Germany's Federal Administrative Court ruled on Tuesday that cities can ban diesel cars to lower air pollution. The verdict could affect millions of drivers in Germany and have significant repercussions for the country's car industry.

Here are five things to know about the case:

WHAT'S THE PROBLEM?

The air in many German cities is worse than the law allows, contributing to respiratory illnesses and thousands of deaths annually.

According to government figures some 6,000 people die prematurely each year in the country because of excessive levels of harmful nitrogen oxides, or NOx.

About 60 per cent of NOx comes from the transportation sector, especially diesel vehicles — an alternative to regular gas that's particularly popular in Europe.

Measurements show that the threshold of 40 micrograms of NOx per cubic meter is regularly surpassed in dozens of German cities including Munich, Stuttgart and Cologne.

IS THIS LINKED TO THE VW EMISSIONS SCANDAL?

Not directly.

Environmental campaigners have been demanding action on air pollution for years, but the revelations about carmakers cheating on diesel emissions tests has stoked the political debate in Germany.

Diesel, once considered a 'cleaner' fuel because it produces fewer greenhouse gas emissions, is now being scrutinised by environmentalists, government agencies, the media and the courts.

WHO BROUGHT THE CASE?

The group Environmental Action Germany, known by its acronym DUH, has been filing legal complaints against cities for years, seeking to have them impose measures to reduce air pollution.

Lower court rulings involving two cities — Stuttgart and Duesseldorf — concluded that driving bans for diesel vehicles would be effective and should be considered. Officials appealed the regional court rulings in Leipzig, arguing there needs to be a nationwide standard for dealing with air pollution.

WHAT HAPPENED ON TUESDAY?

The Leipzig court rejected the appeal, which now means cities are able to impose diesel bans. Cities would have several months to adjust their clean air plans accordingly. Some may choose to ban diesel cars only on certain roads at times when pollution levels are particularly high.

Meanwhile, the verdict could hit sales of diesel cars and the value of second-hand models, which have already been hit by the VW emissions cheating scandal.

Under pressure from the European Commission to act on air pollution, the German government has proposed several other solutions. These include making public transportation free and physically upgrading old vehicles to reduce their emissions. Both options would cost billions of euros and it's unclear who would pick up the tab — taxpayers, car owners or the auto industry.

HOW MANY VEHICLES WOULD BE AFFECTED?

According to official figures there are more than 15 million diesel cars registered in Germany. About 6 million of those meet the Euro 6 emissions standard that would likely be exempt from a driving ban, leaving some 9 million diesel cars affected by the verdict.

Additionally there are several million trucks, buses and other heavily utility vehicles than run on diesel and which could be hit by a ban. City officials are particularly worried about the impact on bus services and tradespeople, who may get special exemptions allowing them to drive diesel vehicles even when a ban is in force.