132 The world of the particles we inhale and how to control it

Abstract We know that nothing is more fundamental than the air we breathe. Deprived of it, we would die within minutes yet keeping it safe for humans remains an aspirational challenge. From ultrafine particles that form in the air through complex physicochemical processes involving gaseous precursors to large respiratory particles containing pathogens, the composition of the air impacts our health. Ambient particulate matter poses a significant risk, as quantified in numerous studies on disease burden and airborne transmission of infections. Particles in indoor and outdoor air come from diverse sources, and scientific knowledge about particle emission, transformation, transport, and impact is advanced, based on decades of research. However, in practice, much of this knowledge is not used to control particle sources or reduce exposure to them. We are not protected from infectious respiratory particles. We have not been able to derive exposure-response relationships for ultrafine particles to develop exposure guidelines, particularly regarding their impact on the brain. We do not control the emissions of volatile organic compounds from numerous consumer products, which interact in indoor air to form secondary particles that we inhale. Pieces are missing from the puzzle of science, technology, applications, and regulations that protect us from the risks of inhaled particles. How can we complete this puzzle?