The World Health Organisation recently declared that air pollution is the greatest environmental threat to health globally. What do cities and governments need to do to clean up their act? Also, we explore how Occam’s razor, a theory from a medieval theologist, has influenced science. And, could music be an effective way to communicate with extraterrestrials? Alok Jha hosts For full access to The Economist’s print, digital and audio editions subscribe at economist.com/podcastoffer and sign up for our new weekly science newsletter at economist.com/simplyscience. Terms and conditions for the book competition featured in this podcast are available at economist.com/podcast-contest. See acast.com/privacy for privacy and opt-out information.