1.2 Skin biophysical properties in different age subgroups
The values of CM, TEWL, EI, ITA°, GLOSS, R2 satisfied Levene's chi-square, ANOVA (LSD and S-N-K methods) was used to test for six skin parameters’ variances among subgroups. The other four skin parameters’ variance among subgroups were performed by Krustal-Wallis H-test. Except for the EI values, skin biophysical parameters were significantly different among the five age subgroups (Fig. 3). The results were generally consistent with the trend of the polynomial fitting. The younger people had significantly higher TEWL and SM values (A1 ~ A2). The intermediate people had significantly lower pH values (A4). The intermediate and older people had significantly higher b* and ITA values and significantly lower R2 values (A4 ~ A5). And the older people had significantly higher MI and significantly lower GLOSS and CM values (A5).
1.3 Comparison of skin biophysical properties between rural and urban females
We previously assessed facial skin aging trends in 300 women aged 18–60 years from five cities in China by noninvasive skin assessment[6]. The sample population of this study was 350 rural women aged 18–65 years, which was similar in sample size and age distribution with above study. Comparing the values of 10 skin biophysical parameters between rural and urban women by independent samples t-test (Table 2), we found that rural women had lower facial skin hydration level and sebum secretion, higher transepidermal water loss and higher melanin content. Urban women had poorer skin gloss, more yellowish skin tone, and poorer elasticity.
By comparing polynomial fitting curve trends, we found that there were various differences in skin aging trends among urban and rural females. In terms of skin barrier levels, the TEWL values of rural women showed a more stable decreasing trend with age, but fluctuated with age in the urban women samples. Sebum secretion showed a similar behavior in rural and urban women, significantly decreasing with age. In terms of color level, the most obvious difference was that skin gloss decreased with age in rural women but increased in urban women. Skin tone both showed deepening and yellowing with age. In terms of skin elasticity, rural women showed a steady decline in R2 values, but urban women showed a slight rebound after age 47.
2 Risky lifestyles for skin biophysical parameters
2.1 Unfavorable value of skin parameter by box-plot delineation
Considering the impact factors on skin biophysical parameters reported in literature, we investigated 18 questions related to four areas, including personal information (self-reported skin type, marital and fertility status), behavioral habits (diet, smoking, alcohol consumption, sleep, allergens), physical/mental health status (mood, bowel movements, menstruation), and cosmetic habits (sunscreen, skincare products), to filter out the risky lifestyles on the skin biophysical parameters.
A quadratic approach was used to categorize people with relatively different skin parameters: (I) For a certain skin parameter, values larger than Q2 are labeled as Upper quartiles, and values smaller than Q3 are labeled as Lower quartiles based on box plots; (II) people with relatively "unfavorable" skin parameters are regarded as the positive group, i.e., the Lower quartile for CM, SM, pH, ITA, GLOSS, R2, and the Upper quartile for TEWL, MI, EI, b* are the positive groups (Table 3). CM ≤ 38.80 indicates lower hydration, CM ≥ 62.60 indicates higher hydration. TEWL ≥ 23.20 indicates higher transepidermal water loss, TEWL ≤ 15.70 indicates lower transepidermal water loss. SM ≤ 1.00 indicates less sebum secretion, SM ≥ 9.00 indicates more sebum secretion. pH ≤ 5.66 indicates stronger acidity, pH ≥ 6.08 indicates weaker acidity. MI ≥ 179.50 indicates higher melanin content, MI ≤ 131.17 indicates lower melanin content. EI ≥ 358.17 indicates higher hemoglobin content, EI ≤ 268.83 indicates lower hemoglobin content. b ≥ 12.71 indicates more yellowish skin tone, b ≤ 9.80 indicates less yellowish skin tone. ITA ≤ 42.33 indicates darker skin tone, ITA ≥ 56.00 indicates brighter skin tone. GLOSS ≤ 5.42 indicates worse glow, GLOSS ≥ 7.10 indicates better glow. R2 ≤ 0.66 indicates lower elasticity, R2 ≥ 0.82 indicates higher elasticity.
2.2 Evaluation of risky lifestyles by crude and adjusted OR
The chi-square test and crude ORs were used to initially evaluate the risky and protective lifestyles related to "unfavorable" skin parameters (Table A2-A11). Due to the tiny sample sizes (N < 10) of the smoking, drinking alcohol, divorced, widowed, pregnant, hysterectomized exposure groups, the effects of these events on skin biophysical parameters were not discussed in this study. The results of the chi-square test showed that allergic histories, mental stress and irritable mood did not have a statistically significant effect on skin biophysical parameters. Then we re-evaluated the risky and protective lifestyles among the other 13 individual lifestyle items for 10 skin biophysical parameters by the logistic regression model and adjusted OR, aiming to control the impact of individual age.
We used transepidermal water loss, pH, hydration, and sebum content as biophysical parameters to assess the skin barrier. After controlling for age, self-reported skin types of oily (OR = 0.099 [95% CI: 0.018, 0.536], P = 0.007) and mixed (OR = 0.253 [95% CI: 0.081, 0.793], P = 0.018) decreased the risk of low sebum secretion to 0.099 times and 0. 253 times, and allergy to food or medicine (OR = 0.259 [95% CI: 0.076, 0.888], P = 0.032) decreased the risk of low sebum secretion to 0.259 times. Self-reporting a skin type of mixed (OR = 5.869 [95% CI: 1.452, 23.729], P = 0.013) increased the risk of high transepidermal water loss to 5.869 times. Use of skincare products (OR = 0.275 [95% CI: 0.018, 0.933], P = 0.038) decreased the risk of low skin hydration content to 0.275 times.
Skin yellowness, individual type angle, melanin content, gloss, and hemoglobin content were used as biophysical parameters characterizing skin tone. Use of skincare products and staying up late were protective factors that decreased the risk of dark skin. First menstruation after age 16, insomnia, and a vegetarian diet were risk factors that increased the risk of dark skin. Women who used sunscreen products had better skin gloss, while those who were menopausal and had children showed poorer skin gloss. For skin melanin content, it was likely to be lower in women using sunscreen products and higher in married, childbearing, and menopausal women. However, these factors did not show statistical significance after controlling for age (P > 0.05). But the risk of higher hemoglobin in women with self-reported oily skin (OR = 7.947 [95% CI: 1.760, 35.887], P = 0.007) was 7.947 times higher than in women with normal skin after controlling for age.
We evaluated skin elasticity by R2 value. The results showed that being married, having children, and being post-menopausal increased the risk of poorer skin elasticity and were risk factors. Higher frequency of the sunscreen decreased the risk of skin elasticity deterioration and was a protective factor. And the use of sunscreen all year round consistently decreased the risk of poorer skin elasticity to 0.242 times compared with no sunscreen use. However, these factors did not show statistical significance after controlling for age (P > 0.05).