A court has ruled that the French state had failed to take sufficient action to curb air pollution in Paris, in what is being hailed as a landmark decision.
The case was brought by a mother and daughter who said their health had deteriorated when living near a busy ring road in the city.
But all their problems disappeared when they moved out, they say.
Air pollution is responsible for 48,000 premature deaths every year, according to the Public Health France agency.
The case, backed by NGOs, was the first brought by individuals against the French state over health problems caused by air pollution.
The mother, 52, and her daughter, 16, said the authorities did not take effective steps against atmospheric pollution, in particular during a bad period in December 2016.
"The state committed a fault by taking insufficient measures concerning the quality of air" when between 2012 and 2016 it failed to reduce the levels of certain polluting gases, the administrative court in Montreuil outside Paris said in a statement.
"For victims of pollution, this is a first," the plaintiffs' lawyer Francois Lafforgue told the AFP news agency.
"From now on, the state will have to take effective measures in the fight against pollution, and the victims can hope to obtain the recognition of their prejudice," he added.
However, the court rejected the mother and daughter's demand for €160,000 ($143,000) in damages, saying it could not find a direct link between their health problems and the state's failure to act.
The two said their respiratory problems - chronic bronchitis and asthma attacks - were worse during episodes of peak pollution. They left Paris to live in Orléans, in the centre of the country, and say their health has markedly improved.
Some 40 cases on the same issue have been brought in the cities of Lyon, Lille or Grenoble, and are awaiting judgement, reports say.
Paris has struggled for years to combat high levels of smog, and the authorities have introduced fines for any vehicle not carrying a "Crit'Air" emissions category sticker - part of a scheme to promote lower-emitting vehicles.
Paris police on Tuesday said only vehicles falling in the Crit'Air 0 to 2 category will be allowed in the city on Wednesday, as ozone levels are forecast to reach peak levels and the country finds itself in the grip of a potentially record-breaking heatwave.
Several routes in the capital now have restrictions on car use and a 3km (1.8-mile) stretch of the Right Bank of the River Seine has become pedestrianised.
NO₂ is a problem gas that is produced primarily by vehicle exhausts and industrial activity via the burning of fossil fuels.