Approximately three billion people use traditional, inefficient and poorly vented indoor cookstoves to meet basic energy needs. Exposure to air pollutant emissions from these cookstoves, experienced primarily by the world's poorest people, accounts for 4.5% of the global burden of disease. This form of combustion is also the second leading source of light-absorbing carbon in the atmosphere. To address this problem, one focus has been to develop solid-fuel combustion technologies (i.e., improved or cleaner-burning stoves) that are more fuel efficient, affordable, and less polluting. The design and dissemination of cleaner stove technologies could have the single largest benefit to human and environmental health since the emergence of distributed water/sanitation systems in the previous century. The prevailing hypothesis is that any cleaner-burning stove (relative to traditional, open fires) will improve health; however, this hypothesis has not been rigorously tested. Improved stoves can vary widely in terms of pollutant emissions. This project, therefore, seeks to develop credible and representative laboratory data as a first step to test the improved stoves hypothesis and to improve our ability to address this massive threat to global health. We propose two specific aims. The first aim will develop a comprehensive profile of gaseous and particulate pollutants emitted from traditional and improved cookstoves. This aim will develop a more complete inventory of the toxic and climate-forcing pollutants emitted from residential cookstove combustion; these emissions are poorly understood and are likely different from well-characterized combustion sources (e.g., motor vehicles). This aim will provide more credible data for risk assessments and climate impact models, inform better stove design, and will aid our interpretation of health effects observed here (Aim 2) and in other studies. The emissions inventory will be made available online in an open-source format for use by researchers, policymakers, and other stakeholders. The second aim is to conduct a controlled human exposure study to investigate acute, subclinical effects of exposure to emissions from prevalent cookstove technologies. We will investigate markers of cardiorespiratory health (blood pressure, heart rate variability, augmentation index, exhaled nitric oxide, and markers of systemic inflammation, oxidative stress, coagulation, and platelet activation) in response to short-term exposures from five different stove types producing emissions that fall into various Tier categories promulgated by the International Standards Organization (ISO); several of the ISO Tier categories will result in air pollution concentrations that greatly exceed regulatory levels i the developed world. Thus, whether the introduction of improved stoves representing each of the Tiers within these newly-promulgated standards will produce meaningful health benefits is unclear. This is the first study to quantify the relationship between markers of cardiorespiratory health and exposure to a broad range of stove technologies. This research will inform future directions for stove programs while providing needed insight into the question, How clean is clean enough?