SMOKING a whole packet of cigarettes in a day once or twice a year would certainly make someone feel ill, but probably would not kill him. Smoking even one cigarette every day for decades, though, might do so. That is the difference between acute and chronic exposure, and it is a difference most people understand. What they may not understand is that the same thing applies to air pollution. On a day-to-day basis, the forecasts most cities offer turn red only when pollution levels rise to a point where they will cause immediate discomfort. That makes sense, for it lets people such as asthmatics take appropriate action. But it might also lead the unwary to assume, if most days in the place he inhabits are green, that the air he is breathing is basically safe. This may well not be the case. In London, for example, a study published last year by researchers at King’s College suggested air pollution shortens the city’s inhabitants’ lives by nine to 16 months.

To investigate the matter, The Economist crunched a year’s worth of data collected from May 2015 onwards in 15 big cities. They were gathered by Plume Labs, a firm based in Paris, which uses them to produce a commercial air-quality app. The three pollutants of most concern in rich countries are nitrogen dioxide (NO 2 , a brownish gas emitted by car exhausts, and particularly by diesels), ozone (a triatomic form of oxygen that irritates lungs) and soot-particles smaller than 2.5 microns across (which makes them tiny enough to get deep into the lungs). These pollutants can cause a variety of medical difficulties, including asthma, heart disease, lung cancer and stunted lung growth in children.

As the chart shows, levels of NO 2 in London and Paris are routinely higher than World Health Organisation (WHO) guidelines about what constitutes a long-term hazard, known as the annual average limit—and that goes, too, for particulate matter. In London, during daytime, the concentration of NO 2 exceeded the WHO’s limit by 41%, on average, over the 12 months examined. In Paris, where the national index said air quality was “good” or “very good” four days out of five, our analysis found that at least one of the three main pollutants exceeded the WHO’s limit at some point almost every day.

A further problem is that setting day-to-day limits is a local matter. So, not only do they rarely take long-term risk into account, they also vary from place to place. In Britain’s index a concentration of NO 2 up to five times the WHO’s annual average limit counts as “low”. America is more conservative. It draws the line at two-and-a-half times the WHO limit. Worse, in some cases there is no pretence of objectivity. The website of Belgium’s BelATMO index, for example, warns that this is “a qualitative representation” of air quality that “has little scientific meaning”. Cities also vary in the way they present pollution data. Most do so on a scale of ten or 100, which is then segmented into four to six bands labelled low, moderate and so on. Some places draw the line between “low” and “moderate” at the level at which pollution starts to cause immediate health effects, reserving the red band for smog that severely affects most people. Others divide the scale into equal chunks, each representing the same additional daily risk of dying or being admitted to hospital because of pollution.