Forty-four male airplane maintenance workers were recruited with informed consent in this study from members of an airplane maintenance factory in mid-Taiwan. All sub-jects work from 8:00 am to 5:00 pm, and occasionally they may work overtime due to extra work. Except for the noon break, all workers must work on the job site, so all workers' exposure time is about 8 to 9 hours a day. According to the actual working conditions observed, subjects' work content and working periods are stable, so the relevant exposure status is also relatively stable. Subjects who had skin disease on the measurement site and long-term drug users were excluded from this study. The information collected by questionnaire included demographic characteristics, lifestyle (such as cigarette smoking and skincare habit), job task, personal hygienic habits (use of mask, glove, and earplug), and health history (skin disease, drug use). The study procedure was approved by the Institutional Review Board (IRB) of China Medical University Hospital before the research proceeded. Each subject in this study conducted a questionnaire with informed consent.
The noise group's job operated the airplane maintenance machinery. The organic solvent group used n-hexane and methyl ethyl ketone (both concentrations > 90%) to soak the cloth and directly clean the paint of aircraft and components. The co-exposure group was operation and cleaning of engines and the section of aircraft. All subjects had been exposed to the machinery noise or organic solvent for several years (2-21 years). Based on the results of working environment measurement once per six months, the equivalent continuous noise level (Leq) of these groups measured by the sound level meter (Lutron SL-4001, Taiwan) was 84-129 dB(A) in the task exposed to noise during several daily work shifts and 48-73 dB(A) in the task without noise exposure. Therefore, all subjects from the factory were divided into four groups depending on the field investigation results: noise exposure group, organic solvent exposure group, co-exposure of noise and solvent group, and control group.
All subjects underwent skin barrier function tests, including basal transepidermal water loss (TEWL), basal water content (WC), skin barrier integrity, and skin barrier recovery at the end of the workday. Each subject should rest at least 15 to 30 minutes before measurement. The barrier function tests were measured on the dominant forearm at room temperature at 22 °C to 25 °C, and the relative humidity ranged from 40% to 60% (du Plessis et al. 2013).
TEWL measurement is the outward diffusion of water through the skin. The Tewameter ® TM 300 (CK, Germany) was used to measure TEWL, and the probe touched on the forearm skin perpendicularly for 1-2 minutes to measure the water loss from the surface skin until the standard deviation less than 0.05 g/m2/h.
WC measurement was used the Corneometer ® CM 825 (CK, Germany), and touched to the subject's forearm skin 15 times successively. The last five measurements were averaged as the stratum corneum water content.
Skin barrier recovery is the repair rate of the stratum corneum after acute disruption (Yosipovitch et al. 2007). The sticky book tape disrupted the measurement site sequentially (Scotch No.845 book tape, 3M, USA) until a TEWL level of at least 20 g/m2/h. TEWL was measured immediately after tape stripping and 3 hours, 6 hours, and 24 hours later. The evaluation of recovery rate is based on the percent change compared with basal TEWL (Tsai et al. 2001).
Skin barrier integrity was the number of successive tape stripping required to disrupt stratum corneum (TEWL at least 20 g/m2/h) in this study,
Shapino-Wilks' W test was performed to examine whether skin barrier function indices were normal distribution. Demographic information and variables comparison among difference exposure groups were computed using one-way ANOVA for the continuous variables and chi-square test for the categorical variables. The skin barrier functions comparison among the four groups were used as one-way ANOVA and the Spjotvoll/Stoline test (Tukey HSD for unequal N test) was used for post hoc comparison. Statistica Software release 6.0 (StatSoft, Tulsa, OK, USA) was used in the statistical analysis. The significance level was set at p < 0.05.