Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine 1059 Mixture Effects of Environmental Contaminants ERIK LAMPA ACTA UNIVERSITATIS UPSALIENSIS ISSN 1651-6206 ISBN 978-91-554-9122-2 UPPSALA urn:nbn:se:uu:diva-237690 2015 Dissertation presented at Uppsala University to be publicly examined in Frödingsalen, Ulleråkersvägen 40A, Uppsala, Friday, 6 February 2015 at 09:00 for the degree of Doctor of Philosophy (Faculty of Medicine). The examination will be conducted in Swedish. Faculty examiner: Professor Jonas Björk (Avdelningen för Arbets- och Miljömedicin, Lunds universitet). Abstract Lampa, E. 2015. Mixture Effects of Environmental Contaminants. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine 1059. 85 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-554-9122-2. Chemical exposure in humans rarely consists of a single chemical. The everyday exposure is characterized by thousands of chemicals mainly present at low levels. Despite that fact, risk assessment of chemicals is carried out on a chemical-by-chemical basis although there is a consensus that this view is too simplistic. This thesis aims to validate a statistical method to study the impact of mixtures of contaminants and to use that method to investigate the associations between circulating levels of a large number of environmental contaminants and atherosclerosis and the metabolic syndrome in an elderly population. Contaminants measured in the circulation represented various classes, such as persistent organic pollutants, plastic-associated chemicals and metals. There was little co-variation among the contaminants and only two clusters of PCBs could be discerned. Gradient boosted CARTs were used to assess additive and multiplicative associations between atherosclerosis, as measured by the intima-media thickness (IMT) and the echogenicity of the intima-media complex (IM-GSM), and prevalent metabolic syndrome. Systolic blood pressure was the most important predictor of IMT while the influence of the contaminants was marginal. Three phthalate metabolites; MMP, MEHP and MIBP were strongly related to IM-GSM. A synergistic interaction was found for MMP and MIBP, and a small antagonistic interaction was found for MIBP and MEHP. Associations between the contaminants and prevalent metabolic syndrome were modest, but three pesticides; p,p’-DDE, hexachlorbenzene and trans-nonachlor along with PCBs 118 and 209 and mercury were the strongest predictors of prevalent metabolic syndrome. This thesis concludes that many contaminants need to be measured to get a clear picture of the exposure. Boosted CARTs are useful for uncovering interactions. Multiplicative and/or additive effects of certain contaminant mixtures were found for atherosclerosis or the metabolic syndrome. Keywords: Mixtures, Environmental Contaminants, Atherosclerosis, Metabolic Syndrome, Boosting, Epidemiology Erik Lampa, Department of Medical Sciences, Occupational and Environmental Medicine, Akademiska sjukhuset, Uppsala University, SE-75185 Uppsala, Sweden. © Erik Lampa 2015 ISSN 1651-6206 ISBN 978-91-554-9122-2 urn:nbn:se:uu:diva-237690 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-237690) Till min familj List of papers This thesis is based on the following papers, which are referred to in the text by their Roman numerals. I E Lampa, L Lind, A Bornefalk-Hermansson, S Salihovic, B van Bavel, and P M Lind. An investigation of the co-variation in circulating levels of a large number of environmental contaminants. Journal of Exposure Science and Environmental Epidemiology, 22(5):476–482, 2012 II E Lampa, L Lind, P M Lind, and A Bornefalk-Hermansson. The identification of complex interactions in epidemiology and toxicology: a simulation study of boosted regression trees. Environ Health, 13(57), 2014 III E Lampa, A Bornefalk-Hermansson, P M Lind, and L Lind. Atherosclerosis in Humans and the Association to Environmental Contaminant Mixtures. Manuscript, 2014 IV E Lampa, A Bornefalk-Hermansson, L Lind, and P M Lind. Mixture Effects of Multiple Environmental Contaminants on the Metabolic Syndrome in a Human Population-based Sample. Manuscript, 2014 Reprints were made with permission from the publishers. Contents 1 Introduction .................................................................................................. 15 1.1 Environmental Contaminants .............................................................. 16 1.1.1 Persistent Organic Pollutants .................................................... 16 1.1.2 Plastic-Associated Chemicals ................................................... 17 1.1.3 Metals ......................................................................................... 18 1.2 Chemical mixtures ................................................................................ 18 1.2.1 Concentration Addition and Independent Action .................... 19 1.2.2 Experimental Studies on Mixtures ............................................ 22 1.2.3 Epidemiological Studies on Mixtures ....................................... 22 1.3 An Unsolved Issue ............................................................................... 23 1.4 Atherosclerosis ..................................................................................... 24 1.4.1 Environmental Contaminants and Atherosclerosis .................. 24 1.5 The Metabolic Syndrome ..................................................................... 25 1.5.1 Environmental Contaminants and the Metabolic Syndrome ... 26 2 Aim ............................................................................................................... 28 3 Material and Methods .................................................................................. 29 3.1 The PIVUS Study .................................................................................. 29 3.1.1 Measured Contaminants ............................................................ 29 3.1.2 Ultrasound Evaluation of the Carotid Artery ........................... 30 3.2 Statistical Methods ............................................................................... 31 3.2.1 Identifying Clusters of Contaminants ....................................... 31 3.2.2 Identifying Suitable Markers ..................................................... 32 3.2.3 Classification and Regression Trees ......................................... 32 3.2.4 Stochastic Gradient Boosting .................................................... 34 3.2.5 Variable Importance and Interpretation .................................... 35 3.2.6 Assessment of Interaction Effects ............................................. 35 3.2.7 Simulating Contaminant Data ................................................... 37 4 Results and Discussion ................................................................................ 39 4.1 Paper I ................................................................................................... 39 4.1.1 Discussion, paper I ..................................................................... 39 4.2 Paper II .................................................................................................. 41 4.2.1 Assessment of Interaction Effects in the Simulated Data ....... 43 4.2.2 Visualizing the Four-way Interaction ....................................... 46 4.2.3 Discussion, paper II ................................................................... 48 4.3 Paper III ................................................................................................ 50 4.3.1 IMT ............................................................................................. 50 4.3.2 IM-GSM ..................................................................................... 53 4.3.3 Discussion, paper III .................................................................. 59 4.4 Paper IV ................................................................................................ 59 4.4.1 Discussion, paper IV .................................................................. 64 5 Summary ...................................................................................................... 65 6 General Discussion ...................................................................................... 66 7 Svensk sammanfattning: Blandningseffekter av Miljögifter .................... 72 7.1 Studie I .................................................................................................. 73 7.2 Studie II ................................................................................................. 73 7.3 Studie III ............................................................................................... 74 7.4 Studie IV ............................................................................................... 74 7.5 Sammanfattning av Studierna .............................................................. 74 8 Acknowledgement ....................................................................................... 75 References ........................................................................................................ 77 List of Tables Table 3.1: Mean values and standard deviations (SD) used in the simulation ......................................................................................................... 38 Table 4.1: Bootstrap validated parameters ..................................................... 43 List of Figures 1.1 A mixture experiment ..................... 20