Visualization of Dust Generation in Outdoor Workplaces Using A Wearable Particle Monitor and Global Navigation Satellite System.

We manufactured a wearable particle monitor (WPM), which is a simple and low-cost dust monitor. We aimed to evaluate the usefulness of the device by using it and location information of a Global Navigation Satellite System (GNSS) to measure dust generation in outdoor workplaces. We used nine WPMs and a particle counter KC-52 to measure in parallel the dust concentration diffusing standard particles in a dust exposure apparatus to evaluate the measurability of the WPM, and visualized dust generation in outdoor workplaces to evaluate its usability. We obtained location information using a GNSS in parallel with measuring with the WPM. The measured values of the WPM followed the measured values of the KC-52, with a strong correlation of the values between the KC-52 and each WPM. The discrepancy among devices tended to increase, however, because the measured values of the WPMs increased. For outdoor measurements, we could create a heat map of the relative values of dust generation by combining two data of the WPM and the GNSS. The methods of using the WPM could overview the conditions needed to produce dust emissions in dust-generating workplaces.

Combining Indoor Positioning Using Wi-Fi Round Trip Time with Dust Measurement in the Field of Occupational Health.

Monitoring of personal exposure to hazardous substances has garnered increasing attention over the past few years. However, no straightforward and exact indoor positioning technique has been available until the recent discovery of Wi-Fi round trip time (Wi-Fi RTT). In this study, we investigated the possibility of using a combination of Wi-Fi RTT for indoor positioning and a wearable particle monitor (WPM) to observe dust concentration during walking in a simulated factory. Ultrasonic humidifiers were used to spray sodium chloride solution inside the factory. The measurements were recorded three times on different routes (Experiments A, B, and C). The error percentages, i.e., measurements that were outside the expected measurement area, were 7% (49 s/700 s) in Experiment A, 2.3% (15 s/660 s) in Experiment B, and 7.8% (50 s/645 s) in Experiment C. The dust measurements were also recorded without any obstruction. A heat map was created based on the results from both measured values. Wi-Fi RTT proved useful for computing the indoor position with high accuracy, suggesting the applicability of the proposed methodology for occupational health monitoring.

[Design and Evaluation of Miniature Cyclones for Dust Indicators].

Dust indicators based on light scattering photometers are widely used to measure aerosol concentrations in work environments. These concentrations at workplaces in Japan are measured by these dust indicators and calibrated by mass concentration in order to control workers' exposure to dust. The mass concentration in a specific point in a workplace is measured simultaneously with a dust indicator. The mass concentration of the respirable fraction of dust particles should be determined by the gravimetric method with low volume air samplers or other devices, but some dust indicators are not equipped with a size separator for respirable fraction, and we used to get unstable results at the calibration. In this study, we designed miniature cyclones for a dust indicator and evaluated their performances of respirable fraction and PM2.5 fraction.

Health Effects of Toner Exposure Among Japanese Toner-Handling Workers: A 10-Year Prospective Cohort Study.

The main objective of this study was to evaluate the risk of the respiratory diseases, i.e. pneumoconiosis, lung fibrosis, granulomatous pneumonitis, lung cancer and bronchial asthma, which have been reported as related to toner exposure. The second main objective was to clarify the association between toner exposure and parameters related with toner-handling worker's health. We conducted a 10-year prospective cohort study from 2004 to 2013 in 296 Japanese toner-handling workers. The evaluation of toner exposure and medical health check were performed once a year. There was no obvious evidence of occurrence of lung diseases. We also investigated several health parameters to recognize the change of respiratory health before onset of pneumoconiosis, lung fibrosis, lung cancer and bronchial asthma. However there were some sporadic statistically significant findings, to bring all health parameters, we did not find obvious evidence that toner exposure would cause adverse health effects as a whole. We concluded that the possibility that toner exposure would cause adverse health effects was quite low.

Usefulness of myeloperoxidase as a biomarker for the ranking of pulmonary toxicity of nanomaterials.

BACKGROUND: In order to examine whether myeloperoxidase (MPO) can be a useful marker for evaluating the pulmonary toxicity of nanomaterials, we analyzed MPO protein in bronchoalveolar lavage fluid (BALF) samples obtained from previous examinations of a rat model. In those examinations we performed intratracheal instillation exposures (dose: 0.2-1.0 mg) and inhalation exposures (exposure concentration: 0.32-10.4 mg/m3) using 9 and 4 nanomaterials with different toxicities, respectively. Based on those previous studies, we set Nickel oxide nanoparticles (NiO), cerium dioxide nanoparticles (CeO2), multi wall carbon nanotubes with short or long length (MWCNT (S) and MWCNT (L)), and single wall carbon nanotube (SWCNT) as chemicals with high toxicity; and titanium dioxide nanoparticles (TiO2 (P90) and TiO2 (Rutile)), zinc oxide nanoparticles (ZnO), and toner with external additives including nanoparticles as chemicals with low toxicity. We measured the concentration of MPO in BALF samples from rats from 3 days to 6 months following a single intratracheal instillation, and from 3 days to 3 months after the end of inhalation exposure. RESULTS: Intratracheal instillation of high toxicity NiO, CeO2, MWCNT (S), MWCNT (L), and SWCNT persistently increased the concentration of MPO, and inhalation of NiO and CeO2 increased the MPO in BALF. By contrast, intratracheal instillation of low toxicity TiO2 (P90), TiO2 (Rutile), ZnO, and toner increased the concentration of MPO in BALF only transiently, and inhalation of TiO2 (Rutile) and ZnO induced almost no increase of the MPO. The concentration of MPO correlated with the number of total cells and neutrophils, the concentration of chemokines for neutrophils (cytokine-induced neutrophil chemoattractant (CINC)-1 and heme oxygenase (HO)-1), and the activity of released lactate dehydrogenase (LDH) in BALF. The results from the receiver operating characteristics (ROC) for the toxicity of chemicals by the concentration of MPO proteins in the intratracheal instillation and inhalation exposures showed that the largest areas under the curves (AUC) s in both examinations occurred at 1 month after exposure. CONCLUSION: These data suggest that MPO can be a useful biomarker for the ranking of the pulmonary toxicity of nanomaterials, especially at 1 month after exposure, in both intratracheal instillation and inhalation exposure.

Biopersistence of NiO and TiO2 Nanoparticles Following Intratracheal Instillation and Inhalation.

The hazards of various types of nanoparticles with high functionality have not been fully assessed. We investigated the usefulness of biopersistence as a hazard indicator of nanoparticles by performing inhalation and intratracheal instillation studies and comparing the biopersistence of two nanoparticles with different toxicities: NiO and TiO2 nanoparticles with high and low toxicity among nanoparticles, respectively. In the 4-week inhalation studies, the average exposure concentrations were 0.32 and 1.65 mg/m³ for NiO, and 0.50 and 1.84 mg/m³ for TiO2. In the instillation studies, 0.2 and 1.0 mg of NiO nanoparticles and 0.2, 0.36, and 1.0 mg of TiO2 were dispersed in 0.4 mL water and instilled to rats. After the exposure, the lung burden in each of five rats was determined by Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES) from 3 days to 3 months for inhalation studies and to 6 months for instillation studies. In both the inhalation and instillation studies, NiO nanoparticles persisted for longer in the lung compared with TiO2 nanoparticles, and the calculated biological half times (BHTs) of the NiO nanoparticles was longer than that of the TiO2 nanoparticles. Biopersistence also correlated with histopathological changes, inflammatory response, and other biomarkers in bronchoalveolar lavage fluid (BALF) after the exposure to nanoparticles. These results suggested that the biopersistence is a good indicator of the hazards of nanoparticles.

The Effectiveness of Specific Risk Mitigation Techniques Used in the Production and Handling of Manufactured Nanomaterials: A Systematic Review.

Many kinds of manufactured nanomaterials (MNMs) have been developed and used as basic materials of industrial products, and they may pose health risks for workers in not only developed countries but also in developing countries. Few studies have looked at the evidence for effects of controls that mitigate the risk of exposure to MNMs. Therefore, we systematically searched the literature from the year 2000 to 2015. We included studies that compared the use of an exposure control to the situation without such a technique and those that measured the exposure to MNMs as the outcome. In order to evaluate the effectiveness of these controls, we used their "protection factor", defined as the ratio between concentrations without and with the control. We located 1,131 references in PubMed and other lists, and out of these references, 41 studies fulfilled our inclusion criteria. We categorized them as engineering controls such as enclosure, local exhaust ventilation or process automation, and as personal protective equipment (PPE). For enclosure systems we found a protection factor beyond 100. For other engineering controls, the better controls scored 10 to 20, but many cases of local exhaust ventilation had a protection factor of less than 10 and some cases even increased exposure. PPE such as N95 or equivalent filtering respirators had a protection factor of approximately 10 tested with nano-sized aerosols. We conclude that there is low quality evidence that specific engineering controls can reduce exposure to MNMs but that enclosure is considerably more effective. For respiratory protection the evidence is of very low quality due to the lack of field studies. This information can be used to decide about controls when exposure to MNMs exceeds proposed occupational exposure limits or when no toxicological information is available for a MNM.

Long-term retention of pristine multi-walled carbon nanotubes in rat lungs after intratracheal instillation.

As a result of the growing potential industrial and medical applications of multi-walled carbon nanotubes (MWCNTs), people working in or residing near facilities that manufacture them may be exposed to airborne MWCNTs in the future. Because of concerns regarding their toxicity, quantitative data on the long-term clearance of pristine MWCNTs from the lungs are required. We administered pristine MWCNTs well dispersed in 0.5 mg ml(-1) Triton-X solution to rats at doses of 0.20 or 0.55 mg via intratracheal instillation and investigated clearance over a 12-month observation period. The pristine MWCNTs pulmonary burden was determined 1, 3, 7, 28, 91, 175 and 364 days after instillation using a method involving combustive oxidation and infrared analysis, combined with acid digestion and heat pretreatment. As 0.15- and 0.38-mg MWCNTs were detected 1 day after administration of 0.20 and 0.55 mg MWCNTs, respectively, approximately 30% of administrated MWCNTs may have been cleared by bronchial ciliary motion within 24 h of administration. After that, the pulmonary MWCNT burden did not decrease significantly over time for up to 364 days after instillation, suggesting that MWCNTs were not readily cleared from the lung. Transmission electron microscopy (TEM) showed that alveolar macrophages internalized the MWCNTs and retained in the lung for at least 364 days after instillation. MWCNTs were not detected in the liver or brain within the 364-day study period (<0.04 mg per liver, < 0.006 mg per brain).

Evaluation of Pulmonary Toxicity of Zinc Oxide Nanoparticles Following Inhalation and Intratracheal Instillation.

We conducted inhalation and intratracheal instillation studies of zinc oxide (ZnO) nanoparticles in order to examine their pulmonary toxicity. F344 rats were received intratracheal instillation at 0.2 or 1 mg of ZnO nanoparticles with a primary diameter of 35 nm that were well-dispersed in distilled water. Cell analysis and chemokines in bronchoalveolar lavage fluid (BALF) were analyzed at three days, one week, one month, three months, and six months after the instillation. As the inhalation study, rats were exposed to a concentration of inhaled ZnO nanoparticles (2 and 10 mg/m³) for four weeks (6 h/day, 5 days/week). The same endpoints as in the intratracheal instillation study were analyzed at three days, one month, and three months after the end of the exposure. In the intratracheal instillation study, both the 0.2 and the 1.0 mg ZnO groups had a transient increase in the total cell and neutrophil count in the BALF and in the expression of cytokine-induced neutrophil chemoattractant (CINC)-1, CINC-2, chemokine for neutrophil, and heme oxygenase-1 (HO-1), an oxidative stress marker, in the BALF. In the inhalation study, transient increases in total cell and neutrophil count, CINC-1,-2 and HO-1 in the BALF were observed in the high concentration groups. Neither of the studies of ZnO nanoparticles showed persistent inflammation in the rat lung, suggesting that well-dispersed ZnO nanoparticles have low toxicity.

Comparison between whole-body inhalation and nose-only inhalation on the deposition and health effects of nanoparticles.

OBJECTIVES: We performed the two inhalation exposures, whole-body inhalation and nose-only inhalation, to investigate the pulmonary deposition and health effects of the two inhalation methods. METHODS: In both methods, we exposed rats to the same TiO2 nanoparticles at almost the same exposure concentration for 6 h and compared the deposited amounts of nanoparticles and histopathological changes in the lungs. Rats were exposed to rutile-type TiO2 nanoparticles generated by the spray-dry method for 6 h. The exposure concentration in the whole-body chamber was 4.10 ± 1.07 mg/m(3), and that in nose-only chamber was 4.01 ± 1.11 mg/m(3). The particle sizes were 230 and 180 nm, respectively. A control group was exposed to fresh air. RESULTS: The amounts of TiO2 deposited in the lungs as measured by ICP-AES after acid digestion just after the exposure were: 42.6 ± 3.5 µg in the whole-body exposure and 46.0 ± 7.7 µg in the nose-only exposure groups. The histopathological evaluation was the same in both exposure groups: no infiltration of inflammatory cells in the alveolar space and interstitium, and no fibrosis. CONCLUSION: The two inhalation methods using the same material under the same exposure conditions resulted in the same particle deposition and histopathology in the lung.

Expression of cytokine-induced neutrophil chemoattractant in rat lungs following an intratracheal instillation of micron-sized nickel oxide nanoparticle agglomerates.

OBJECTIVE: In our previous study, we reported that the micron-sized nickel oxide nanoparticle agglomerates induced neutrophil infiltration and the gene expression of the cytokine-induced neutrophil chemoattractant (CINC)-2αß in a rat lung. In this study, we examined the expression of the CINCs family in the lung using the same rat model exposed to micron-sized nickel oxide nanoparticle agglomerates. METHODS: The count median diameter of nickel oxide nanoparticle agglomerates suspended in saline was 1.34 µm (primary diameter: 8.41 nm). Male Wistar rats received an intratracheal instillation of 1 mg (3.3 mg/kg) of nickel oxide nanoparticles and were dissected at 3 days, 1 week, 1 month, 3 months, and 6 months after the instillation. The negative control group received an instillation of saline. The concentration of CINC-1 in the lung and the bronchoalveolar lavage fluid (BALF), CINC-2αß in the BALF, and CINC-3 in the lung and the BALF was examined. RESULTS: The concentration of CINC-1 was elevated at 3 days, 3 months, and 6 months in the lung tissue and from 3 days to 6 months in the BALF. The concentration of CINC-2αß was elevated from 3 days to 3 months in the BALF. The concentration of CINC-3 was also elevated at 3 days, 1 week, 3 months, and 6 months in the lung tissue. Infiltration of neutrophil and alveolar macrophage was observed mainly in the alveoli during the observed time period. CONCLUSION: These results suggest that CINC-1 to -3 were totally involved in the lung injury caused by micron-sized nickel oxide nanoparticle agglomerates.

Analysis of pulmonary surfactant in rat lungs after intratracheal instillation of short and long multi-walled carbon nanotubes.

Multi-walled carbon nanotubes (MWCNTs) are interesting new materials, but there is some concern about their harmfulness due to their fibrous nature. To determine the difference in the biological effects of MWCMTs by fiber length, we prepared two MWCNT samples from one bulk sample. One consisted of cut up short fibers (Short; average length=0.94 µm) and the other was just dispersed (Long; average length=3.4 µm). The samples were administered to male Wistar rats by intratracheal instillation at doses of 0.2 mg and 1 mg/animal (Short) and 0.2 mg and 0.6 mg/animal (Long). The animals were sacrificed at time points from 3 d to 12 months after administration. Bronchoalveolar lavage fluid (BALF) was taken from the lungs and pathological specimens were prepared. The concentrations of phospholipids, total protein and surfactant protein D (SP-D) in the pulmonary surfactant of the BALF were determined, the surface tension of BALF was measured, and the inflammation score was determined by the point-counting method to assess pulmonary tissue inflammation. The present study suggests that inflammatory response in the lung was slightly higher for long MWCNTs than for short MWCNTs when compared at the same mass dose. The correlation between pulmonary surfactant components and BALF surface tension was also evaluated. The Spearman's rank correlation coefficients obtained for the phospholipid, total protein and SP-D concentrations were -0.068 (p=0.605), -0.360 (p=0.005) and -0.673 (p=0.000), respectively. Surface tension, measured by a simple method, should be reflected in the change of a surfactant protein, such as SP-D.

Comparison of dose-response relations between 4-week inhalation and intratracheal instillation of NiO nanoparticles using polimorphonuclear neutrophils in bronchoalveolar lavage fluid as a biomarker of pulmonary inflammation.

Inhalation studies and intratracheal instillation studies using laboratory animals are commonly conducted for pulmonary toxicity tests of nanomaterials. In our study, male Wister rats were exposed to nickel oxide (NiO) particles including a nano-scale, even for aerosols and suspensions, in a 4-week inhalation and intratracheal instillation. Using polymorphonuclear neutrophils (PMNs) in bronchoalveolar lavage fluid as a biomarker of inflammation, we attempted to quantify the relationship between responses to inhalation and intratracheal instillation of the nanoparticles, based on surface area doses. Four kinds of NiO suspension samples with different specific surface areas were singly injected via the tracheas of the rats. The relationship between the instilled doses and PMN production was examined 3 days and 1 month after the instillation. In parallel, 4-week inhalation studies, using two of the suspensions, were conducted for aerosols generated by a pressurized nebulizer. NiO samples induced PMN responses 3 days after instillation according to the surface area doses, but not the mass doses, as has been reported in many studies. When the same NiO samples were tested in a 4-week inhalation and intratracheal instillation, the amount of pulmonary deposition of the sample after the 4-week inhalation, and an intratracheally instilled dose about ten-times higher, induced similar PMN responses 3 days after termination of inhalation and instillation. Using the relationship between these responses to 4-week inhalation and intratracheal instillation, it may be possible to estimate what aerosol concentrations of other nanomaterials might cause the same responses of PMN production as intratracheal instillation tests.

[Working environmental control for industrial physicians].

Forty years has passed after the promulgation of the Industrial Safety and Health Law, Japan. In this paper, working environmental control based on the law was overviewed. The working environmental control includes evaluation and engineering controls of hazardous elements in the work environment. It is an important task for industrial physicians to conduct, similar to workers' health control. The annual cycle of evaluation and engineering control for environments is also similar to the cycle of diagnosis and clinical treatment for workers for the workers' health control. Historically, the working environmental control system was based on a list of worksites, using chemicals listed in the regulations, and the concentration of contaminants in the air at each work site. Recently, control banding of newly developed chemicals without administrative control levels, was introduced as a method for the working environmental control.

Binding of titanium dioxide nanoparticles to lactate dehydrogenase.

OBJECTIVE: Measurement of released lactate dehydrogenase (LDH) activity, a commonly used marker of lethal cell injury in both in vitro and in vivo screenings, has been used to assess the cytotoxicity of nanoparticles (NPs), chemical compounds, and environmental factors. We have recently demonstrated that titanium dioxide (TiO2) particles bind to several serum proteins. In the present study we investigated the binding of TiO2 NPs to LDH. METHODS: Purified LDH was incubated with TiO2 NPs at 37°C for 1 h. The particles were then sedimented by centrifugation, and the activity and quantity of LDH in the supernatant and precipitated fraction were analyzed. RESULTS: Incubation with TiO2 reduced the LDH activity in the supernatant in a dose-dependent manner, while LDH activity in the precipitated fraction increased in a dose-dependent manner. Moreover, sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis revealed a TiO2 dose-dependent reduction in the quantity of LDH protein in the supernatant and an increase of LDH in particulate re-suspensions. CONCLUSIONS: These findings, although based on a purified form of LDH, suggest that TiO2 NPs bind to LDH, and consequently, TiO2 NP-induced toxicity could be underestimated by the LDH activity assay.

[Biological effect of fullerene (C60) to lung by inhalation or instillation].

Fullerene (C60) is a newly generated nanoparticle that has a soccer-ball structure with 60 carbon atoms. Fullerenes are expected to have a multitude of uses, for example as energy device materials or information technology materials. The biological effects of nanoparticles are now being discussed, and there are ongoing animal studies using various nanoparticles. Here we review some reports about the toxicity of C60, and in addition we present the results of our 2-year follow up studies of instillation and inhalation of C60 via the trachea.

Foam cell formation of alveolar macrophages in Clara cell ablated mice inhaling crystalline silica.

We investigated the function of Clara cells in vivo during exposure to inhaled crystalline silica by morphological and immunohistochemical examination of intra-alveolar cells and alveolar macrophages in Clara cell-ablated mice. The Clara cells of male FVB/n mice (8-12 weeks old) were ablated by intraperitoneal administration of naphthalene (300 mg/kg). The mice were then exposed to crystalline silica (Min-U-Sil-5, 97.1 ± 9.5 mg/m³, 6 hours/day, 5 days/week) for up to two weeks. The lungs were assessed by morphometry, as well as by immunohistochemistry of CD36, lectin-like oxygenated low-density lipoprotein receptor (LOX)-1, and matrix metalloproteinases (MMPs) -2, -9 and -12. There was a significantly greater number of intra-alveolar cells in Clara cell-ablated mouse groups than in wild-type mouse groups that were exposed to crystalline silica. A marked number of foamy alveolar macrophages were only detected in the Clara cell-ablated group exposed to crystalline silica, indicating that Clara cells inhibit infiltration and foam cell formation of alveolar macrophages. Immunohistochemical analysis indicated that foamy alveolar macrophages in the Clara cell-ablated group that inhaled crystalline silica overexpress CD36 and LOX-1, indicating upregulation of scavenger receptors of alveolar macrophages. These cells also express MMP-2, -9 and -12, suggesting increased gelatinolytic and elastolytic activities. Our findings suggest that Clara cells not only inhibit infiltration of alveolar macrophages but also their phagocytotic and gelatinolytic functions in silica-induced pulmonary injury.

Biopersistence of potassium hexatitanate in inhalation and intratracheal instillation studies.

An inhalation study and an intratracheal instillation study were conducted to evaluate the biological effects of the new chemical, potassium hexatitanate (PH). For the inhalation study, Wistar male rats were exposed to PH for 6 h a day, 5 days a week for a period of 3 months. The mass median aerodynamic diameter of PH in the exposure chamber was 4.9 µm (1.8) and the mean concentration during the exposure was 2.3 ± 0.1 mg/m(3). After the 3-month inhalation period, rats were dissected at 3 days, 1 month, 3 months, 6 months, and 12 months. The initial PH burden was 0.17 ± 0.03 mg/lung, and this decreased exponentially up to 6 months after inhalation. After 6 months, the rate at which the burden decreased slowed. The biological halftime up to 6 months after exposure was 2.3 months. No difference was found in the dimension of PH fibers in the lung during the observation period and the histopathological examination found no remarkable inflammation or fibrosis. For the intratracheal instillation study, the rats were given a single 2-mg dose of PH suspended in a 0.4 ml saline solution. The geometric mean diameter was 4.3 µm (2.3). After instillation, the rats were dissected at 3 days to 12 months. The PH burden in the lungs decreased exponentially and the biological halftime was 3.1 months. The results of the dimension of PH and histopathological findings were the same as those for the inhalation study. These data suggest that the toxicity of PH in the lung is low in these doses.

Pathological features of rat lung following inhalation and intratracheal instillation of C(60) fullerene.

We evaluated the pulmonary pathological features of rats that received a single intratracheal instillation and a 4-week inhalation of a fullerene. We used fullerene C(60) (nanom purple; Frontier Carbon Co. Ltd, Japan) in this study. Male Wistar rats received intratracheal dose of 0.1, 0.2, or 1 mg of C(60), and were sacrificed at 3 days, 1 week, 1 month, 3 months, 6 months, and 12 months. In the inhalation study, Wistar rats received C(60) or nickel oxide by whole-body inhalation for 6 h/day, 5 days/week, 4 weeks, and were sacrificed at 3 days, 1 month, and 3 months after the end of exposure. During the observation period, no tumors or granulomas were observed in either study. Histopathological evaluation by the point counting method (PCM) showed that a high dose of C(60) (1 mg) instillation led to a significant increase of areas of inflammation in the early phase (until 1 week). In the inhalation study of the C(60)-exposed group, PCM evaluation showed significant changes in the C(60)-exposed group only at 3 days after exposure; after 1 month, no significant changes were observed. The present study demonstrated that the pulmonary inflammation pattern after exposure to well-characterized C(60) via both intratracheal and inhalation instillation was slight and transient. These results support our previous studies that showed C(60) has no significant adverse effects in intratracheal and inhalation instillation studies.

Biopersistence of inhaled MWCNT in rat lungs in a 4-week well-characterized exposure.

It is important to conduct a risk assessment that includes hazard assessment and exposure assessment for the safe production and handling of newly developed nanomaterials. We conducted an inhalation study of a multi-wall carbon nanotube (MWCNT) as a hazard assessment. Male Wistar rats were exposed to well-dispersed MWCNT for 4 weeks by whole body inhalation. The exposure concentration in the chamber was 0.37 ± 0.18 mg/m³. About 70% of the MWCNTs in the chamber were single fiber. The geometric mean diameter (geometric standard deviation, GSD) and geometric mean length (GSD) of the aerosolized MWCNTs in the chamber were 63 nm (1.5) and 1.1 µm (2.7), respectively. The amounts of MWCNT deposited in the rat lungs were determined by the X-ray diffraction method and elemental carbon analysis. The average deposited amounts at 3 days after the inhalation were 68 µg/lung by the X-ray diffraction method and 76 µg/lung by elemental carbon analysis. The calculated deposition fractions were 18% and 20% in each analysis. The amount of retained MWCNT in the lungs until 3 months after the inhalation decreased exponentially and the calculated biological half times of MWCNT were 51 days and 54 days, respectively. The clearance was not delayed, but a slight increase in lung weight at 3 days after the inhalation was observed.