ABSTRACT
OBJECTIVES: This study was conducted to assess the development of posttraumatic stress disorder (PTSD) after occupational physical injuries and the risk factors in occupational accident patients for PTSD. METHODS: Forty-seven occupational accident patients with physical injuries were administered BDI, STAI I and II, SCL-90-R, IES and questionnaires for risk factors and Clinician-Administered PTSD Scale(CAPS). RESULTS: 1) 12 patients(25%) were diagnosed as PTSD. 2) The BDI, STAI I and II and IES scores are significantly high(<0.05) in PTSD group than non-PTSD group. 3) The subscales of SCL-90-R, with an exception of PAR subscale, showed higher scores(<0.05) in PTSD group. 4) The loss of consciousness(LOC) was only significant risk factor in the occupational accident-related factors(<0.05). CONCLUSION: The results of this study show the development of PTSD among occupational accident patients. The loss of consciousness(LOC) was the only signiticant risk factor for PTSD in occupational accident patients.
Subject(s)
Humans , Accidents, Occupational , Surveys and Questionnaires , Risk Factors , Stress Disorders, Post-TraumaticABSTRACT
OBJECTIVE: The heart diseases are known as a major cause of sudden death, as well as a cause of poor life-quality of school-age children. But there have been few mass screening of heart diseases in these children in Korea. This study was done to estimate the prevalence of heart diseases of these population. METHODS: We screened all elementary students(grade 1) in 12 cities and 16 counties(Gun) in Kyonggi province from 1992 to 1995. The first screening was done by auscultation of doctors and simultaneously by checking using 'auto-interpreter of EKG-cardiac sound'(Fukuda Densi ECP 50A). We conducted futher examinations to whom classified as being abnormal condition in first screening, by using EKG, chest x-ray, doppler echocardiograpy(if needed). RESULTS: The total number of examined students was 161,308(92% of the population), the male were 83,238 and female were 78,070. The congenital heart diseases(CHD) patients were 290(18 per 10,000) - male 155(18.6 per 10,000) and female 135(17.3 per 10,000). The most frequent disease was ventricula septal defect(VSD, 45.5%), Atrial septal defect(ASD, 14.8%), Tetralogy of Follot(TOF, 11.7%), and Patent Dutus Arteriosis(PDA, 7.6%) in order. In female, the order was VSD(48.1%), ASD(13.3%), TOF(11.1%), and PDA(10.4%). The total number of EKG abnormality were 433(62.7 per 10,000) among 69,056 screened children in 1995. The complete right bundle branch block(CRBBB) and paroxymal ventricular contraction(PVC) were frequent(26.6%, 26.3% in each), and incomplete right bunddle branch block(IRBBB,14.6%), paroxymal atrial contraction(PAC, 6.7%), abnormal Q(5.8%), Wolf-Pakinson-White syndrom (5.5%) in order. In female, the most frequent abnormality was PVC(29.8%), and CRBBB(19.9%) in order. CONCLUSION: We could present the stable prevalence of the rare heart disease. The prevalence of congenital heart diseases was 18.0 per 10,000 and of EKG abnormality was 62.7 per 10,000 among school children.
Subject(s)
Child , Female , Humans , Male , Auscultation , Death, Sudden , Electrocardiography , Heart Diseases , Heart , Korea , Mass Screening , Prevalence , ThoraxABSTRACT
The multifactorial hypothesis is proposed as a working hypothesis which encompass both the genetic and environmental factors known to participate in the etiology of congenital heart malformations. So, it is believed that avoidance of suspected environmental factors in early pregnancy is the most certain preventive measure of congenital heart malformations. This study has been undertaken in order to find the possible environmental risk factors for congenital heart malformations in Korea. A total of 114 mothers of first graders of the elementary schools with congenital heart malformations confirmed through the screening program in Kyonggi Province from 1992 to 1995 were included as cases. And 206 mothers of healthy students matched by sex and elementary schools comprised the control group. Environmental risk factors including drug use during the first trimester of pregnancy, and other confounders were collected by telephone interview using standardized questionnaires by well trained interviewers. The result of multivariate logistic regression analysis showed that congenital heart malformation were associated with family history of congenital heart malformations(OR=2.94, 95% CI: 1.08, 7.96), the order of birth(OR=0.49, 95% CI: 0.31, 0.79). And the coffee consumption over 14 cups/week during early pregnancy showed marginal significance(OR=3.52, 95%CI: 0.98, 12.62). The mother's age at the subject birth and father's smoking at home were significant in linear trend test(p<0.05). It is recommended that the genetic counselling and the avoidance of known environmental risk factors in early pregnancy were needed to prevent congenital heart malformations.
Subject(s)
Female , Humans , Pregnancy , Case-Control Studies , Coffee , Heart , Interviews as Topic , Korea , Logistic Models , Mass Screening , Mothers , Odds Ratio , Parturition , Pregnancy Trimester, First , Risk Factors , Smoke , Smoking , Surveys and QuestionnairesABSTRACT
Sakai's method has been known as the simplest one for determination of erythrocyte pyrimidine 5'-nucleotidase (P5N) activity using high performance liquid chromatography(HPLC). However the drawback of the method is that it is difficult to wash the erythrocyte for isolation. To search for the simpler method, we compared Sakai's method with other methods using whole blood treated with heparin and concanavalin A or whole blood treated with EDTA-2K instead of washing the erythrocyte. The mean concentrations of lead in blood samples collected from 44 male and 16 female workers who are healthy without any exposure to lead in their workplace were 4.30 +/- 1.31 microgram /dl (mean +/-standard deviation), which were measured by frameless atomic absorption spectrophotometer. Erythrocyte P5N activities were measured by 3 methods; Sakai's method(Method I), using whole blood treated with heparin and concanavalin A (Method II), and using whole blood treated with EDTA-2K (Method III). The results were obtained as follows ; 1. The mean of erythrocyte P5N activity by Sakai's method(Method I) were 12.7 +/-2.47 amole uridine/hr/gm of Hb. 2. The mean of erythrocyte P5N activity by the method using heparinized whole blood treated with concanavalin A(Method II) were 13.1 +/-2.41 micromole uridine/hr/gm of Hb. 3. The difference of mean erythrocyte P5N activity between Method I and Method was not significant. 4. The erythrocyte P5N activity by the method using whole blood treated with EDTA-2K (Method III) was significantly different from Method I. We thought that omission of incubation period which was required on Method III using EDTA-2K caused the difference between Method I and Method III. 5. Simple linear regression equation for erythrocyte P5N activity between Method I (Y) and Method II(X) was significant: Y = -0.012 + 0.9724 X. These results suggest that the method using whole blood treated with heparin and concanavalin A is simpler to examine the erythrocyte P5N activity as a biological indicator of lead intoxication than Sakai's method.
Subject(s)
Female , Humans , Male , 5'-Nucleotidase , Absorption , Concanavalin A , Erythrocytes , Heparin , Linear ModelsABSTRACT
Sakai's method has been known as the simplest one for determination of erythrocyte pyrimidine 5'-nucleotidase (P5N) activity using high performance liquid chromatography(HPLC). However the drawback of the method is that it is difficult to wash the erythrocyte for isolation. To search for the simpler method, we compared Sakai's method with other methods using whole blood treated with heparin and concanavalin A or whole blood treated with EDTA-2K instead of washing the erythrocyte. The mean concentrations of lead in blood samples collected from 44 male and 16 female workers who are healthy without any exposure to lead in their workplace were 4.30 +/- 1.31 microgram /dl (mean +/-standard deviation), which were measured by frameless atomic absorption spectrophotometer. Erythrocyte P5N activities were measured by 3 methods; Sakai's method(Method I), using whole blood treated with heparin and concanavalin A (Method II), and using whole blood treated with EDTA-2K (Method III). The results were obtained as follows ; 1. The mean of erythrocyte P5N activity by Sakai's method(Method I) were 12.7 +/-2.47 amole uridine/hr/gm of Hb. 2. The mean of erythrocyte P5N activity by the method using heparinized whole blood treated with concanavalin A(Method II) were 13.1 +/-2.41 micromole uridine/hr/gm of Hb. 3. The difference of mean erythrocyte P5N activity between Method I and Method was not significant. 4. The erythrocyte P5N activity by the method using whole blood treated with EDTA-2K (Method III) was significantly different from Method I. We thought that omission of incubation period which was required on Method III using EDTA-2K caused the difference between Method I and Method III. 5. Simple linear regression equation for erythrocyte P5N activity between Method I (Y) and Method II(X) was significant: Y = -0.012 + 0.9724 X. These results suggest that the method using whole blood treated with heparin and concanavalin A is simpler to examine the erythrocyte P5N activity as a biological indicator of lead intoxication than Sakai's method.
Subject(s)
Female , Humans , Male , 5'-Nucleotidase , Absorption , Concanavalin A , Erythrocytes , Heparin , Linear ModelsABSTRACT
Malaria, one of the compulsory notifiable diseases, has been diappeared from Korea based on that fact no notification on malaria case was received from local health agencies during the last decade or so. Recently, Indigenous malaria has been re-emerged since 1993 and 549 cases was notificated till 1996. We conducted a surveillance system on the resurgent malaria outbreaks in the northern area of Kyonggi Province around the Imjin River. Malaria Surveillance Networks(MSNs) were established in Paju and Yoncheon between August 1996 and December 1996. When a febrile patient visits a clinic or a hospital, clinician takes a blood sample and refer to district malaria laboratory for the sample. The blood sample is examined in the malaria laboratory(public health center), and if malaria parasites are found, a radical or curative treatment is offered to patients. MSNs took 94 febrile cases and identified 23 malaria cases(24.5%). All malaria cases were infected by the indigenous vivax malaria. In Paju, 14 of 62 febrile cases(22.6%) were malaria outbreaks and 9 of 32 febrile cases(28.1%) in Yoncheon. In Korea resurgent malaria, malaria surveillance system should be operated for a program based on the district public health center with the coupled laboratory and dispensary.
Subject(s)
Humans , Disease Outbreaks , Korea , Malaria , Malaria, Vivax , Parasites , Public Health , RiversABSTRACT
This study was conducted on 367 male workers to evaluate the change of vibration perception threshold (VPT) according to exposure degree to carbon disulfide. VPT was measured on both metacarpal bones of index and little fingers and on fibular malleoli by Vibrometer (Rion, on 125Hz). The exposure degree was classified by the name of department. Workers in department 1 had been exposed to high concentration of carbon disulfide, those in. department 2 had been exposed to carbon disulfide moderately or intermittently, and those in department 3 had been exposed to undetectable concentration of carbon disulfide. The results were as follows: 1. The mean values of VPT of metacarpal(MC) bones showed dose-response pattern, i.e., the mean values of VPT at department 1 were highest, and those of VPT at department 3 were lowest with similar work duration or age. But the mean values of VPT of fibular malleoli (FM) showed dose-response pattern only in workers having above 5 years of work duration or whose age were above 30. 2. The mean values of VPT of MC bone of workers increased by age. 3. There were statistically significant difference in mean values of VPT of MC bone only in workers having 5-10 year work duration. And there were statistically significant difference of mean values of VPT of left 2nd and 5th MC bone in workers whose age was 30's, of right 2nd MC bone in those whose age was 40's, and of left 2nd MC bone in those whose age was 50's. 4. The variables affecting VPT significantly were department and age (or age group) both in multiple regression and general linear model. The department showed to have more effect to VPT than age in multiple regression, but age group showed to have more effect to VPT than department in general linear model. 5. The variables were transformed to indicator variable to do logistic regression analysis. Department 1, 2, 3 were classified into high exposure (spinning department) and low exposure (other department), age was classified into old (above 30) and young, and work duration was classified into long(above 10 years) and short. The exposure and age were significant variables by logistic regression analysis. Odds ratios of department and VPT on metacarpal bones or malleoli were 2.7-3.2 and those of age and VPT were 2.6 or 2.7. Odds ratio of age and abnormality of VPT on'any point was 3.3(1.59-6.;8), and that of department and VPT was 2.8(1.5-5.5). We concluded that exposure of carbon disulfied was significantly associated with reduction of VPT regardless of age; and that the test of VPT would be useful tool for screening and early detection in neuropathy by carbon disulfide or other neurotoxic chemicals,even though it is simple and cheap.
Subject(s)
Humans , Male , Carbon Disulfide , Carbon , Fingers , Linear Models , Logistic Models , Mass Screening , Metacarpal Bones , Odds Ratio , VibrationABSTRACT
Biological monitoring for exposures permits estimation of organ doses or body burdens from exposures through all relevant portals of entry. Biological monitoring data may be used to estimate environmental concentrations when the latter cannot be measured directly. Biological indices are usually surrogates for the concentration of a chemical or its metabolites or its effect at the true receptors. Mercury concentration in urine has-been most-coinmoialy-recommended as a biological exposure index of mercury. For data based on urine analysis, variation in urine volume is the most significant. The urinary concentration related to excretion of the solute provides some correction for fluctuation of urine output. Sampling time must be carefully observed because distribution and elimination of a chemical are kinetic events. This study has evaluated mercury concentration in spot urine compared to the results of 24 hour collected urine by the adjustment methods (specif ic gravity, creatinine) and sampling time. The subjects were 43 workers who had been exposed to the metallic mercury. The results were as follows: 1. The correlation coefficients between mercury concentration in 24 hour urine and that in spot urine were 0.639-0.715 and were not different by adjustment methods. 2. In the high exposure group who were over lOOug/1 of urinary mercury, the correlation coefficients between mercury concentration in 24 hour urine and that in spot urine were 0. 687-0.824 and were not different by adjustment methods. 3. Mercury concentration in spot urine were very variable by sampling time or exposure time. The correlation coefficients between mercury concentration in 24 hour urine and that in spot urine were most highest as 0.85-0.91 at first voiding urine in the morning, and were 0. 77-0.86 at urine collected within four hours before end of shift. In the biological monitoring to exposure of mercury, sampling of spot urine were most proper at first voiding urine in the morning, and then at urine collected within four hours before end of shift. But the adjustment methods of specific gravity and creatinine were no difference of the results.
Subject(s)
Body Burden , Creatinine , Environmental Monitoring , Gravitation , Specific GravityABSTRACT
We investigated the 1,004 workers who worked in a automobile factory to study the epidemiologic characterist of dermatoses due to cutting oils. Among the workers, 667(66.4%) answered the questionnaire. They are belong to 5 departments of the factory-the Engine-Work(86), Power train Assembly(17). We measured the oil mist concentration in air of the departments and examined the workers who had dermatologic symptoms. The results were follows; 1) Oil mist concentration; Of all measured points(52), 9 points(17.2%) exceeded 5mg/m3-the time-weighed PEL- and one department had a upper confidence limit(95%) higher than 5mg/m3. 2) Dermatologists examined 213 workers. 172 of them complained any skin symptoms at that time-itching(32.5%), papule(21.6%), scale(15.7%), vesicle(12.5%) in order. The abnormal skin site found by dermatologist were palm(29.3%), finger & nail(24.6%), forearm(16.2%), back of hand(8.4%) in order. 3) As the result of physical examination, we found that 160 workers had skin diseases. Contact dermatitis was the most common; 69 workers had contact dermatitis alone(43.1%), 11 had contact dermatitis with acne(6.9%), 10 had contact dermatitis with folliculitis(6.3%), 1 had contact dermatitis with acne & folliculitis, and 1 had contact dermatitis with abnormal pigmentation. Others were folliculitis(9 workers, 5.6%), acne(8, 5.0%), folliculitis & acne(2, 1.2%), keratosis(1, 0.6%), abnormal pigmentation(1, 0.6%), and non-specific hand eczema(47, 29.3%). 4) The prevalence of any skin diseases was 34.0 per 100 in cutting oil users, and 13.3 per 100 in non-users. Especially, the prevalence of contact dermatitis was 23.0 per 100 in cutting oil users and 4.3 per 100 in non-users. 5) We tried patch test(standard series, oil series, organic solvents) on 49 patients to differentiate allergic contact dermatitis from irritant contact dermatitis and found 20 were positive. 6) In a multivariate analysis(independent=age, tenure, kinds of cutting oil), the risk of skin diseases was higher in the water-based cutting oil user and both oil user than non-user or neat oil user(odds ratio were 2.16 and 2.78, respectively). And the risk of contact dermatitis was much higher at the same groups(odds ratio were 5.16 and 6.82, respectively).
Subject(s)
Humans , Acne Vulgaris , Automobiles , Dermatitis, Allergic Contact , Dermatitis, Contact , Fingers , Folliculitis , Hand , Oils , Physical Examination , Pigmentation , Prevalence , Surveys and Questionnaires , Skin , Skin DiseasesABSTRACT
This study was conducted on 103 male workers exposed to local vibration (exposure workers) and 23 male clergical workers not exposed to local vibration (reference workers) to evaluate the effect of local vibration in a automobile industry workers who had used vibratory tools such as electric bolt impacter or grinder. The assessment methods of this study were some tests recommended by Wasserman and Taylor. None of reference workers complained sensorineural symptoms, but 43.7% of exposed workers complained over one of those symptoms. The proportion of non-recovery type, which means that there was no full recovery of temperature of finger tips, was from 0% to 31. 3% in reference workers and from 10.7% to 15.5% in exposure workers. There was statistically significant difference of recovery time of plethysmogram after cold provocation between mean recovery time of 0.44 minutes in reference workers and that of 3.05 minutes in exposure workers. The proportion of bad plethysmogram was from 0% to 4.4% in reference workers and from 3.9% to 7.8% in exposure workers. There was statistically significant difference of discrimination sense between mean length of 2.04 mm in reference workers and that of 2.9 mm in exposure workers. There were also statistically significant difference of pain sense between mean weights of from 5.02 g to 5.26 g in reference workers and those of from 6.81 g to 7.6 g in exposure workers. By the results of multiple stepwise regression analysis the statistically significant variable affecting discrimination and pain sense was exposure and those affecting vibration sense were exposure and age. We concluded that exposure of local vibration in automobile workers was significaritly associated with decrease of sensorineural sense regardless of age and other, possible related variables.
Subject(s)
Humans , Male , Automobiles , Discrimination, Psychological , Fingers , Hand-Arm Vibration Syndrome , Vibration , Weights and MeasuresABSTRACT
This study was conducted on 103 male workers exposed to local vibration (exposure workers) and 23 male clergical workers not exposed to local vibration (reference workers) to evaluate the effect of local vibration in a automobile industry workers who had used vibratory tools such as electric bolt impacter or grinder. The assessment methods of this study were some tests recommended by Wasserman and Taylor. None of reference workers complained sensorineural symptoms, but 43.7% of exposed workers complained over one of those symptoms. The proportion of non-recovery type, which means that there was no full recovery of temperature of finger tips, was from 0% to 31. 3% in reference workers and from 10.7% to 15.5% in exposure workers. There was statistically significant difference of recovery time of plethysmogram after cold provocation between mean recovery time of 0.44 minutes in reference workers and that of 3.05 minutes in exposure workers. The proportion of bad plethysmogram was from 0% to 4.4% in reference workers and from 3.9% to 7.8% in exposure workers. There was statistically significant difference of discrimination sense between mean length of 2.04 mm in reference workers and that of 2.9 mm in exposure workers. There were also statistically significant difference of pain sense between mean weights of from 5.02 g to 5.26 g in reference workers and those of from 6.81 g to 7.6 g in exposure workers. By the results of multiple stepwise regression analysis the statistically significant variable affecting discrimination and pain sense was exposure and those affecting vibration sense were exposure and age. We concluded that exposure of local vibration in automobile workers was significaritly associated with decrease of sensorineural sense regardless of age and other, possible related variables.
Subject(s)
Humans , Male , Automobiles , Discrimination, Psychological , Fingers , Hand-Arm Vibration Syndrome , Vibration , Weights and MeasuresABSTRACT
To investigate the effect of carbon disulfide on blood pressure, the BP measurements in the periodic health examination results and the medical records of factory clinic were reviewed The study subjects were composed of 1336 male and 544 female workers, who were categorized into three groups by the exposure status-highly exposed, moderately exposed and non-exposed group. The results of the study were as follows; 1. The age-adjusted mean systolic and diastolic BP of male workers were 122.35 mmHg/79.11mmHg in highly exposed, 121.57mmHg/79.05mmHg in moderately exposed and 122.67mmHg/82.27mmHg in non-exposed group. For female workers, BP were 115.13mmHg/74.49mmHg in moderately exposed and 113.48mmHg/74.30mmHg in non-exposed group. 2. In multiple regression analysis of maximum BP against Age and tenure, the slope coefficients of age and tenure on the systolic BP were 0.379, 0.667 respectively and those on the diastolic BP were 0.331, 0.405 respectively in highly exposed male workers. Tenure was a significant variable in this study. For female workers, however, the slope coefficients of tenure on BP were significant only for systolic BP of moderately- expected group. 3. In multiple regression analysis of Bp against age, cumulative exposure index(CEI), cholesterol, all the variables showed significant slope coefficients in male, but age and CEl on systolic BP were significant for female workers (P<0.05). 4. In the multiple analysis of the amount of Bp change and the velocity of Bp change among male workers, the slope coefficients of tenure tended to increase as exposure level increased. Among female workers, the slope coefficients of tenure were significant on the amount of Bp change and the velocity of Bp change in moderately exposed group.
Subject(s)
Female , Humans , Male , Blood Pressure , Carbon Disulfide , Carbon , Cholesterol , Medical RecordsABSTRACT
This study was conducted to evaluate the relationships between the environmental exposure and biological monitoring among workers exposed to metallic mercury. We interviewed each workers to get the medical history including previous hazardous occupational history we measured the respiration rate and tidal volume of each worker in order to calculate the 8-hour inhaled mercury of workers. And we wanted to evaluate the effect of exposure duration to mercury concentrations in blood and urine as biologic exposure indices of metallic mercury. The regression and correlation analysis were done to the relationships of 8-hour inhaled mercury are mercury in blood and urine. The results were as follows; 1. The subjects were 35 fluorescent lamp manufacturing workers. The mean age of subjects was 24.8 years old, and the mean work careers of workers was 1.19 years. 89% of the total was consisted man. 2. The correlation coefficients between 8-hour inhaled mercury and mercury in blood and urine were higher than that of only considered air mercury concentration. 3. The correlation coefficients of 8-hour inhaled mercury and mercury in blood and urine were above 0.9 in workers who had exposed to mercury more than 1 year. 4. The R-square value and -value of regression analysis between the 8-hour inhaled mercury and mercury inn blood and urine was also higher in workers who had exposed to mercury over 1 year than in workers who had less than 1 year working experience. The important results or this study were that relationships between the 8hr-inhaled mercury and mercury in blood and urine was very high than that with air mercury concentration only. And the results were very apparent when considering workers 1 year or more. Therefore we concluded that the work career and respiratory volume of each individuals should be considered in evaluation the results of biological monitoring of workers exposed to metallic mercury.
Subject(s)
Environmental Exposure , Environmental Monitoring , Inhalation , Respiratory Rate , Tidal VolumeABSTRACT
In this study, we measured the activity of the erythrocyte pyrimidine 5'-nucleotidase(P5N) for 83 workers exposed to lead in their workplace, and analyzed the correlation of the activity with other biological exposure indices of lead. The measurement was performed by using a high performance liquid chromatography (HPLC) with a reverse phased ODS column. The results are as follows; 1. The correlation of the erythrocyte P5N activity with the concentration of blood lead(PbB) was shown to be statistically significant (r=-0.71, p=0.0001). 2. For a group of subjects whose PbB was less than 10microgram/dl, the erythrocyte P5N activity was 14.9+/-1.5 micromole uridine/h/g Hb. 3. For medical surveillance, this study suggests the erythrocyte P5N activity of 12 micromole uridine/h/g Hb is equivalent to 40 microgram/dl of PbB. 4. The correlation coefficients of the erythrocyte P5N activity with other biological exposure indices of lead such as PbB, ZPP, ALA-U, PBU, CP-U, ALAD, and log ALAD were -0.71, -0.64, -0.57, -0.51, -0.50, 0.46 and 0.64, respectively. 5. The correlation coefficients of the PbB with other biological exposure indices of lead for ALAD, P5N, ZPP, PBU, CP-U, and ALA-U were -0.76, -0.71, 0.68, 0.59, 0.42, and 0.41, respectively. The erythrocyte P5N activity can be used as a reliable biological exposure index of lead.
Subject(s)
5'-Nucleotidase , Chromatography, Liquid , ErythrocytesABSTRACT
The cohort study is an observational epidemiological study which selects the specific study population attempting to study the relatiqnship between an exposure to the purported cause and the subsequent risk of developing disease in accordance of time direction. Since Doll and Hill had studied the association between lung cancer and smoking behavior among the British medical doctors in the years of the last fifties, the defined group cohort studies analysing the relationship between an exposure and the occurence of a disease have become very popular. Cohort studies can be classified as either prospective or retrospective, depending on the temporal relationship between the initiation of the study and the occurence of the disease. Cohort studies are admitted as the very valuable studies for demonstrating the association between an exposure and a disease because it is possible to drive relative and attributable risks and often incidence measures. They can even examine multiple effects of a single exposure. However, they are usually expensive to carry out and large cohorts are required for rare diseases in addition to the time consum ing works. There are also very significant problems associated with selection of appropriate groups to be studied as far as complete ascertainment of disease occurence in them. Usually it is necessary that we must compromise to provide the opportunity for various types of bias such as selection bias, follow-up bias, information bias or misclassification, confounding bias and post hoc bias to occur which can result in incorrect conclusions. Only the success of a cohort study would be expected when the investigator pay the deep care in recognizing and correcting for these biases.