particles in the chamber were classified using a cascade impactor (NL-3-2.5c0.5c, Tokyo Dylec). Fractions of 0.5 − 2.5 μm and <0.5 μm were collected for 30 sec and 15 min, respectively, at a flow rate of 0.3 L/min. The fractionated particles were collected on a silicon wafer (SI-500452, Nilaco Co., Tokyo, Japan) or a collodion film-patched mesh (200 mesh Cu; Nisshin EM Co., Tokyo, Japan) by electrostatic force using a sampler for suspended particle matter, SSPM-100 (Shimadzu Co., Kyoto, Japan). To determine the form and elemental component of the particles, they were observed by a field emission-type scanning electron microscope/energy-dispersive X-ray spectrometer (FE-SEM/EDS) (JSM-6500 F, JEOL Ltd., Akishima, Tokyo, Japan) with accelerating voltage of 15.0 kV and a transmission electron microscope (TEM: JEM-1200EX II, JEOL) with accelerating voltage of 90.0 kV.
Exposure of mice to particles released from an ultrasonic humidifier
Experimental conditions for analysis of the effect of particles generated by ultrasonic humidifier on mouse lung
8 hr/day, 7 days
Tap water/Ultrapure water
8 hr/day, 7 days
High-silica water/Ultrapure water
24 hr/day, 7 days
Tap water/Ultrapure water
24 hr/day, 7 days
High-silica water/Ultrapure water
24 hr/day, 14 days
High-silica water/Ultrapure water
BALF cell analysis
The cells in BALF were collected by centrifugation at 1,000 × g for 10 min and resuspended in phosphate-buffered saline (pH 7.4). After counting the total number of cells, they were observed in a Giemsa-stained smear on a glass slide under a light microscope (BX51; Olympus Co., Tokyo, Japan). Statistical analysis was performed using an unpaired t-test and the level of significance was set at P < 0.05.
Total RNA isolation
Lung tissue was homogenized in Isogen (Nippon Gene Co., Ltd., Tokyo, Japan). Total RNA was isolated by chloroform, purified by isopropanol, precipitated in 70% ethanol according to the manufacturer’s protocol and then suspended in RNase-free water. The RNA quantity was determined by absorption spectrophotometry at OD260 in a BioPhotometer plus (Eppendorf, Hamburg, Germany). Isolated RNA from each sample was provided for quantitative RT-PCR and microarray analyses.
Complementary DNA microarray
Total RNAs (n = 4 − 5/group) were pooled (45 μg) for each group and purified by RNeasy Micro Kit (Qiagen, Hilden, Germany). The integrity of RNA was evaluated by Bioanalyzer 2100 (Agilent Technologies Inc., Santa Clara, CA, USA). Complementary-DNA (cDNA) of each of the RNA samples was labeled by Cy3 and hybridized to the SurePrint G3 Mouse Gene Expression 8 × 60 K Microarray (Agilent Technologies) consisting of 62976 spots (containing probes for 24163 mRNAs) according to the protocol of Oncomics Co. Ltd. (Nagoya, Aichi, Japan). The microarray was then washed using Gene Expression Wash Pack (Agilent Technologies) and scanned by a DNA Microarray Scanner (Agilent Technologies). Scanner output images were normalized and digitalized by Feature Extraction software (Agilent Technologies) according to the Minimum Information About a Microarray Experiment (MIAME) guidelines .
Hierarchical cluster analysis
To extract gene sets for which differential expression was induced by airborne particles generated by the ultrasonic humidifier, microarray data from experiments 1–5 (Table 1) were hierarchically clustered using a complete linkage algorithm and Euclidean distance as the distance metric . The analysis was performed using Cluster 3.0  and the result was visualized by Java TreeView .
Functional analysis of microarray data
To better understand the biological meaning of the microarray results, functional analysis was performed using gene annotation by gene ontology (GO) and pathway. Genes were annotated with GO and pathway using an annotation file (gene2go.gz) provided by the National Center for Biotechnology Information (NCBI; Bethesda, MD, USA)  and c2.cp.v3.1.symbols.gmt by the Broad Institute (Cambridge, MA, USA) . The annotations were updated on March 6, 2013. Enrichment factors for each GO and pathway were defined as (nf/n)/(Nf/N), where nf is the number of flagged (differentially expressed) genes within the category, Nf is the total number of genes within that same category, n is the number of flagged genes on the entire microarray, and N is the total number of genes on the microarray. Statistical analysis was done with Fisher’s exact test based on a hypergeometric distribution and then the GO and pathways with enrichment factors ≥3, nf ≥3 and P <0.01 were extracted.
Total RNA (1 μg) for each sample was treated by Dnase (Promega Co., Fitchburg, WI, USA) and then by M-MLV reverse transcriptase (Invitrogen Co., Carlsbad, CA, USA) to obtain first strand cDNA according to the manufacturer’s instructions. Quantitative PCR was performed in a 96-well plate and an Mx3000P (Agilent Technologies) with SYBR Green Realtime PCR Master Mix (Thunderbird; Toyobo Co., Ltd., Osaka, Japan) and specific primers (Fasmac Co. Ltd., Atsugi, Kanagawa, Japan) or with Probe qPCR Master Mix (Thunderbird; Toyobo Co.) and primer (Fasmac)/probe (Biosearch Technologies Japan, Inc., Tokyo, Japan) sets for indicated genes (Additional file 1: Table S1). Statistical analysis was performed using an unpaired t-test and the level of significance was set at P < 0.05.
Lung tissue was fixed in phosphate-buffered (pH7.4) 4% formaldehyde, and then cryoprotected in phosphate-buffered 30% sucrose solution for 24–48 hr. Next, brains were frozen and cut into 4-μm sections using a cryostat (Tissue-Tek Cryo; Sakura Finetek Japan). Immunohistochemical visualization of a macrophage marker, F4/80, was performed using antibodies and avidin − biotin-peroxidase methods. After blocking endogenous peroxidase and preincubation with 10% normal horse serum, sections were treated overnight with primary rat monoclonal anti-F4/80 antibody (ab6640; Abcam plc, Cambridge, UK), secondary biotinylated donkey anti-rat IgG (AP189B; Merck Millipore) for 2 hr, and finally treated with an avidin − biotin-peroxidase complex (Vectastain ABC peroxidase kit, Vector Laboratories, CA, USA) for 4 hr. Sections were then reacted for peroxidase activity in a solution of 0.04% 3,3′-diaminobenzidene (DAB) in 0.1 M Tris–HCl buffer (pH 7.6) and 0.01% hydrogen peroxide water. Immunoreactive cells on sections were observed under BX51 light microscope. Quantitative analysis was performed on all sections. Statistical analysis was performed using an unpaired t-test and the level of significance was set at P < 0.05.
Electron microscopic analysis
Lung tissues and BALF cells were prefixed in cacodylate-buffered 2.5% glutaraldehyde (pH 7.4) for 24 hr, washed in cacodylate buffer, and postfixed with 2% osmium tetroxide (Nisshin EM) for 1 hr. After washing in cacodylate buffer, the tissue samples were dehydrated using a graded series of ethanol (up to 100%) and propylene oxide (Nisshin EM), and then embedded in Quetol 812 (Nisshin EM). Ultra-thin sections (80-nm thick) were cut on the ultra-microtome EMUC6 (Leica Microsystems K.K., Tokyo, Japan). Some sections were double-stained with uranyl acetate and lead citrate. They were then observed under the TEM (JEM-1200EX II) with accelerating voltage of 80–90 kV.
Mineral and silica concentration of each water type
Mass concentration of mineral and silica in each type of water
Concentration and size distribution of particles released from humidifier
Concentration and size of airborne particles released from ultrasonic humidifier for each type of water
Mass concentration (mg/m 3)
Number concentration (#/cm 3)
Peak of size distribution (nm)
0.46 ± 0.03
(5.0 ± 1.1) × 104
183 ± 9
0.58 ± 0.04
(3.6 ± 0.0) × 104
180 ± 3
CaCl2 (400 mg/L Ca)
1.66 ± 0.16
(6.3 ± 0.5) × 104
CaCl2 (200 mg/L Ca)
1.26 ± 0.19
(4.2 ± 0.3) × 104
356 ± 11
CaCl2 (80 mg/L Ca)
0.46 ± 0.11
(5.0 ± 0.1) × 104
283 ± 9
CaCl2 (40 mg/L Ca)
0.12 ± 0.03
(4.5 ± 0.0) × 104
222 ± 4
CaCl2 (10 mg/L Ca)
0.08 ± 0.01
(3.4 ± 0.1) × 104
123 ± 4
CaCl2 (4 mg/L Ca)
0.02 ± 0.01
(2.7 ± 0.3) × 104
108 ± 2
(4.4 ± 0.6) × 104
90 ± 2
(0.7 ± 0.0) × 104
To investigate the correlations between mineral concentration in water and each parameter of generated particle, we characterized the particles released from the ultrasonic humidifier using water containing serial concentrations of calcium chloride solution (4–400 mg/L Ca) (Table 3). The concentration of calcium chloride in water was positively correlated with the mass and number concentrations (Figure 1C and D) and the peak size distribution (Figure 1B and E) of generated particles. The correlation of the concentration of dissolved material in water with the mass concentration of particles generated was linear (r =0.97) (Figure 1C), whereas the correlation with the number concentration was logarithmic (r =0.98) (Figure 1D). These correlations were also observed in particles from the humidifier with serial concentrations of sodium chloride solution (4–400 mg/L Na) (data not shown). The mass concentration of the aerosol particles was also correlated with total mineral concentration of tap water, high-silica water and RO water (Figure 1A).
Form and elemental composition of particles released from humidifier
Effect of particles released from ultrasonic humidifier on mRNA expression in the lung
Significantly enriched GO categories from the microarray data
Regulation of axonogenesis
Antigen processing and presentation of peptide or polysaccharide antigen via MHC class II
Chromosome, centromeric region
MHC class II protein complex
Peptide antigen binding
Gamma-aminobutyric acid signaling pathway
Retinol metabolic process
Adenylate cyclase-activating G-protein coupled receptor signaling pathway
Antigen processing and presentation of exogenous peptide antigen via MHC class II
Significantly enriched pathways from the microarray data
CENP-A NAC-CAD complex (MIPS)
PLK1 pathway (PID)
Cell adhesion molecules (KEGG)
Viral myocarditis (KEGG)
Allograft rejection (KEGG)
Mitotic prometaphase (Reactome)
Autoimmune thyroid disease (KEGG)
Graft versus host disease (KEGG)
Antigen processing and presentation (KEGG)
Type I Diabetes Mellitus (KEGG)
Effect of particles released from ultrasonic humidifier on histology of the lung and alveolar macrophage
Tap water contains dissolved solid composed of calcium, sodium, other minerals and anions. The secondary water quality standard of total dissolved solids (TDS) is established as 500 mg/L by the Ministry of Health, Labour and Welfare (Japan) and the Environmental Protection Agency (US). Water also has ‘hardness’, which is definable as the concentration of resistant solid matter determined as the equivalent concentration of calcium carbonate. Since the concentration and size distribution of particles from ultrasonic humidifier were well correlated with the concentration of minerals, they can be estimated by the TDS and hardness of water provided to the humidifier.
Humidifiers are usually operated with tap water, and in these experiments we found that operation with tap water generated submicron-sized particles (100–1000 μm), which contribute to the mass concentration, and that the humidifier released them into the air. In contrast, the particles from the humidifier operated with water containing a low concentration of minerals (4 mg/L Na or Ca) were mostly nano-sized (<100 nm); therefore they had only a small mass (<0.03 mg/m3) but a large number concentration (>2 × 104/cm3). This is the first study showing the characteristics of particles from the humidifier with tap water as well as other water with a low concentration of minerals. In the study using a series of calcium chloride solutions, the mass and the number concentrations were not well correlated. We concluded the cause of this worse correlation was another correlation between the mineral concentration of water and the particle size; i.e., the size was larger when the humidifier was operated with a water with higher concentration of minerals. Our data for the concentration of particles from ultrasonic humidifiers were measured in an experimental chamber (0.765 m3, ventilation ratio 11.5 m3/hr), whereas 0.59 mg/m3, which is much greater than daily PM2.5 standard (35 μg/m3), was also reported when the humidifier was operated in an actual residence environment with tap water containing 303 mg/L TDS .
The health effect of airborne particles is one of the main issues in environmental toxicology. The size distribution data on the fog also indicated that it substantially contained submicron-sized particles, which can pass the nasal airways and penetrate down to the lung . Since a case report describing lung injury by “white dust”  suggested that the particles from ultrasonic humidifiers may exert adverse effects on the lung, we examined the effects in detail using a mouse model. Although the bacterial contamination was not investigated in the present study, chlorine-disinfected tap water, a commercial mineral water for drinking, and purified water were used for the experiments to minimize the contamination. In general, the insoluble component of the particles are not dissolved and deposit in pulmonary surfactant, where they are taken up by alveolar macrophages [25, 26]. The inhaled particles taken up by the macrophages are transferred to intracellular vesicle and phagolysosome , exposed to acidic milieu, and can cause lysosome destabilization leading to inflammation . TEM observation in the present study showed that the particles were being dissolved in the phagosome, and also indicated that the particles generated by the ultrasonic humidifier disintegrated in the phagolysosome of the macrophages. This observation was consistent with our microarray data suggesting that genes dysregulated by the particles from the ultrasonic humidifier were enriched in GO categories associated with MHC molecules and endocytosis. However, expression of H2-Eb1 was suppressed by inhalation of particles from the humidifier with tap water or high-silica water. The result was in opposition to the effect of pulmonary exposure (intratracheal instillation) to titanium dioxide nanoparticles  and fullerenes  on the lung, suggesting that 7- and 14-day inhalation of the particles from the humidifier induced a cellular response from the alveolar macrophages but did not cause acute or sub-acute toxic effects on the lung tissue. This observation was supported by RT-PCR data suggesting that no significant difference in Ccl2, Cxcl1, and Tnf expression between the tap water or high-silica water group and each control (ultrapure water) group. A previous study showed that inhalation of silica-coated nanoparticles (10 mg/m3; 2 hr/day for 4 days) enhanced Cxcl1 and Tnf expression in mouse lung . Even though the total exposure period in the present study was longer than this previous study, the concentration of airborne particles containing silica from the humidifier was lower. The genes differentially expressed by inhalation of particles from the humidifier were enriched in mitosis, meiosis and related GO and pathways. However, histopathology (including cell proliferation) and macrophage location were not affected by the particles derived from tap water or high-silica water. Our data on the mononuclear cells in BALF indicated that alveolar macrophages tended to be activated by the aerosol particles. The results suggested that 7- and 14-day inhalation of the particles from the humidifier did not cause acute or sub-acute toxic effects on the lung of mice without pre-existing respiratory infection, disease or malfunction.
The present study showed the characteristics of particles generated from ultrasonic humidifiers operated with tap water, a commercial mineral (high-silica) water, and other types of water. This study also showed the effect of 7- and 14-day inhalation of particles released from the humidifier on the lung in a mouse model. The particles were composed of multiple elements including sodium, magnesium, silicate, sulfate, and calcium. Mass and number concentrations and the peak size of the particles were positively correlated to concentration of dissolved mineral components in water provided to the humidifier. Inhalation of particles caused dysregulation of genes related to mitosis, cell adhesion molecules, MHC molecules and endocytosis followed by antigen processing, but did not induce any signs of inflammation or tissue injury in the lung. We conclude that the particles released from the humidifier operated with tap water and commercial mineral water induced a cellular response from the alveolar macrophages but did not cause acute or sub-acute toxic effects on the pulmonary organs in the mouse model. Finally, since the mass concentration of particles generated is linearly correlated with the concentration of dissolved material in water, cautions may be needed to use the humidifier in some areas with hard water. Long exposure to the aerosol particle from the humidifier may occur in home and working environment. High mineral content tap water is not recommended and de-mineralized water should be recommended in order to exclude any adverse effects.
Bronchoalveolar lavage fluid
Energy-dispersive X-ray spectrometer
Field emission-type scanning electron microscope
Inductively coupled plasma mass spectrometry
Total dissolved solids
Transmission electron microscope.
We thank the graduate and undergraduate students in the Takeda laboratory, especially Mr. Ryuhei Shimizu and Mr. Hiroshi Hori, for their technical assistance. This work was supported in part by a MEXT-Supported Program for the Strategic Research Foundation at Private Universities (Ken Takeda; 2011–2015) and a JSPS KAKENHI Grant Number 24790130 (Masakazu Umezawa; 2012–2013).
- Klepeis NE, Nelson WC, Ott WR, Robinson JP, Tsang AM, Switzer P, Behar JV, Hern SC, Engelmann WH: The National Human Activity Pattern Survey (NHAPS): a resource for assessing exposure to environmental pollutants. J Expo Anal Environ Epidemiol 2001, 11: 231–252. 10.1038/sj.jea.7500165View ArticlePubMed
- Norbäck D, Torgén M, Edling C: Volatile organic compounds, respirable dust, and personal factors related to prevalence and incidence of sick building syndrome in primary schools. Br J Ind Med 1990, 47: 733–741.PubMed CentralPubMed
- Harrison J, Pickering CA, Faragher EB, Austwick PK, Little SA, Lawton L: An investigation of the relationship between microbial and particulate indoor air pollution and the sick building syndrome. Respir Med 1992, 86: 225–235. 10.1016/S0954-6111(06)80060-0View ArticlePubMed
- Carrer P, Maroni M, Alcini D, Cavallo D: Allergens in indoor air: environmental assessment and health effects. Sci Total Environ 2001, 270: 33–42. 10.1016/S0048-9697(00)00791-9View ArticlePubMed
- Oeder S, Dietrich S, Weichenmeier I, Schober W, Pusch G, Jörres RA, Schierl R, Nowak D, Fromme H, Behrendt H, Buters JT: Toxicity and elemental composition of particulate matter from outdoor and indoor air of elementary schools in Munich, Germany. Indoor Air 2012, 22: 148–158. 10.1111/j.1600-0668.2011.00743.xView ArticlePubMed
- Fang L, Wyon DP, Clausen G, Fanger PO: Impact of indoor air temperature and humidity in an office on perceived air quality, SBS symptoms and performance. Indoor Air 2004,14(Suppl):74–81.View ArticlePubMed
- Melikov AK, Skwarczynski MA, Kaczmarczyk J, Zabecky J: Use of personalized ventilation for improving health, comfort, and performance at high room temperature and humidity. Indoor Air 2013, 23: 250–263. 10.1111/ina.12012View ArticlePubMed
- Environmental protection Agency: Indoor Air Facts No.8 Use and Care of Home Humidifiers. 1991. http://www.epa.gov/iaq/pdfs/humidifier_factsheet.pdf
- Kane GC, Marx JJ, Prince DS: Hypersensitivity pneumonitis secondary to Klebsiella oxytoca, A new cause of humidifier lung. Chest 1993, 104: 627–629. 10.1378/chest.104.2.627View ArticlePubMed
- Suda T, Sato A, Ida M, Gemma H, Hayakawa H, Chida K: Hypersensitivity pneumonitis associated with home ultrasonic humidifiers. Chest 1995, 107: 711–717. 10.1378/chest.107.3.711View ArticlePubMed
- Tyndall LR, Lehman SE, Bowman KE, Milton KD, Barbaree MJ: Home humidifiers as a potential source of exposure to microbial pathogens endotoxins and allergens. Indoor Air 2004, 5: 171–178.View Article
- Cheong HK, Ha M, Lee JH: Unrecognized bomb hidden in the babies’ room: fetal pulmonary damage related with use of biocide in humidifiers. Environ Health Toxicol 2012, 162: e2012001.View Article
- Rodes C, Smith T, Crouse R, Ramachandran G: Measurements of the size distribution of aerosols produced by ultrasonic humidification. Aerosol Sci Technol 1990, 13: 220–229. 10.1080/02786829008959440View Article
- Highsmith VR, Rodes CE, Hardy RJ: Indoor particle concentrations associated with use of tap water in portable humidifiers. Environ Sci Technol 1988, 22: 1109–1112. 10.1021/es00174a019View ArticlePubMed
- Daftary AS, Deterding RR: Inhalational lung injury associated with humidifier “white dust”. Pediatrics 2011, 127: e509-e512. 10.1542/peds.2010-1312View ArticlePubMed
- Brazma A, Hingamp P, Quackenbush J, Sherlock G, Spellman P, Stoeckert C, Aach J, Ansorge W, Ball CA, Causton HC, Gaasterland T, Glenisson P, Holstege FC, Kim IF, Markowitz V, Matese JC, Parkinson H, Robinson A, Sarkans U, Schulze-Kremer S, Stewart J, Taylor R, Vilo J, Vingron M: Minimum information about a microarray experiment (MIAME)-toward standards for microarray data. Nat Genet 2001, 29: 365–371. 10.1038/ng1201-365View ArticlePubMed
- Quackenbush J: Computational analysis of microarray data. Nat Rev Genet 2001, 2: 418–427. 10.1038/35076576View ArticlePubMed
- Eisen MB, Spellman PT, Brown PO, Botstein D: Cluster analysis and display of genome-wide expression patterns. Proc Natl Acad Sci USA 1998, 95: 14863–14868. 10.1073/pnas.95.25.14863PubMed CentralView ArticlePubMed
- Saldanha AJ: Java treeview–extensible visualization of microarray data. Bioinformatics 2004, 20: 3246–3248. 10.1093/bioinformatics/bth349View ArticlePubMed
- DATA folder in FTP site of National Center for Biotechnology Information ftp://ftp.ncbi.nih.gov/gene/DATA/
- MSigDB/Download in the GSEA site of Broad Institute http://www.broadinstitute.org/gsea/downloads.jsp#msigdb
- Hung DV, Tong S, Nakano Y, Tanaka F, Hamanaka D, Uchino T: Measurements of particle size distributions produced by humidifiers operating in high humidity storage environments. Biosyst Eng 2010, 107: 54–60. 10.1016/j.biosystemseng.2010.07.001View Article
- Rossi EM, Pylkkänen L, Koivisto AJ, Vippola M, Jensen KA, Miettinen M, Sirola K, Nykäsenoja H, Karisola P, Stjernvall T, Vanhala E, Kiilunen M, Pasanen P, Mäkinen M, Hämeri K, Joutsensaari J, Tuomi T, Jokiniemi J, Wolff H, Savolainen K, Matikainen S, Alenius H: Airway exposure to silica-coated TiO 2 nanoparticles induces pulmonary neutrophilia in mice. Toxicol Sci 2010, 113: 422–433. 10.1093/toxsci/kfp254View ArticlePubMed
- Oberdörster G, Oberdörster E, Oberdörster J: Nanotoxicology: an emerging discipline evolving from studies of ultrafine particles. Environ Health Perspect 2005, 113: 823–839. 10.1289/ehp.7339PubMed CentralView ArticlePubMed
- Geiser M, Kreyling WG: Deposition and biokinetics of inhaled nanoparticles. Part Fibre Toxicol 2010, 7: 2. 10.1186/1743-8977-7-2PubMed CentralView ArticlePubMed
- Scherbart AM, Langer J, Bushmelev A, Van Berlo D, Haberzettl P, Van Schooten FJ, Schmidt AM, Rose CR, Schins RP, Albrecht C: Contrasting macrophage activation by fine and ultrafine titanium dioxide particles is associated with different uptake mechanisms. Part Fibre Toxicol 2011, 8: 31. 10.1186/1743-8977-8-31PubMed CentralView ArticlePubMed
- Geiser M, Casaulta M, Kupferschmid B, Schulz H, Semmler-Behnke M, Kreyling W: The role of macrophages in the clearance of inhaled ultrafine titanium dioxide particles. Am J Respir Cell Mol Biol 2008, 38: 371–376. 10.1165/rcmb.2007-0138OCView ArticlePubMed
- Donaldson K, Schinwald A, Murphy F, Cho WS, Duffin R, Tran L: The biologically effective dose in inhalation nanotoxicology. Acc Chem Res 2013, 46: 723–732. 10.1021/ar300092yView ArticlePubMed
- Park EJ, Yoon J, Choi K, Yi J, Park K: Induction of chronic inflammation in mice treated with titanium dioxide nanoparticles by intratracheal instillation. Toxicology 2009, 260: 37–46. 10.1016/j.tox.2009.03.005View ArticlePubMed
- Park EJ, Kim H, Kim Y, Yi J, Choi K, Park K: Carbon fullerenes (C 60 s) can induce inflammatory responses in the lung of mice. Toxicol Appl Pharmacol 2010, 244: 226–233. 10.1016/j.taap.2009.12.036View ArticlePubMed
This article is published under license to BioMed Central Ltd. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.