Air pollution is the presence of substances in the atmosphere that are harmful to humans and other living beings, or cause damage to the environment. Air pollution can be chemical, physical or biological. There are many different types of air pollutants, such as gases (including ozone, nitrogen oxides, sulfur dioxide, carbon monoxide, ammonia, and methane), particulates (such as soot), lead and biological molecules. Air pollution can cause diseases, allergies, and even death; it can also cause harm to animals and crops and damage the natural environment (for example, climate change, ozone depletion or habitat degradation) or built environment (for example, acid rain). Air pollution can occur naturally or be caused by human activities. Air pollution causes around 7 or 8 million deaths each year. It is a significant risk factor for a number of pollution-related diseases, including heart disease, stroke, chronic obstructive pulmonary disease (COPD), asthma and lung cancer. It is the fourth-largest risk factor overall for human health as 99% of people are exposed to harmful levels of air pollution. Outdoor particulate pollution (PM2.5) is the largest cause of death (4.7 million), followed by indoor air pollution (3.1 million) and ozone (0.5 million). The World Bank has estimated that welfare losses (premature deaths) and productivity losses (lost labour) caused by air pollution cost the world economy over $8 trillion per year. Air quality is closely related to the Earth's climate and ecosystems globally. Many of the contributors of local air pollution are also sources of greenhouse emission i.e., burning of fossil fuel. Many different technologies and strategies are available for reducing air pollution. National air quality laws have often been highly effective, notably the 1956 Clean Air Act in Britain and the US Clean Air Act, introduced in 1963. Some of these efforts have been successful at the international level, such as the Montreal Protocol, which reduced the release of harmful ozone-depleting chemicals, while others, such as international action on climate change, have been less successful. Source: Wikipedia (en)