Cont ents Acknowledgements (Agradecimientos) i Scientific papers v Summary vii Resumen ix Chapter 1. Introduction and objectives 1 1.1. Introduction 3 1.2. Objectives 9 1.3. Relevance of the thesis 1.4. Structure of the thesis Chapter 2. Influence of population density on the concentration and speciation of metals in the soil and road dust from urban areas Abstract 2.1. Introduction 2.2. Material and methods 2.2.1. Study area and sampling collection 2.2.2. Analytical methodology 2.2.3. Statistical analysis 2.3. Results and discussion 2.3.1. Effect of population density on physical-chemical characteristics of soil and road dust 2.3.2. Population density and total metal concentrations in soil and road dust 2.3.3. Effect of population density on chemical speciation of trace metals 2.3.4. Behaviour of heavy metals in the soil and road dust 2.4. Conclusions