Atmospheric pollution, or air pollution, is the introduction of harmful particulates, biological molecules or chemical molecules into the Earth's atmosphere. Air pollution occurs when certain gases and particles known as pollutants build up in the atmosphere to such levels that they can cause harm to our health and also the environment around us. Air pollution is caused by solid and liquid particles and certain gases that are suspended in the air. These particles and gases can come from car and truck exhaust, factories, dust, pollen, mould spores, volcanoes and wildfires. The solid and liquid particles suspended in our air are called aerosols. Outdoor air pollution involves exposures that take place outside of the built environment. Examples include fine particles produced by the burning of coal, noxious gases such as sulphur dioxide, nitrogen oxides and carbon monoxide; ground-level ozone and tobacco smoke. The harmful effects of the presence of greenhouse gasses in the atmosphere are global warming, acid rain, climate change, ozone depletion, sea-level rise, adverse effects on biodiversity etc.