Mainly emitted by diesel vehicles, nitrogen dioxide (NO2) is considered to be a very harmful gas for the environment. With emission levels stagnating, the Court of Justice of the European Union (CJEU) has set a limit on pollution in recent years. Indeed, any country exceeding this limit in a "systematic and persistent manner" will be fined more than €11 million. Today, several countries in Europe are being monitored. These include Italy, the United Kingdom, Hungary, Romania and Germany. The latter has decided to react by launching a universal ticket at 9 euros. Focus on this new measure, which marks a huge turning point in the world of public transport.