California has become the first US state to classify second-hand tobacco smoke as a toxic air pollutant.
The decision by the California Air Resources Board puts drifting smoke in the same category as diesel exhaust, and could lead to tougher regulation.
The agency said many scientific studies had linked passive smoking to a range of cancers and respiratory diseases.
California pioneered smoking bans in the workplace, and later in restaurants and bars.
John Froines, chairman of the Air Resources Board's Scientific Review Panel, said Thursday's ruling put "California way ahead".
The decision to declare second-hand smoke as a pollutant relied on a September report that found a sharply increased risk of breast cancer in young women exposed to it.
It also linked second-hand smoke to premature births, asthma, and numerous health problems in children.
The study found that about 16% of all Californians smoked, but that 56% of adults and 64% of adolescents were exposed to second-hand smoke.
Some health experts say the ultimate impact of California's decision to classify second-hand smoke as a toxin could reach beyond the US.
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