Petter Kristensen

Birth defects among offspring of Norwegian farmers, 1967-1991.

We investigated birth defects (N = 4,565) reported to the Medical Birth Registry of Norway among 192,417 births between 1967 and 1991 to parents identified as farmers in five agricultural and horticultural censuses between 1969 and 1989. The prevalences at birth of all and specific birth defects deviated little from those among 61,351 births to non‐farmers in agricultural municipalities. We classified exposure indicators on the basis of information provided at the agricultural censuses. The main hypotheses were that parental exposure to pesticides was associated with defects of the central nervous system, orofacial clefts, some male genital defects, and limb reduction defects. We found moderate increases in risk for spina bifida and hydrocephaly, the associations being strongest for exposure to pesticides in orchards or greenhouses [spina bifida: 5 exposed cases, odds ratio (OR) = 2.76, 95% confidence interval (CI) = 1.07–7.13; hydrocephaly: 5 exposed cases, OR = 3.49, 95% CI = 1.34–9.09]. Exposure to pesticides, in particular in grain farming, was also associated with limb reduction defects (OR = 2.50; 95% CI = 1.06–5.90). We also saw an association with pesticides for cryptorchism and hypospadias. We found less striking associations for other specific defects and pesticide indicators, animal farming, and fertilizer regimens.

Explaining the Relation Between Birth Order and Intelligence

Negative associations between birth order and intelligence level have been found in numerous studies. The explanation for this relation is not clear, and several hypotheses have been suggested. One family of hypotheses suggests that the relation is due to more-favorable family interaction and stimulation of low-birth-order children, whereas others claim that the effect is caused by prenatal gestational factors. We show that intelligence quotient (IQ) score levels among nearly 250,000 military conscripts were dependent on social rank in the family and not on birth order as such, providing support for a family interaction explanation.

Bias from nondifferential but dependent misclassification of exposure and outcome.

When misclassification of exposure and disease is nondifferential but not independent of one another, bias away from the null can result. For dichotomous variables, misclassification is nonindependent when the probability of misclassification of one variable is dependent on the correctness of classification of the other variable. One plausible form of nonindependent misclassification may result from variation in the threshold for reporting exposure and outcome by study subjects. The odds ratio after dependent misclassification can be expressed as a function of the true odds ratio,the prevalences of exposure and outcome, and the probabilities of misclassification. When prevalences of exposure and outcome are low, bias may be considerable even at low probabilities of misclassification. The nonindependent misclassification described in this article will result in a positive bias in the odds ratio and is therefore of prime concern when questioning the validity of an observed effect. The core of the problem lies in the study design and can be solved by eliminating the common link that makes nonindependent errors possible. (Epidemiology 1992;3:210-215)

Respiratory symptoms and ventilatory function in workers exposed to Portland cement dust

The healthy worker effect and incomplete exposure information have been problems in former studies regarding the association between exposure to Portland cement dust and respiratory effects. We included former workers and made an individual estimation of particle exposure to reduce the selection bias in this controlled cross-sectional study on the effects of cement dust exposure on respiratory symptoms and ventilatory function in long-term exposed Norwegian cement plant workers. A total of 119 workers from the largest cement plant in Norway and 50 workers from a nearby control plant, born 1918 to 1938, performed spirometry and gave information on respiratory symptoms in 1998 and 1999. The prevalence of symptoms and mean pulmonary function indices were similar for exposed workers and controls. There was no dose–response-related increase in symptoms or decrease in lung function indices. The estimated power to detect a true difference between forced expiratory volume in one second (FEV1) in the two groups of 0.3l was 0.90, assuming 95% significance level. The prevalence of chronic obstructive pulmonary disease was 14.3% in the exposed group and 14.0% among the controls. These findings do not support the hypothesis that cement dust exposure has a negative impact on lung function or gives an increase in respiratory symptoms.

Suicide in young Norwegians in a life course perspective: population-based cohort study

Background Suicide is a leading cause of death in young adults. Several risk factors are well known, especially those related to adult mental health. However, some risk factors may have their origin in the very beginning of life. This study examines suicide in the general Norwegian population in a life course perspective, with a main focus on early life factors. Methods In this study, several national registers were linked, supplying personal data on biological and social variables from childhood to young adult age. Participants were all Norwegians live born during the period 1967–1976, followed up through 2004. Persons who died or emigrated before the year of their 19th birthday, at which age follow-up started, were excluded. Thus, the study population comprised 610 359 persons, and the study outcome was completed suicide. Results 1406 suicides (0.23%) were recorded, the risk being four times higher in men than in women. Suicide risk factors included not being firstborn (adjusted HR in men and women (95% CIs): 1.19 (1.05 to 1.36) and 1.42 (1.08 to 1.88)), instability of maternal marital status during childhood, parental suicide (mainly in women), low body mass index (only investigated in men), low education and indications of severe mental illness. Conclusions Suicide in young adults may be rooted in early childhood, and the effect is likely to act through several mechanisms, some of which may be linked to the composition and stability of the parental home. A life course perspective may add to our understanding of suicide.

Influence of physical, mental and intellectual development on disability in young Norwegian men.

BACKGROUND Few studies have examined the effects of educational level, intellectual performance, mental function, body mass index and height as well as their interrelationship on the risk of disability pension (DP), taking other life course factors into account. METHODS We linked several national registers, comprising the Medical Birth Registry, the Central Population Register, the Education Register, the Norwegian Labour and Welfare Organisation and the Norwegian Armed Forces Personnel Data Base, providing personal data on biological and social variables from childhood to young adult age. Participants were all males live born during the period 1967-76, followed up through 2003. Men were excluded who died, emigrated or were granted a DP until age 23 years (when follow-up started) and persons who did not become gainfully employed during the study period. Thus, the study population comprised 302 330 men, and the study outcome was being granted a DP after age 23 years. RESULTS 3651 men (1.2%) were granted a DP. The DP rate was inversely associated with both educational level and intellectual performance. The adjusted population attributable risks (PAR) values for these two variables were 47% and 35%, respectively. The effect of the other variables was modest. Over- and underweight and short stature were associated with DP, but the effect was largely reduced after adjusting for intellectual performance. Impaired mental function seemed to have an independent effect. CONCLUSION Receiving an early DP is dependent on several factors acting at different stages of life, above all educational level and intellectual performance. High education can modify some of the effects of low intellectual performance.

Job adjustment and absence from work in mid-pregnancy in the Norwegian Mother and Child Cohort Study (MoBa)

Background: Pregnant women at work have special needs, and sick leave is common. However, job adjustment in pregnancy is addressed in European legislation. Our main objective was to examine if job adjustment was associated with reduced absence. Methods: This study is based on the Norwegian Mother and Child Cohort Study (MoBa) conducted by the Norwegian Institute of Public Health. 28 611 employed women filled in questionnaires in weeks 17 and 30 in pregnancy. The risk of absence for more than 2 weeks was studied among those who were not absent in week 17 (n = 22 932), and the probability of return to work in week 30 among those who were absent in week 17 (n = 5679). Data were based on self-report. The influence of job adjustment (three categories: not needed, needed but not obtained, needed and obtained) was analysed in additive models in multivariable binomial regression. Associations with other job characteristics and work environment factors were also analysed. Results: The risk of absence for more than 2 weeks was 0.308 and the probability of return to work was 0.137. Compared with women who needed but did not achieve job adjustment, obtained job adjustment was associated with a 0.107 decreased risk of absence (95% confidence interval 0.090 to 0.125) in a model including other job characteristics and work environment factors. Job adjustment was correspondingly associated with a 0.041 (0.023 to 0.059) increased probability of return to work. Absence was associated with adverse work environment, whereas the opposite pattern was found for return to work among those who started off being absent. Conclusions: Job adjustment was associated with reduced absence from work in pregnancy. Results should be interpreted cautiously because of low participation in MoBa and potential information bias from self-reported data.

Risk factors for long-term absence due to psychiatric sickness: a register-based 5-year follow-up from the Oslo health study.

Objective: To identify individual and work-related predictors of long-term (>8 weeks) sickness absence with psychiatric diagnoses (LSP). Methods: Data from the Oslo Health Study (response rate 46%) were linked to public registers. A total of 8333 subjects were followed from 2001 through 2005. Cox regression was used to compute hazard ratios for LSP. Results: At least one LSP was present in 7.8% of women and 3.9% of men. Poor support from superior had an independent and moderate effect. Path and linear regression analyses indicated that the effect of support from superior was mediated through mental distress and not the other way around. Self-reported mental distress had a strong independent effect. Conclusions: Women had a higher risk of LSP than men. Low education and poor support from superior and mental distress were found to be determinants of LSP.

Maternal reproductive history: a registry based comparison of previous pregnancy data derived from maternal recall and data obtained during the actual pregnancy

Objective. To compare the quality of data on previous pregnancies, based on maternal recall, to that of actual data in previous birth records, and to quantify to which extent recall based data bias results in epidemiologic studies on adverse reproductive outcomes.

Epidemiology of trauma: a population-based study of geographical risk factors for injury deaths in the working-age population of Norway.

INTRODUCTION Trauma is a major global cause of morbidity and mortality. Population-based studies identifying high-risk populations and regions may facilitate primary prevention and the development of optimal trauma systems. This study describes the epidemiology of adult trauma deaths in Norway and identifies high-risk areas by assessing different geographical measures of rurality. METHODS All trauma-related deaths in Norway from 1998 to 2007 among individuals aged 16-66 years were identified by accessing national registries. Mortality data were analysed by linkage to population and geographical data at municipal, county and national levels. Three measures of rurality (centrality, population density and settlement density) were compared based on their association with trauma mortality rates. RESULTS The study included 8466 deaths, of which 78% were males. The national annual trauma mortality rate was 28.7 per 100,000. Population density was the best predictor of high-risk areas, and there was a consistent inverse relationship between mortality rates and population density. The most rural areas had 52% higher trauma mortality rates compared to the most urban areas. This difference was largely due to deaths following transport-related injury. Seventy-eight per cent of all deaths occurred in the prehospital phase. Rural areas and death following self-harm had higher proportion of prehospital deaths. CONCLUSION Rural areas, as defined by population density, are at a higher risk of deaths following traumatic injuries and have higher proportions of prehospital deaths and deaths following transport-related injuries. The heterogeneous characteristics of trauma populations with respect to geography and mode of injury should be recognised in the planning of preventive strategies and in the organisation of trauma care.