Photochemical smog, commonly observed in urban environments, is formed through a series of complex chemical reactions. It primarily involves nitrogen oxides (NOx), volatile organic compounds (VOCs), and atmospheric oxygen. These precursors react in the presence of sunlight, leading to the formation of ground-level ozone (O₃), peroxyacetyl nitrate (PAN), and other oxidants. These substances are irritants and contribute to respiratory problems.