Mexico City's infamous smog problem is slowly improving, a government minister has claimed .

Click here to watch the full interview

Claudia Sheinbaum, the environment minister, says levels of ozone - often linked to the development of asthma - have fallen.

We've had some important successes in the past decade

Claudia Sheinbaum

But she acknowledged that general levels of pollutants in the air are still above international health standards.

She told the BBC's Hardtalk programme: "In Mexico City we've had some important successes in the past decade. In the past four or five years ozone pollution has been reduced.

"It doesn't mean that we are OK - 80-90% of the days we are still over the health standards. But in general terms ozone is reducing."

Deadly

Mexico City is one of the largest and most polluted capitals in the world. From November to May, a thick layer of smog hangs over the city.

Mexico City: Smog is a major problem

Recent figures suggested that tens of thousands of residents may be dying prematurely because of smog-related health problems.

In 2001, the Mexico City government announced a ten-year air improvement plan.

Measures included restrictions on emissions and lead and sulphur content in petrol, development of bus lanes, installation of computerised traffic lights and investment in better public transport.

Congestion

However, plans to build a double-decker motorway over the city's ring road have sparked controversy.

Critics say Mexico City's mayor, Andres Lopez Obrador, has put forward the $900m scheme to attract the vote of middle class car owners ahead of presidential elections in 2006.

New roads doesn't necessarily mean new private cars

Claudia Sheinbaum

Ms Sheinbaum denied the accusation, saying the road formed part of an integrated, long term strategy for Mexico City.

She added: "Models show us that we would have better mobility in the city if we were able to better develop the whole transportation policy.

"In the last 15 years, even though we haven't constructed any new roads, private transportation has increased at 6% per year.

"The construction of new roads doesn't necessarily mean new private cars."

The interview can be watched in full on Thursday 29 August on BBC World and BBC News 24 at the following times:

BBC News 24 (times shown in BST) 0430, repeated 2230

BBC World (times shown in GMT) 0330, repeated 0830, 1130, 1530, 1830, 2330