MEXICO’S capital is not a great place to be an asthmatic. Its levels of ozone, a pollutant which can damage lung tissue and cause breathing difficulties, are the highest of any city in Mexico and well beyond the recommended limits of the World Health Organisation. They are worst in the first few months of the year, because of the heat and the absence of rain. But in 2016 they have been particularly high. Indeed, in March, Mexico City endured its first “ozone alert” in almost 14 years. This was followed by further alerts in early April and early May (these were triggered by lower ozone levels, because the regulations had changed). The government responded to the first alarm by introducing new driving rules for those in the wider metropolitan area. From April to June, every car must be kept off the road for one day of the working week and one Saturday a month. The later alerts saw 40% of cars and motorbikes removed from the roads, for up to three days.

The city is cleaner now than in it has been in the past. It was named the most polluted in the world by the United Nations in 1992. Progress has been made since then thanks to the installation of catalytic converters in cars, the removal of lead from fuel, the shifting of a refinery outside the city and a reduction in cars’ polluting impact. A scheme called Hoy No Circula (“Don’t drive today”) originally took all vehicles off the roads for a day a week, before adherence to the rules was made contingent on the car’s age, and then contingent merely on emissions. But, as the recent alerts show, Mexico City still has a problem and the government is struggling to come up with a solution.

There are several reasons for this. The biggest difficulty is the sheer number of cars: about 5.5m in a city of over 20m souls. And they do not move quickly. A recent index released by TomTom, a navigation specialist, named Mexico City the most congested city in the world. Idling cars add to the pollution problem. Under the usual rules of Hoy No Circula, cars are meant to have emissions below certain thresholds if they are to drive every day. But some scientists argue that an emissions-based scheme is open to corruption—unlike one based purely on the vehicle’s age—and that many cars on the roads have not actually passed the tests. Moreover, privately operated microbuses are responsible for over 50% of all the journeys taken in the city: most are over 20 years old, and they are egregious polluters. The city’s altitude, at around 2,200 metres, also causes difficulties. Ozone is created more readily at such elevation, and engines work less efficiently and spit out more pollutants. The fact that the city has mountains around it does not help either, as the polluted air cannot escape easily, especially during the windless days the city has experienced recently.

Since the city cannot change its location it has to focus on mitigating the other problems. How to encourage drivers and passengers out of their cars and microbuses, for example. Money has been invested in public transport: in subway extensions, a “bus rapid transit” system and a bike-sharing scheme. But more is needed at a time when government budgets are being cut. The regional body that deals with the capital’s environment has put an annual price of 40 billion pesos ($2.2 billion) on sorting out the city’s pollution problems—far beyond city or federal means. New bus services could certainly replace the most popular microbus lines. A stricter set of planning codes would hold up the city’s spread; this in turn would make the provision of transport easier and cars less necessary. And finding a way to price the cost of environmental damage into the use of cars is leading to calls for a congestion charge. In the meantime, the spluttering of both engines and residents will continue.