Background and aims: Noise is an important source of physical and psychological stress that is considered as the most important physical detrimental factor in developed and developing countries. In the United States, more than 30 million workers are exposed to dangerous noise, and 7. 4 to 10. 2 million industrial workers are at risk of hearing loss due to exposure to industry noise, but what is certain is that the noisy environment causes carelessness in activities. The brain becomes inconsistent in intellectual work and so on. Noise is an environmental stressors and in combination with other stressors can cause or exacerbate mental disorders and even under certain conditions can affect performance to the extent that WHO considers accidents as one of the indicators of performance decline. Recognizes noise-induced and recognizes ambient noise as a direct cause of mental disorders. Noise is the most common occupational hazard in various industries, especially petrochemicals and related companies. Noise exposure causes a wide range of the discomforts, disorders and occupational diseases and effects including focus loss, long-term memory loss, anger, increasing stress and etc. Cognitive performance such as consistent attention, comprehension, speed of reaction as well as correct information processing and correct decision making play a key role in performing many tasks. In some occupations, in order to react appropriately and in a timely manner, the person needs to process the information completely and without defects. Therefore, even temporary defects in cognitive and mental performance can lead to serious consequences in individuals, especially when an accurate and immediate response is needed. The results of various studies conducted in the oil and gas industry and related companies indicate that sometimes noise contact is outside the permissible range (85 dBA). Basically, in the oil and petrochemical industries, a lot of noise is produced, which can be said that the main noise produced is related to the movement of fluids and related piping on the one hand, and on the other hand, the era of engines and compressors in refining operations. It is essential to maintain human health and safety of the work system at the highest possible level, and any error on the part of operators can lead to unintended and harmful consequences or accidents, or reduce the quantity and quality of products and irreparable economic losses and considering that the accuracy, speed of action and ability to perform skills by individuals; are the most important factors affecting the efficiency and increasing the productivity of human resources in improving the level of production and its quality, so evaluate the effects of different Sound pressure level cognitive performance has been performed on petrochemical industry workers. Methods: This descriptive-analytical cross-sectional study was conducted in 1398 in Kavian Petrochemical Company located in South Pars region (Assaluyeh). In this study, before collecting information, the purpose of the study was explained to the participants. Demographic data collected included: age, work experience, body mass index and metabolism. Exclusion criteria included: less than one year of work experience, use of hypnotic and caffeinated substances, having any mental disorders, sleep disorders, major systemic diseases, etc. Subjects were divided into three groups (one control group and two case groups). There were 30 people in each group. In the study of the effects of Sound pressure levels below the allowable limit, control room operators were selected and participants in 2 case groups were selected from site operators who were exposed to more than the allowable noise level. Metabolism of participants in three groups was determined according to ISO 8996 standard. Participants in the study had a work schedule of one week working day and one week working night. In this study, day workers who were active from 7 am to 7 pm were selected. To investigate the effect of noise on workers' cognitive performance, first the average equivalent Sound level during the shift (beginning, middle and end of the shift) was measured. Since the Sound pressure level is different in different parts of the workplace and the company workers normally commute in different parts of the environment, so to determine the average Sound level equivalent to their exposure, the dosimeter is the most reliable method for measuring and evaluating. It is an individual encounter. The dosimetry was performed by TES-1345 dosimeter made in Taiwan, in accordance with ISO 9612 standard. Before using, the device was calibrated by a CEL110. 2 calibrator made by Casella Company. In the studied units, measurements were performed using a regular grid pattern and then the cognitive performance of individuals at the beginning, middle and end of the shift was measured using CPT and N-back tests. The Continuous Performance Test (CPT) is used to measure people's attentive attention and alertness. It is a valid test that is used to find sustained attention deficit impaired performance. In the continuous performance test (CPT), 150 numbers appear at regular intervals and a stimulus is determined as the target stimulus, and the participant must press the relevant key on the computer screen as soon as possible by observing the desired numbers. The variables measured in the CPT test were commission error, omission error, and response time (milliseconds), and working memory test (N-back) is used to measure working memory of individuals, since this task includes both cognitive information retention and manipulation, it is known to be very suitable for measuring working memory performance. And repeatedly used for this purpose, in the working memory performance test (N-back), a sequence of numbers, step by step, appears randomly on the screen as a visual stimulus. . The subject should then examine whether the current stimulus presented is consistent with the N-stimulus preceded by it. The variables measured in the working memory performance test (N-back) were reaction time (milliseconds) and mean correct response. The collected data were analyzed with SPSS V. 20. Then descriptive methods (mean, standard deviation and frequency) were used to summarize the data. The normality assumption for the data was performed using the Kolmogorov-Smirnov test. The main basis of data analysis was based on analysis of variance (ANOVA). Significance level in data analysis was considered less than 0. 05. Results: Comparison of the results in the Continuous Performance Test (CPT) showed that the average equivalent Sound level has no effect on the commission error, but at the end of the shift the commission error is significantly affected (P=0. 040). However, the average equivalent Sound level has no effect on the omission error and only the omission error in the middle and end of the shift is statistically significant (P<0. 001, P=0. 002). But the average equivalent Sound level has a significant effect on workers' response time (P=0. 006). It was also found that the response time of workers in groups that were exposed to more than the permissible noise was statistically significantly different (P=0. 025, P=0. 007). Also, shift time affects response time and workers' response time in the middle and end of the shift is statistically significant (P=0. 002, P=0. 008). According to the working memory performance test (N-back), it was found that the average equivalent Sound level has a significant effect on workers' reaction time (P=0. 006). Also, workers' reaction time is affected by shift time, so that the reaction time of people in the middle and end of the shift was statistically significant (P<0. 001). Also, the reaction time was statistically significant in the groups that were exposed to more than the allowable noise (P=0. 001, P=0. 006). But the average equivalent Sound level had no effect on the average correct answer of the workers (P>0. 05) and only the correct response in the middle and end of the shift was statistically significant. The highest reaction time and the lowest mean correct response belonged to the third group who were faced with a Sound pressure level of 91. 83± 1. 78 dBA. Accordingly, the number of correct responses of the groups that were exposed to more than the allowable noise was significantly less than the group that was exposed to less than the allowable noise. Also, the number of errors in these groups was more than that group. According to the Continuous Performance Test (CPT) and also according to the Memory Performance Test (N-Back), the Sound pressure level increases the reaction time of the subjects (P<0. 05). Conclusion: In general, the results of the present study showed that in cases where people are exposed to more noise than allowed, the number of correct response is less than when they are exposed to less than the allowable noise, and with increasing Sound pressure level, the number of correct responses decreases and the amount error, response time and reaction time increases. So noise has a negative effect on subject’ s cognitive performance in such a way that it can disrupt the cognitive performance of the site operators by significantly increasing the response time and reaction time. Therefore, due to the high job sensitivity of petrochemical workers, exposure to various harmful factors and also hard environmental conditions, their impaired cognitive performance can be very dangerous in terms of safety and control strategies should be given more importance than before. Therefore, the present study can be used by safety and health managers to implement an effective strategy and improve the cognitive performances of different groups of workers while on duty.