4.2 million people die prematurely each year from diseases caused by ambient air pollution (WHO 2018). Of these deaths, 38% to ischaemic heart disease, 20% to stroke, 18% to chronic obstructive pulmonary disease (COPD), 18% to acute lower respiratory infections and 6% to lung cancer. In fact, exposure, in particular long-term, to air pollution can cause a variety of adverse health outcomes. It increases the risk of respiratory infections, heart disease and lung cancer. Emerging evidence suggests that exposure to ambient air pollution may additionally cause cerebrovascular, neurodevelopmental and neuropsychiatric disorders, male and female reproduction and fertility, pregnancy and birth outcomes and diabetes type 2. More severe impacts affect people who are already ill, and children, the elderly and poor people are more susceptible.  The most health-harmful pollutants – closely associated with excessive premature mortality –  are fine PM2.5 particles that penetrate deep into lung passageways. Other air pollutants such as nitrogen dioxide, ozone, carbon monoxide and sulfur dioxide need to be monitored to prevent adverse health effects (see Air Quality Guidelines).