Leaded fuel is a type of gasoline that contains tetraethyl lead, a compound added to improve engine performance by increasing octane ratings. This type of fuel was commonly used in vehicles and aircraft until the late 20th century, when its harmful effects on health and the environment became widely recognized. Due to the toxicity of lead, many countries phased out the use of leaded fuel in favor of unleaded alternatives. The transition has significantly reduced lead pollution, benefiting public health and the environment, particularly in urban areas where vehicle emissions are a major concern.