Air is all around us. It's invisible and most of the time we don't think much about it. But when the air is polluted, it's deadly. Even when it doesn't kill us, polluted air increases respiratory illnesses, strokes, and Alzheimers; it may even be making us dumb. Air pollution is behind the deaths of at least 4.5million people a year worldwide, the vast majority harmed by tiny particles of soot emitted by burning fossil fuels in cars and factories or by burning wood or coal for cooking. So what can we do? Ritula Shah talks to health and public policy experts about the risks posed by polluted air. How can we clean up our air to have healthier bodies and brains and build better communities?