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- Oudin, Anna, et al.
(författare)
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Efficiency of two-phase methods with focus on a planned population-based case-control study on air pollution and stroke
- 2007
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Ingår i: Environmental Health. - 1476-069X. ; 6:34
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Tidskriftsartikel (refereegranskat)abstract
- ABSTRACT: We plan to conduct a case-control study to investigate whether exposure to nitrogen dioxide (NO2) increases the risk of stroke. In case-control studies, selective participation can lead to bias and loss of efficiency. A two-phase design can reduce bias and improve efficiency by combining information on the non-participating subjects with information from the participating subjects. In our planned study, we will have access to individual disease status and data on NO2 exposure on group (area) level for a large population sample of Scania, southern Sweden. A smaller sub-sample will be selected to the second phase for individual-level assessment on exposure and covariables. In this paper, we simulate a case-control study based on our planned study. We develop a two-phase method for this study and compare the performance of our method with the performance of other two-phase methods. METHODS: A two-phase case-control study was simulated with a varying number of first- and second-phase subjects. Estimation methods: Method 1: Effect estimation with second-phase data only. Method 2: Effect estimation by adjusting the first-phase estimate with the difference between the adjusted and unadjusted second-phase estimate. The first-phase estimate is based on individual disease status and residential address for all study subjects that are linked to register data on NO2-exposure for each geographical area. Method 3: Effect estimation by using the expectation-maximization (EM) algorithm without taking area-level register data on exposure into account. Method 4: Effect estimation by using the EM algorithm and incorporating group-level register data on NO2-exposure. RESULTS: The simulated scenarios were such that, unbiased or marginally biased (< 7 %) odds ratio (OR) estimates were obtained with all methods. The efficiencies of method 4, are generally higher than those of methods 1 and 2. The standard errors in method 4 decreased further when the case/control ratio is above one in the second phase. For all methods, the standard errors do not become substantially reduced when the number of first-phase controls is increased. CONCLUSION: In the setting described here, method 4 had the best performance in order to improve efficiency, while adjusting for varying participation rates across areas.
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| 2. |
- Oudin, Anna, et al.
(författare)
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Long-term exposure to air pollution and hospital admissions for ischemic stroke. A register-based case-control study using modelled NOx as exposure proxy
- 2009
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Ingår i: BMC Public Health. - : Springer Science and Business Media LLC. - 1471-2458. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- Background: Long-term exposure to air pollution is a hypothesized risk factor for ischemic stroke. In a large case-control study with a complete study base, we investigated whether hospital admissions for ischemic stroke were associated with residential concentrations of outdoor NOx, as a proxy for exposure to air pollution, in the region of Scania, Southern Sweden. Methods: We used a two-phase case-control study design, including as first-phase controls all individuals born between 1923 and 1965 and residing in Scania in 2002 (N=556 912). We defined first-phase cases as first-time ischemic stroke patients residing in Scania and registered in the Swedish stroke register between 2001 and 2005 (N=4 904) and second-phase cases as cases for whom we had information on smoking status, diabetes, and medication for hypertension (N=4 375). For the controls, information on these covariables was collected from a public health survey, resulting in 4 716 second-phase controls. With a geographical information system and an emission database, individual residential outdoor annual mean NOx concentration was modelled. The data were analyzed with logistic regression. Results: We found no evident association between NOx and ischemic stroke. For example, the odds ratio for ischemic stroke associated with the NOx category 20-30 mu g/m(3) compared to the reference category of <10 mu g/m(3) was 0.95 (95% CI 0.86-1.06). Conclusion: In this study area, with generally low levels of air pollution, using a complete study base, high-quality ascertainment of cases, and individually modelled exposure, we did not observe any clear association between NOx and ischemic stroke hospital admissions.
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| 3. |
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| 4. |
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| 5. |
- Oudin, Anna
(författare)
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Short-term and Long-term Exposure to Air pollution and Stroke risk
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Background: Stroke is a major cause of mortality and morbidity across the world. Air pollution has become an acknowledged risk factor for stroke, especially for acute effects. There is less support for effects by chronic exposure due to the methodological challenges inherent in such studies. Such challenges can regard missing individual level data or residual confounding and can partly be overcome by employing two-phase methods. The general idea with such methods is to incorporate study subjects with partly missing data into the analysis. The main aims in this work were to investigate if air pollution increase stroke risk in Scania (Sweden), where pollution levels are rather low, and to investigate the performance of different two-phase methods. Methods: Nitrogen oxides (NOx) concentrations were modelled with high resolution. Data from the National Stroke Register (Riks-stroke) were obtained on hospital admissions for ischemic and hemorrhagic stroke during the years 2001 to 2006. The association between annual mean levels of NOx (as a marker for long-term exposure to air pollution) and ischemic stroke was studied in a two-phase case-control design. The short-term association between daily levels of inhalable particulate air pollution (PM10), ozone, NOx and temperature and the number of strokes occurring each day was studied with time series methods. The performance of several different two-phase methods was compared in a simulated case-control study. Results: No association between annual mean NOx and ischemic stroke was observed. Regarding short-term effects, high levels of PM10 increased the risk of ischemic stroke the next day, whereas high levels of temperature decreased the risk the next day. For hemorrhagic strokes, a similar, but less precise pattern was observed. Two-phase methods seemed to benefit from incorporating group-level data on exposure. The method which was most suitable for incorporating such data could be generalised to polytomous exposure categories and to include confounding factors. Conclusions: No evidence for long-term exposure to air pollution to increase ischemic stroke risk in Scania was observed. Short-terms effects by PM10 were documented, contributing to evidence for risk increases even in low-level areas. The role of temperature should be investigated further. The use of two-phase methods was illustrated in both a methodologial study and an empirical study.
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| 7. |
- Stroh, Emilie, et al.
(författare)
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Are associations between socio-economic characteristics and exposure to air pollution a question of study area size? An example from Scania, Sweden
- 2005
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Ingår i: International Journal of Health Geographics. - 1476-072X. ; 4:30
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Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND: Numerous studies have shown that exposure to air pollutants in the area of residence and the socio-economic status of an individual may be related. Therefore, when conducting an epidemiological study on the health effect of air pollution, socio-economy may act as a confounding factor. In this paper we examine to what extent socio-economic status and concentrations of NO2 in the county/region of Scania, southern Sweden, are associated and if such associations between these factors differ when studying them at county or city level. To perform this study we used high-resolution census data and modelled the annual exposure to NO2 using an emission database, a dispersion modelling program and a geographical information system (GIS). RESULTS: The results from this study confirm that socio-economic status and the levels of NO2 in the area of residence are associated in some cities. The associations vary considerably between cities within the same county (Scania). Even for cities of similar sizes and population bases the associations observed are different. Studying the cities together or separately yields contradictory results, especially when education is used as a socio-economic indicator. CONCLUSION: Four conclusions have been drawn from the results of this study. 1) Adjusting for socio-economy is important when investigating the health effects of air pollution. 2) The county of Scania seems to be heterogeneous regarding the association between air pollution and socio-economy. 3) The relationship between air pollution and socio-economy differs in the five cities included in our study, depending on whether they are analysed separately or together. It is therefore inadvisable to determine and analyse associations between socio-economy and exposure to air pollutants on county level. This study indicates that the size and choice of study area is of great importance. 4) The selection of socio-economic indices (in this study: country of birth and education level) is important.
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| 8. |
- Stroh, Emilie, et al.
(författare)
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Geographical patterns in blood lead in relation to industrial emissions and traffic in Swedish children, 1978-2007
- 2009
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Ingår i: BMC Public Health. - : Springer Science and Business Media LLC. - 1471-2458. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- Background: Blood lead concentrations (B-Pb) were measured in 3 879 Swedish school children during the period 1978-2007. The objective was to study the effect of the proximity to lead sources based on the children's home and school location. Methods: The children's home address and school location were geocoded and their proximity to a lead smelter and major roads was calculated using geographical information system (GIS) software. All the statistical analyses were carried out using means of generalized log-linear modelling, with natural-logarithm-transformed B-Pb, adjusted for sex, school year, lead-exposing hobby, country of birth and, in the periods 1988-1994 and 1995-2007, parents' smoking habits. Results: The GIS analysis revealed that although the emission from the smelter and children's BPb levels had decreased considerably since 1978, proximity to the lead smelter continued to affect levels of B-Pb, even in recent years (geometric mean: near smelter: 22.90 mu g/l; far from smelter 19.75 mu g/l; p = 0.001). The analysis also revealed that proximity to major roads noticeably affected the children's B-Pb levels during the period 1978-1987 (geometric mean near major roads: 44.26 mu g/l; far from roads: 38.32 mu g/l; p = 0.056), due to the considerable amount of lead in petrol. This effect was, however, not visible after 1987 due to prohibition of lead in petrol. Conclusion: The results show that proximity to the lead smelter still has an impact on the children's B-Pb levels. This is alarming since it could imply that living or working in the vicinity of a former lead source could pose a threat years after reduction of the emission. The analysis also revealed that urban children exposed to lead from traffic were only affected during the early period, when there were considerable amounts of lead in petrol, and that the prohibition of lead in petrol in later years led to reduced levels of lead in the blood of urban children.
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