Special Report on Mexico: When the smog lifts to reveal a hidden beauty: A city which has air thick with petrol fumes is committed to the fight against pollution, writes Colin Harding

ONE Sunday last month the air in Mexico City was so clear that you felt you could reach out and touch the distant wooded hills that ring the valley. It's not supposed to be like that. More often than not the sky is overcast, the atmosphere thick with petrol fumes and particles that stick in the throat and sting the eyes. The mountains are usually hidden behind an impenetrable curtain of smog.

On 20 March, the air pollution reached crisis pitch, with the ozone levels measured at 360 in the city centre. A reading of more than 100 is regarded as 'unsatisfactory' by internationally accepted standards, and more than 200 is 'dangerous'. But that was at the driest time of the year, when the dreaded thermal inversion strikes: the hot, stagnant air hangs over the city like a malignant grey blanket, leaving no escape route for the 17 million people below.

It's not as though they're unaware of the problem. There is an entire department of the city government (DDF) dedicated to reducing air pollution, which has produced a stream of measures ranging from keeping cars off the roads one day a week to planting trees on the eroded hillsides. Air pollution bulletins are issued three times a day, and show that all the main contaminants except ozone - lead and sulphur dioxide, carbon monoxide and nitrogen oxide - are under control, according to Fernando Menendez Garza, technical secretary of the commision set up in January to co-ordinate policy on atmospheric pollution in the Valley of Mexico. 'I don't know any other city in the world that's managed that,' he said.

The walls of the city are plastered with posters and slogans, urging people to keep their cars off the road and take the metro, have their engines tuned, take care of trees and so on. There are several green pressure groups and even 'ecological taxis' in this acutely environmentally-aware city: green VW Beetles with catalytic converters.

The rest of the world has been so impressed with Mexico City's clean-up efforts that the mayor, Manuel Camacho Solis, received a special award at the Earth Summit in Rio last month. But the sad fact is that it is hard to find anybody who believes the DDF has made much impression on the pollution problem. Mexico City has about three million vehicles on its tangled roads (compared with six million in Tokyo), and about 30,000 factories, or 40 per cent of the country's manufacturing industry, are located in the Valley of Mexico. Whatever the government may be doing, the only solution perceived by those with the means is to get out for as long and as often as possible. Anybody with money has a weekend home south of the city, in the neighbouring states of Mexico or Morelos; the traffic jams on the motorway south on a Sunday night are horrendous.

And then, just when you feel you've had about enough of all the smog and scramble, there is a day like that Sunday, limpid and blue, giving an inkling - just an inkling - of what it must have been like before the internal combustion engine and the industrial revolution brought a messy end to Mexico City's state of grace. Coyoacan, a southern suburb, is the only place to be if you are stuck in Mexico City on a sunny Sunday. The beautiful main square is thronged with strolling families, eating ice cream and watching the street theatre shows put on for children, before a late lunch at one of the many restaurants. The place is packed, but the atmosphere is peaceful and relaxed. It boasts elegant cobbled streets lined with whitewashed colonial houses and a marvellous craft market.

Coyoacan and neighbouring San Angel are a marvellous, and very necessary oasis amid the urban clamour of Mexico City. That is how its inhabitants see it, too, but they are under siege. The Sanborns supermarket chain is building a store in Coyoacan's main square, and other developments could be on the way. For the first time neighbourhood action groups are being formed, in a belated realisation that there is a lot worth saving.

(Photograph omitted)