The authorities in Mexico City are taking action against pollution levels that have become quite literally breathtaking.

The city's health officials declared a general pollution alert when they found the ozone levels had reached 272 micrograms per cubic metre - almost triple the 100 micrograms that most industrialised countries consider to be the maximum acceptable.

Cars contribute to the problem

It is highly unusual for time of year. As some places experience wet and dry seasons, so Mexico City goes through its pollution season.

But now is the time when people normally feel they can breathe easy.

Meteorologists say an unusually clear dry spell has helped attract huge amounts of ozone into the Mexico City basin.

Buildings several blocks away are now just shadowy outlines.

But the authorities have acted: by declaring the alert, they triggered an all too familiar drill.

They have ordered hundreds of thousands of cars off the streets and told factories to work at 30% capacity to fight soaring pollution levels. Hundreds of thousands of cars are off the streets and factories have cut production.