Maynard et al. (2004) and Han et al. (2008) reported measured concentrations of U0126 ic50 SWCNTs and MWCNTs in the research facilities, respectively. Maynard et al. (2004) reported that atmospheric concentrations of SWCNTs, which were estimated using an indicator of metal catalysts, were in range of 0.7–53 μg/m3 during the collection and cleaning process, based on the investigation of SWCNT research facilities with laser-abrasion or the high pressure carbon monoxide (HiPco) method. Han et al. (2008) reported

that the atmospheric mass concentration of total dust (including MWCNTs) was in range of 210–430 μg/m3 during the blending process and in range of 37–190 μg/m3 during the weighing and spraying process, based on the investigation

of MWCNT research facilities with the thermal chemical vapor deposition (CVD) method. They also reported that the number concentration of MWCNTs was in range of 172.9–193.6 × 106 tubes/m3 during these processes. Based on the atmospheric mass concentration AC220 mw or number concentration of CNTs reported in these studies, deposition amounts of MWCNTs to the lungs of humans working for 8 h/day and 5 days/week without any exposure protection can be calculated as follows. Assuming that average daily exposure time is 8 h/day × 5 days/week × 60 min/h = 343 min/day, the deposition fraction of inhaled MWCNTs into the lungs is 0.1 (10%) based on the study of Miller (2000), the respiratory minute volume is 25 L/min,

and body weight is 60 kg, then pulmonary deposition amounts of MWCNTs are calculated to be 0.01 and 6.2 μg/kg/day, based on the medroxyprogesterone atmospheric concentration of 0.7 μg/m3 (Maynard et al., 2004) and 434.5 μg/m3 (Han et al., 2008), respectively. Therefore, instillation exposure of 1.0 mg/kg MWCNTs corresponds to pulmonary deposition amounts of 160–1300 days (i.e., several months to several years) in the working environment without any exposure protection when the maximum atmospheric concentration of MWCNTs is used in the calculation. Based on the number concentration of CNTs, deposition of MWCNTs into the lungs per day per kg body weight were calculated to be 2.47–2.77 × 106 tubes/kg/day based on the atmospheric number concentration of 172.9–193.6 × 106 tubes/m3 (Han et al., 2008). Therefore, 2.4 × 1011 tubes/kg (1.0 mg/kg) of instillation exposure of MWCNTs corresponds to a pulmonary deposition amount of 85,000–95,000 days, which is longer than the average human lifespan. Collectively, our data indicated that the pulmonary inflammatory responses to MWCNT deposition in the lungs were dose dependent, and the responses were weak and transient under approximate pulmonary deposition amounts comparable to the work environment. Chronic inflammatory responses such as pulmonary fibrosis or angiogenesis were not observed.