Background: The increasing interest in Indoor Air Quality (IAQ) of educational buildings has been underpinned by the rising incidence of asthma and respiratory disease among children, who spend a substantial amount of their lives on the school premises. The susceptibility of children compared with adults has led to the formulation of guidelines regulating IAQ in school buildings. WHO guidelines provide the scientific basis for legally enforceable standards for non-industrial environments. Reflecting the relative difficulty and expense of obtaining measurements of specific pollutants, guidelines for the provision of adequate IAQ in UK schools have been typically framed around thermal conditions, carbon dioxide (CO2) levels, and estimated ventilation rates as a primary indicator of IAQ. Aim: Drawing on detailed monitoring data from 15 primary and three nursery London classrooms, this thesis sets to evaluate if indoor CO2 levels in classrooms are a good indicator in ensuring a healthy and satisfactory school environment. To fully answer this question this thesis aims to associate levels of specific indoor pollutants with CO2 levels and ventilation rates after controlling for environmental and behavioural factors; to identify specific exposures in the classroom that may affect asthma prevalence, self-reported health symptoms and perceived IAQ. Method: The study was organised as a case-crossover study of the heating and non-heating season, and employed a multi-disciplinary methodology, including direct-reading instrumental sampling, passive sampling for long-term measurements, and determination of microbiological contaminants with molecular methods. The monitored data were matched with school and classroom characteristics, self-reported health symptoms and IAQ perception of 376 primary school students attending 15 classrooms with standardised questionnaires. The integrated database was analysed with Bayesian multilevel models that provide a concordance between theoretical approaches and statistical analysis, while taking into account the hierarchy of the data. Results: Indoor CO2 levels and estimated ventilation rates were a reliable predictor for some outcomes, such as indoor temperature, Particulate Matter (PM) and Volatile Organic Compounds (VOCs) levels. Overall evidence from this study suggests that limiting CO2 levels below 1000 ppm (which is lower than current guideline values of BB101 performance standard in England (DfE, 2014)) is necessary in order to achieve indoor PM levels in classrooms below WHO 2010 annual guideline values, after removing indoor furnishing acting as dust reservoirs. A strong relationship between indoor temperatures and Total VOCs (TVOCs) levels emerged, and the predictive models estimated that after removing indoor TVOCs sources, keeping indoor temperatures below 26 °C, and preferably below 22 °C depending on season, may keep indoor TVOCs levels below 250ppb. Based on the self-reported satisfaction with IAQ at baseline and follow-up period, it was found that keeping indoor temperatures below 26 °C and CO2 levels below 1000ppm, may additionally reduce predicted percentage of dissatisfaction with IAQ below 30%. The air was perceived as less acceptable with increasing indoor temperature and CO2 levels, stressing the importance for an integrated approach for the simultaneous provision of thermal comfort and IAQ. However, indoor CO2 levels were a poor predictor of traffic related pollutants, such as indoor NO2 levels, which were significantly associated with the high asthma prevalence reported in this study (OR: 1.11, 95% CI: 1.04-1.19). Exposure to traffic-related pollution levels was additionally associated with increased IAQ dissatisfaction, and higher prevalence and incidence of Sick Building Syndrome SBS symptoms. SBS describes a constellation of non-specific health symptoms including mucosal, dermal, respiratory and general, that have no clear aetiology and are attributable to exposure to a particular building environment. Recommendations for future research: The methodological framework used in this study could be potentially applied to large scale investigations enhancing our understanding of the factors affecting indoor pollution levels in educational settings. More research is necessary to validate the predictive model of satisfaction with IAQ in different climatic and geographical areas. Implications for policy: This study shows that complaints about poor air quality and health symptoms were related to deficiencies in the indoor school environment, and identified that management and operation of classrooms are key in creating healthy and comfortable school buildings. Greening programmes around school buildings, simple passive measures of the building envelope, altering ventilation strategies among seasons, and timely control of ventilation may improve perceived IAQ and alleviate SBS symptoms. Together with increasing average and background ventilation rates, elimination of indoor sources that impact IAQ is necessary.