*2.5. Analytical Method*

In this paper, noise levels and exposure time were independent variables; skin conductivity and heart rate were dependent variables. A bar graph was used to show the distribution of all indicators and the variation trend of their mean values. SPSS26.0 was used to perform paired sample *t*-test analysis. It could determine when the hands were statistically significant and when they were not under different noise levels. We checked the normality of the physiological data before the paired sample *t*-test because we can perform a paired sample *t*-test only when statistics show normal distribution.

### **3. Results**

#### *3.1. Skin Conductivity*

Figure 3 shows how skin conductivity changed with time under five noise levels. We can see that overall skin conductivity increased exposure time. Skin conductivity under all five noise levels changed relatively rapidly at a particular time, but the specific time when the rapid changes occurred was somehow different. When the noise was at 60 dB, skin conductivity sharply rose in 20–25 min. At 70 dB and 80 dB, skin conductivity did not sharply rise until 10–15 min. At 90 dB and 100 dB, skin conductivity sharply increased in the first 0–5 min. We further determined the statistical significance of the above changes through the paired sample *t*-test results to speculate the reasons for the trend change.

**Figure 3.** Bar graph of skin conductivity under five noise levels. "ns" means no significant difference, "\*\*\*" means have significant differences.

Table 3 shows the normality test results of skin conductivity. The null hypotheses H0 are the skin conductivity of the control group, 0–5 min at 60 dB/70 dB/80 dB/90 dB/100 dB, 10–15 min at 60 dB/70 dB/80 dB/90 dB and 20–25 min at 60 dB/70 dB/80 dB/90 dB, and they were normally distributed. We can see that under the test standard α = 0.05, the *p*-value of the S-W test for skin conductivity in different noise levels and their different test periods was greater than 0.05; thus, the null hypotheses H0 were accepted. That is to say, the distribution of skin conductivity was normal, and we were able to perform the paired sample *t*-test.


**Table 3.** Normality test results of skin conductivity.

Table 4 shows the results of the paired sample *t*-test analysis of skin conductivity. The null hypotheses H0 were that there was no significant difference between the skin conductivity of 0–5 min/10–15 min/10–15 min at 60 dB/70 dB/80 dB/90 dB/100 dB and that of the control group. We can see that the higher the noise levels were, the shorter the contact time it took for skin conductivity to present significant changes. In detail, under the test standard α = 0.05, *p* < 0.05 was in 20–25 min at 60 dB; *p* < 0.05 was in 10–15 min at both 70 dB and 80 dB; *p* < 0.05 was in the first 0–5 min at both 90 dB and 100 dB. This shows that the changes in the box plot in Figure 3 were statistically significant. Thus, it can be imagined that the increase in noise intensity and exposure time makes the human body easily nervous, anxious, and mentally sweating, resulting in a decrease in skin resistance and an increase in skin conductivity.

#### *3.2. Heart Rate*

Figure 4 shows how heart rate changed with time under five noise levels. We can see that, overall, heart rate increased with the rise of exposure time, but this trend was not as strong as skin conductivity. The heart rates under all five noise levels changed relatively rapidly at a particular time, but the specific time when the rapid change occurred was somehow different. When noise was at 60 dB, the heart rate slightly increased, but there was no drastic change within 25 min. At 70 dB, the heart rate sharply rose in 20–25 min. At 80 dB and 90 dB, heart rate drastically grew in 10–15 min. At 100 dB, the heart rate sharply grew in the first 0–5 min. We further determined the statistical significance of the above changes through the paired sample *t*-test results to speculate the reasons for the trend change.


**Table 4.** Paired sample *t*-test results of skin conductivity.

**Figure 4.** Bar graph of heart rate under five noise levels. "ns" means no significant difference, "\*\*\*" means have significant differences.

Table 5 shows the normality test results of heart rate. The null hypotheses H0 are the heart rate of the control group, 0–5 min at 60 dB/70 dB/80 dB/90 dB/100 dB, 10–15 min at 60 dB/70 dB/80 dB/90 dB, and 20–25 min at 60 dB/70 dB/80 dB/90 dB, and they were normally distributed. We can see that, under the test standard α = 0.05, the *p*-value of the S-W test for heart rate in different noise levels and their different test periods was greater than 0.05; thus, the null hypothesis H0 was accepted. That is to say, the distribution of heart rate was normal, and we were able to perform the paired sample *t*-test.

Table 6 shows the results of the paired sample *t*-test analysis of heart rate. The null hypotheses H0 were that there was no significant difference between the heart rate of 0–5 min/10–15 min/10–15 min at 60 dB/70 dB/80 dB/90 dB/100 dB and that of the control group. We can see that the higher the noise levels were, the shorter the contact time it took for the heart rate to present significant changes. In detail, under the test standard α = 0.05, when the noise was at 60 dB, the *p*-value was greater than 0.05 within 25 min. At 70 dB, *p* < 0.05 was in 20–25 min. At 80 dB and 90 dB, *p* < 0.05 was 10–15 min in both. At 100 dB, *p* < 0.05 was in the first 0–5 min. Thus, the changes in the box plot in Figure 4 were statistically significant. Thus, it can be imagined that the increase in noise intensity and exposure time increases the psychological load on humans, making it easy to be nervous and impetuous and resulting in an increase in heart rate.


**Table 5.** Normality test results of heart rate.

**Table 6.** Paired sample *t*-test analysis results of heart rate.


#### **4. Discussion**

In the high-risk coal mine environment, due to the particularity of their work, miners are under much greater pressure than ordinary workers, and the potential unintended consequences of accidents caused by stress disorders are even greater. As external influences and internal psychological conditions are easy to damage human performance, whether high-risk coal mine workers can take effective measures to mitigate and adjust their emotions and attitudes under noise conditions in a timely manner is crucial to recover losses, reduce losses, and control the scale of disasters. In risk assessment, environmental factors must be scientifically assessed.

Previous studies explored the impact of noise on miners through cognitive level data of attention, reaction time, and mental fatigue level. For a more in-depth study, we analyzed the impact of noise on miners based on data produced, making use of the principles of neuroscience. Through studying whether and how short-term noise exposure affects human physiological indicators, we compared when dramatic changes occurred in skin conductivity and heart rate under different noise levels and found that: (a) When the noise was at 60 dB, skin conductivity did not present significant growth until 20–25 min, while heart rate showed no significant change within 25 min. (b) When noise was at 70 dB, skin conductivity significantly increased in 10–15 min, while heart rate significantly rose in 20–25 min. (c) When noise was at 80 dB, skin conductivity and heart rate both presented as significantly increased in 10–15 min. (d) When the noise was at 90 dB, skin conductivity significantly changed in 0–5 min, while heart rate significantly changed in 10–15 min. (e) When the noise level was at 100 dB, skin conductivity and heart rate presented significant changes in 0–5 min. Relevant studies [21,23] also found that human heart rate, heart rate variability, skin conductivity, and other physiological indicators all change in a noisy environment. The center rate, heart rate variability, skin conductivity, and respiratory rate can reflect human comfort. Physiological indicators are widely used in the study of environmental factors.

In short, the noise intensity standard is not absolute, and it cannot be said that a noise environment lower than the noise intensity standard will not affect humans. An intensity of 60 dB~80 dB affects skin conductivity and heart rate relatively short-term, although this range is not beyond what is required by occupational health standards. Moreover, sometimes time spent working in a noisy environment is not long, but it may still have an impact on the human body. Additionally, noise protection is necessary despite shorter working hours or an environment of low-level noise(60 dB~80 dB), and the noise level should preferably be kept no higher than 90 dB in coal mines. We hope that this study can provide scientific basis for formulating noise occupational health standards and preventing accidents.

Considering the experimental operation, the researchers just selected the mining college students as volunteers. By reason of the influence of the cultural level, physiological and psychological quality, work experience, and other factors, students' reactions to coal mine accidents may be different from miners' reactions, and the changes of physiological indicators may also be different under different noise levels. In future research, the experiment can be further expanded to make the selected research volunteers closer to the actual situation of miners. In real coal mines, the walls have noise-absorptive and reflective effects, and this experiment did not consider this. However, it still can be proven that the collected coal mine noise is an adequate stressor. This aspect needs further research to use virtual reality technology to simulate a natural coal mine noise environment.

This research can also be further explored later. For example, in real accidents, individuals may suffer from physical injury and irritable psychological and irrational factors, which may lead to accidents. The possibility and extent of operational errors, inattention, and unreasonable behaviors caused by physiological and psychological changes of volunteers under different noise levels were not taken into account.

#### **5. Conclusions**

Based on human factor engineering, we selected two physiological indicators and studied how five noise levels affect the indicators in a short time. The results are as follows:


d We compared how much time it takes for skin conductivity and heart rate to present significant changes. We found that skin conductivity needs relatively less time to show substantial change under the five noise levels.

To conclude, from the perspective of accident prevention, we suggest coal mines pay attention to the noise protection of miners even though miners are in an environment of low-level noise. Additionally, noise protection is necessary despite shorter working hours. Notably, the noise level should preferably be kept no higher than 90 dB in coal mines without protection. These can effectively reduce accidents.

**Author Contributions:** J.L. designed the study. J.L. and Z.C. wrote the manuscript. H.L. performed data collection and data analysis. Y.X. performed the reviewing and editing of the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the 2 Batch of 2022 MOE of PRC Industry University Collaborative Education Program (Program No. 220705329280548, Kingfar-CES "Human Factors and Ergonomics" Program) and the Fundamental Research Funds for the Central Universities (2022YJSAQ20). The funding body played no role in the design of the study and collection, analysis, and interpretation of data and in writing the manuscript.

**Institutional Review Board Statement:** All research procedures were approved prior to the commencement of the study by the China University of mining and technology (Beijing). All participants signed an informed consent form. Our research received ethics approval from China University of Mining and Technology (Beijing) and it conformed to the ethics guidelines of the Declaration of Helsinki.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The datasets used and/or analysed during the current study are available from the corresponding author upon reasonable request.

**Conflicts of Interest:** The authors declare that they have no competing interest.
