The 100 most-polluted cities in the world are all in Asia. Severe air contamination persists across South Asia. In November 2024, residents of major Indian cities, including the capital, New Delhi, and parts of Pakistan were advised to remain indoors and schools and outdoor construction work were suspended as a result of smog events.
Such air pollution is common in the world's most-populated and built-up cities. And exposure to polluted air can happen anywhere: Whether a person is walking through a city full of factories, stuck in rush-hour traffic or in a rural area that relies on wood fires for heating.
Taking precautions to avoid poor-quality air is, however, for many people in the worst-affected parts of the world, easier said than done.
Smog is a portmanteau of "smoke" and "fog," which gives a first indication of how this dirty, chemical haze arises.
It forms when ground-level pollutants, such as ozone, particulate matter, sulfates, nitrates and other toxic chemicals combine with fog under sunlight.
Smog and pollution are dangerous because they are so easily inhaled.
Combustive processes — whether in an industrial factory, the engine of your car or the furnace of your home wood fire — release toxic gases into the atmosphere.
Often suspended within the smoke and gas are microscopic particles that result from complex chemical reactions between the substances that we burn.
Particulate matter is labelled by size. For........