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A growing number of apps, such as London Air and AIRNow, are available for metropolitan residents worried about air pollution. Most air-quality apps rely on official daily indices. These are typically designed to alert their users to the immediate effect of various pollutants, which for most people are significant only when pollution is extremely high. In Britain’s index, for example, the concentration of nitrogen dioxide above which pollution is classified as "moderate" is set at a level where it becomes, according to the WHO, "a toxic gas which causes significant inflammation of the airways".
   The resulting dichotomy between daily and long-term health advisories can be huge, according to an analysis of a year’s worth of data. In London, for example, the daily index says air pollution is low on about six out of seven days. But daytime levels of nitrogen dioxide exceed the WHO’s one-year limit almost 80% of the time. Similarly, France’s index tells Parisians that the quality of the air they breathe is "good" or "very good" on about four out of five days — whereas the levels of nitrogen dioxide, particulate matter or both are above that annualised limit during 82% of daytime hours. In fact, the harm from chronic exposure to amounts of pollution that are typical for many rich-world cities can include asthma, heart disease and lung cancer. In children it can lead to stunted growth of the lungs, heart and brain.