dental wear is a complex multifactorial process that involves the loss of hard dental tissue , specifically enamel and dentin 1 . the prevalence of dental wear , being as sign of normal ageing , has increased , as the average life expectancy of the population is also increasing with natural teeth being retained longer than in past generations . the mechanical causes and the parafunctional habits ( like bruxism ) are the most commonly reported factors associated with dental wear . mechanical factors , such as toothbrush abrasion , tongue friction and attrition have been accepted as causes of enamel loss 2 . additionally , chronic regurgitation of gastric acids in patients with gastro - esophageal reflux disease may cause dental erosion , which can lead , in combination with attrition or bruxism , to extensive loss of coronal tooth tissue 3 . tooth grinding may also cause severe damage not only to enamel and dentin , but also on the patient 's temporomandibular joint 4,5 . parafunctional habits have been reported by many researchers as a self - destructive process caused by stress . according to these studies 7 - 11 , excessive stress results in hyperactivity of masticatory muscles , which manifests itself through various parafunctional activities , in particular teeth grinding , leading to progressive abrasion of the teeth 6 . emotional problems , such as anxiety , anger and depression , may result in bruxism behavior . sleep bruxism is common in the general population and represents the third most frequent parasomnia 12 . bruxism has numerous consequences , which are not limited to dental or muscular problems . among the associated risk factors , patients with anxiety and sleep disordered breathing have a higher number of risk factors for sleep bruxism 12 . slavicek stated that the explanations of parafunctions are concerned with viewing and tolerating the masticatory organ as a  psychic stress valve  13 . there is sufficient evidence , from large - scale epidemiological studies 14 - 16 , linking the populations exposure to environmental noise to adverse health effects . according to the results from the environmental burden of disease ( ebd ) in europe , a project in six european countries 17 reported at the world health organization ( who ) ministerial conference held in parma in march 2010 , traffic noise , rich in low frequency noise ( lfn ) components , was ranked second among the selected environmental stressors , evaluated in terms of their public health impact in those six european countries . the trend is that noise exposure is increasing in europe compared to other stressors ( e.g. exposure to second hand smoke , dioxins and benzene ) , which have been declining . on the other hand , lfn is present in professional , residential , and leisure environments , and is not adequately measured with standard methods . typically , lfn is not assessed during routine noise evaluations that are measured in dba ; according to the who 17 , lfn should be measured in dblin ( unweighted ) and defined as low spectral energy , bandwidth up to 500 hz and amplitude of 80 - 110 db . it is currently known that lfn affects several organs and systems , like the heart 18 , lung 19 , stomach or duodenum 20,21 , inducing the proliferation of extracellular matrices and cellular degeneration in the absence of inflammatory process . as an environmental  stressor agent , the effects of lfn can not be ignored and its effects on the teeth must be considered . our aim was to study the effects of industrial noise on dental wear over the time . our null hypothesis is that industrial noise does not affect crown area loss ( enamel and dentin ) . 39 male wistar rats were used , from a spanish producer ( charles river laboratories espaa sa , spain ) . all animals were fed normally and had free access to water , received treatment in accordance with the laws of the european union for the protection of animals ( 1005/92/23out ) and the interdisciplinary principles and guidelines for the use of animals in research . they were kept under normal conditions and placed in groups of two inside a plastic box ( 42x27x16 cm ) with a steel cover . the environmental noise of a cotton - mill room from a large textile factory was used as the paradigm of the occupational noise . the noise present in this cotton mill room was recorded and reproduced according to the procedure used by oliveira et al . the spectrum of frequencies and amplitude of noise used is documented in ( figure 1 ) . thirty rats were exposed to noise , and nine were used as age - matched controls . rats exposed to noise were submitted to different periods of noise exposure , ranging from 1 to 7 months , according to an occupationally simulated time schedule ( 8h / day , 5 days / week with the weekends in silence ) . the thirty rats were divided into three groups and sacrificed after 1 , 4 and 7 months of noise exposure . they were , therefore , 3 , 6 and 9 months old when they were sacrificed . the remaining 9 wistar rats were used as age - matched controls ( no noise exposure ) and thus sacrificed when they were 3 , 6 and 9 months old . after the reported periods , rats were sacrificed , together with 3 rats from the control group , with a lethal intraperitoneal injection of sodium pentobarbital . 126 out of the 240 extracted teeth were studied ; the remaining was discarded due to fractures of the crown and/or root , during the extraction procedures . the area of the three main cusps of each tooth was measured : the mesial cusp , the central cusp and the distal cusp . all the images of the sections were obtained with a magnification of 40x using a leica dmlb microscope ( leica microsystems cms gmbh , wetzlar , germany ) . individual cusp base areas were measured by tracing the outline of the cusp , following the occlusal surface to the tangential line cross the highest point of the pulp horn ( figure 2 ) . each blinded measurement was repeated 3 times , in three different days , and the average was calculated and used to statistically analyze the data . a two - way anova model was fit to the data after checking model assumptions , in which the dependent variable was defined as the average of the 3 cusp areas . the two independent factors were time ( 1 , 4 and 7 months ) and group ( exposed and control groups ) . the assumption of homogeneity of variance was checked using the levene test . concerning normality of the distribution of the dependent value , and given the small size of the classes , it was sufficient to confirm the symmetry of the variable , as the f ratio is robust in such conditions . 39 male wistar rats were used , from a spanish producer ( charles river laboratories espaa sa , spain ) . all animals were fed normally and had free access to water , received treatment in accordance with the laws of the european union for the protection of animals ( 1005/92/23out ) and the interdisciplinary principles and guidelines for the use of animals in research . they were kept under normal conditions and placed in groups of two inside a plastic box ( 42x27x16 cm ) with a steel cover . the environmental noise of a cotton - mill room from a large textile factory was used as the paradigm of the occupational noise . the noise present in this cotton mill room was recorded and reproduced according to the procedure used by oliveira et al . , 
 22 , 23 . the spectrum of frequencies and amplitude of noise used is documented in ( figure 1 ) . thirty rats were exposed to noise , and nine were used as age - matched controls . rats exposed to noise were submitted to different periods of noise exposure , ranging from 1 to 7 months , according to an occupationally simulated time schedule ( 8h / day , 5 days / week with the weekends in silence ) . the thirty rats were divided into three groups and sacrificed after 1 , 4 and 7 months of noise exposure . they were , therefore , 3 , 6 and 9 months old when they were sacrificed . the remaining 9 wistar rats were used as age - matched controls ( no noise exposure ) and thus sacrificed when they were 3 , 6 and 9 months old . after the reported periods , rats were sacrificed , together with 3 rats from the control group , with a lethal intraperitoneal injection of sodium pentobarbital . 126 out of the 240 extracted teeth were studied ; the remaining was discarded due to fractures of the crown and/or root , during the extraction procedures . the area of the three main cusps of each tooth was measured : the mesial cusp , the central cusp and the distal cusp . all the images of the sections were obtained with a magnification of 40x using a leica dmlb microscope ( leica microsystems cms gmbh , wetzlar , germany ) . individual cusp base areas were measured by tracing the outline of the cusp , following the occlusal surface to the tangential line cross the highest point of the pulp horn ( figure 2 ) . each blinded measurement was repeated 3 times , in three different days , and the average was calculated and used to statistically analyze the data . a two - way anova model was fit to the data after checking model assumptions , in which the dependent variable was defined as the average of the 3 cusp areas . the two independent factors were time ( 1 , 4 and 7 months ) and group ( exposed and control groups ) . the assumption of homogeneity of variance was checked using the levene test . concerning normality of the distribution of the dependent value , and given the small size of the classes , it was sufficient to confirm the symmetry of the variable , as the f ratio is robust in such conditions . the he sections of the control groups 1 and 4 displayed a normal appearance of the teeth , namely intact cusps without signs of wear ( figure 3 ) . the teeth of control group 7 showed a total crown loss of 17.3% compared to control 1 and of 17.5% when compared to control group 4 ( figure 4 and table 1 ) . table 2 displays the two - way anova results for the average area of all cusps . the results show that the average area differs significantly between exposed and non - exposed animals , that the area also varies significantly with time , and that variation is significantly different between groups . moreover , the model fit to the data is good , as expressed by the r squared value ( 0.668 ) . table 1 presents the estimated mean values of average areas in the classes defined by main factors , also depicted in figure 4 . there is a significant interaction , identified by the different slopes of the lines in figure 4 , which made it necessary to conduct planned contrasts for the interactive and main effects . in what concerns the interactive effect , the planned contrasts show that , at month 1 , the mean cusp area does not differ significantly between exposed and non - exposed animals ( p=0.331 ) , whereas at months 4 and 7 , the mean cusp area is significantly lower in exposed animals ( p < 0.001 ) . regarding time variations , the planned contrasts show that , between month 1 and 4 , the observed variation in control cusp areas ( 0.3% increase ) is significantly different ( p<0.001 ) from that observed in the exposed group ( 39.2% decrease ) . between month 4 and month 7 , the observed variation in cusp areas in the control group ( 17.5% decrease ) is not significantly differently ( p=0.212 ) from the exposed group ( 12.0% decrease ) . from the magnitude and statistical significance of the contrasts , the most noticeable differences occurred between month 1 and 4 . the total crown loss from month 1 to month 7 was a 17.3% decrease in the control group and 46.5% decrease in the exposed group , and the differences between these variations were statistically significant ( p<0.001 ) . tooth wear is irreversible and , unless the causes are detected and dealt with , it may progress in severity with age 25 , 26 . early diagnosis , prevention and intervention are the basis for tooth wear management 27 . the mechanical causes and parafunctional habits are the most reported in the literature . among the deleterious habits , bruxism , associated with abrasion , bruxism is an oral habit characterized by a rhythmic activity of the temporomandibular muscles that causes a forced contact between dental surfaces . it is accompanied by tooth clenching or grinding that can be loud enough to be heard 12 . the grinding of teeth has long been held as one physical manifestation of stress and anxiety 28 . our study revealed a serious problem of tooth wear amongst the rats exposed to lfn . the occlusal wear found in the aged - matched controls is certainly related to the normal ageing process as the animals were fed the same food and were in a silent environment . there was a noticeable and significant crown wear rate in the group exposed to lfn for 4 months , which resulted in a 39% reduction in the average areas of the molar cusps . other studies have recognized the relationship between lfn and stress 29 - 31 . in most studies , the relationship between noise as a  stressful  agent and the activity of the muscles of mastication suggests that the prevalence of dental abrasion in individuals subjected to noise is very high . studies in workers of the textile industry 32 reported significantly higher probability ratios of teeth with abrasion in the groups exposed to noise in comparison with the control group . the first stress response induced pathway is the activation of the hypothalamic - pituitary - adrenal ( hpa ) axis , which promotes the liberation of hormones that stimulate the release of glucocorticoids from the adrenal cortex 31 . the second response pathway is the sympathetic , promoting the liberation of adrenaline and noradrenaline from the adrenal medulla 31 . several studies clearly separate acute and chronic stress conditions , referring an adaptation / habituation state 33,34 . habituation is defined as a behavioral response decrement that results from repeated stimulation and that does not involve sensory adaptation / sensory fatigue or motor fatigue 35 . it is also known that the magnitude of the hpa activation occurring in response to a stressor declines with repeated exposure to that same stressor 23,36 . these studies may explain our results , specifically the decline in tooth wear with exposure time . moreover , it can not be excluded a defensive response to pain or discomfort , caused by wear , with less contact force between teeth . repairing , tertiary dentin formation may also play a role , concerning our results . unlike secondary dentin , which is physiological and forms throughout the vital life of the tooth , the formation of tertiary dentin is localized in the pulp chamber wall corresponding to the area of the stimulus 37 . the tissue that is deposited on the pulpal aspect in response to external stimuli , such as abrasion , attrition , caries , ultra - sonic scaling , among others , is called reactionary tertiary dentin 37 . these external factors stimulate an increased rate of matrix secretion by the existing odontoblasts exposed to the influence . according to lovschall et al . 38 the cusp wear disturbs the odontoblasts at the tip of the pulp horn , stimulating the deposition of tertiary dentin continuously . this response , in a certain way , compensates the loss of hard tissues from the crown and might justify the attenuation of the values of the wear rate observed in our study after the third month . this wear attenuation , observed from month 4 to month 7 , may also be explained by a decrease in the response of reactionary dentinogenesis with increasing thickness of tertiary dentin being deposited over the pulp chamber wall 37 . the distance between odontoblast cells , responsible for producing tertiary dentin , and the dentin tubules , may limit further odontoblast stimulation and cause the measured attenuation in tertiary reactionary dentin production . lfn induces morphological and functional alterations on the parotid gland from the first week of exposure , which affects the saliva conditions , namely quantitative and qualitative 39,40 . being saliva a very important maintainer of oral homeostasis 41 and , consequently is a tooth protector 42 , the susceptibility of teeth to mechanical wear may be increased when secretion of saliva is compromised . lfn also causes lesions similar to periodontitis , as described in a previous study 43 . this periodontal condition can also contribute to the decrease in wear rate , as it can cause discomfort and lead to a defensive response with less contact force . based on our data , there is significant tooth wear in animals exposed to industrial noise . this wear correlates with exposure time and is significantly higher in the first 4 months of exposure , probably due to mechanisms of adaptation / habituation , defensiveness ( periodontitis , pain ) , to continuous deposition of reactionary tertiary dentin 44 , or to a combination of all these factors . our results , and the growing presence of noise in the environment , point to the consideration of this stimulus in the pathogenesis of tooth wear .
tooth wear is a complex multifactorial process that involves the loss of hard dental tissue . 
 parafunctional habits have been mentioned as a self - destructive process caused by stress , which results in hyperactivity of masticatory muscles . 
 stress manifests itself through teeth grinding , leading to progressive teeth wear . 
 the effects of continuous exposure to industrial noise , a  stressor  agent , can not be ignored and its effects on the teeth must be evaluated.aims : the aim of this study was to ascertain the effects of industrial noise on dental wear over time , by identifying and quantifying crown area loss.material and methods : 39 wistar rats were used . 
 thirty rats were divided in 3 experimental groups of 10 animals each . 
 animals were exposed to industrial noise , rich in lfn components , for 1 , 4 and 7 months , with an average weekly exposure of 40 hours ( 8h / day , 5 days / week with the weekends in silence ) . 
 the remaining 9 animals were kept in silence . 
 the areas of the three main cusps of the molars were measured under light microscopy.statistical analysis used : a two - way anova model was applied at significance level of 5%.results : the average area of the molar cusps was significantly different between exposed and non - exposed animals . 
 the most remarkable differences occurred between month 1 and 4 . 
 the total crown loss from month 1 to month 7 was 17.3% in the control group , and 46.5% in the exposed group , and the differences between these variations were significant ( p<0.001).conclusions : our data suggest that industrial noise is an important factor in the pathogenesis of tooth wear .