Air pollution is also harming also harms our natural environment. It decreases the oxygen supply in our oceans, makes it harder for plants to grow, and contributes to climate change.Around 90 per cent of people go through their daily lives breathing harmful polluted air, which has been described by the United Nations as the most important health issue of our time. To mark the first International Day of Clean Air for blue skies, on 7 September, UN News explains how bad it is and what is being done to tackle it.