In late November, some 40,000 politicians, delegates, scientists and environment experts will descend on Paris to discuss what is billed by many as humanity’s last hope of saving the planet from irreversible climate change.

As the largest diplomatic event ever held in France, the UN conference would seem the perfect opportunity for the city’s socialist government to put its own record on display.

Except that when it comes to the environment, Paris has been attracting the wrong sort of attention: for a fleeting moment in March, the city was the most polluted in the world.

Local government scrambled. As the Eiffel Tower disappeared behind a cloud of poisonous gas, authorities imposed a partial car ban and waived charges on the Métro. During a pollution peak last December, the air contained so many hazardous fine particles that inhaling it was comparable to passive smoking, a study found.

But just how polluted is the air in Paris on an average day? According to one app I downloaded recently, it is not good at all. It is 10.29am on a weekday morning and the app, Plume, tells me there is “high pollution”, which means that levels exceed the daily World Health Organisation recommendations.

London is also polluted. But on the day earlier this week when I looked, it registered just 37 points compared with 55 for Paris and I could still exercise outside in the UK capital without any problem. “Go for it,” Plume advises. By contrast, I should “take it easy” if I am thinking of going for a run in Paris. Over an entire year, the app says, the French capital averages 41 points on its open-ended index versus 31 for London.

The main benefit of Plume is not only that it cuts through typically baffling pollution data to provide readers with one clean number comparable across cities; it also uses Big Data and artificial intelligence to predict what local pollution levels will be in the next few hours — useful if you are able to choose when you exercise, for example.

“Pollution levels change much more than we think — and, while there is generally more traffic in the mornings, not every morning is the same,” Romain Lacombe tells me. The young French entrepreneur and mathematician created the app with a university friend. “If we can give people access to timely information, they can start to change their life.”

Pierre-Emmanuel Burg of Airparif, the association in charge of monitoring air quality in the Paris region, explains that there are many contributing factors to the capital’s pollution levels. One is the country’s love affair with diesel cars. Diesel, now in the firing line after an emissions scandal at Volkswagen, has long been a favourite with French motorists. Diesel engines are cheaper to run than petrol ones because they are more efficient and the fuel is less expensive in France. But they are also much more polluting, in particular older models.

A second source of pollution is residential heating. And a third comes from surrounding farmland, where the ammonia used in crop fertilisers forms a chemical reaction with the fumes from the city’s fleet of cars.

To be fair, it is not all doom and gloom. Mr Burg points out that pollution levels in Paris have improved consistently in the past 20 years or so. Some pollutants, such as lead, have all but disappeared. Meanwhile, the city’s government is growing more serious about tackling pollution. It has just banned the circulation of heavy goods vehicles registered before October 2001. Similar bans will follow next year for old cars, according to the Paris mayor’s office.

There is even a five-year, €150m plan to turn Paris into a world cycling capital. In keeping with the French spirit, subsidies will be available to encourage people to buy electric bicycles. In the next five years, the city’s authorities also aim to create 30 hectares of green space and to cover the equivalent of 100 hectares of the capital’s roofs and façades — a third of which will be dedicated to urban agriculture. Parisians will probably breathe a little easier as early as this Sunday, the city’s first car-free day.

But none of this is going to bring about overnight change, which means that Parisians will have to keep closer tabs on the data. Fancy a run tonight? Plume says 7pm is better than 8.