Science and Technology: Air-pollution, Breathtaking(2)
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 NO2 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.
Offcial indices also fail to capture patterns of variation within a day.
These can be important—and people might be able to modify their behavior if they understood them.
Our analysis suggests, for example, that Parisians who head out for work at 9am and return at 6pm could reduce their average daily commuting intake of NO2 by 16% by travelling both ways an hour earlier.
Going two hours earlier would cut the intake by 28%.
Weekly cycles also exist.
Parents in Brussels and Paris might be wise to schedule their children's indoor activities, such as swimming lessons, on Saturdays and outdoor stuff like football practice on Sundays.
That is because, during daytime hours, the concentration of NO2 in those cities was, on average, about 20% lower on Sundays.
In Amsterdam it was 16% lower.
In all three places, fine-particle pollution also fell on Sundays, as did ozone in the summer months.
The best pollution advice of all to people in these cities, though, is: move to America.
In New York, levels of NO2 were 20% below the WHO limit, and that is pretty typical of places in the United States, where diesels are less common than in Europe.
As the inscription under the Statue of Liberty has it, "Give me your huddled masses, yearning to breathe free."