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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.
一天抽完一整包煙，一年這麼抽個一兩天，必然會讓人感覺不適，但也許還不足以致命。
Smoking even one cigarette every day for decades, though, might do so.
但如果每天抽一支，數十年如一日下去，就可能會讓你喪命了。
That is the difference between acute and chronic exposure, and it is a difference most people understand.
這就是急性暴露和慢性暴露的區別，大多數人都能理解這兩者的不同之處。
What they may not understand is that the same thing applies to air pollution.
但是同樣的情況放到空氣汙染上，他們可能就沒弄明白了。
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.
大多數城市的每日天氣預報裡，只有當空氣汙染達到能讓人當即感到不適的程度時，才會出現紅色的警示。
That makes sense, for it lets people such as asthmatics take appropriate action.
這些警示有一定的作用，可以供如哮喘病一類的患者因時制宜。
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.
但也會讓沒有戒備心的人誤以為，既然當地絕大多數時候空氣質量狀況指標為綠色，那麼他們呼吸的空氣基本上就是安全無害的。
This may well not be the case.
但事實很可能並非如此。
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.
比如在倫敦，國王學院的研究人員去年發表的一項研究稱，空氣汙染會使倫敦居民的壽命減少9至16個月。
To investigate the matter, The Economist crunched a year’s worth of data collected from May2015 onwards in 15 big cities.
為了探明究竟，《經濟學人》雜誌用電腦處理了來自15個大城市於2015年5月開始收集、為期一年的資料量。
They were gathered by Plume Labs, a firm based in Paris, which uses them to produce a commercial air-quality app.
這些資料由巴黎的Plume Lab公司收集，並用於製作一款商業化的空氣質量應用軟體。
The three pollutants of most concern in rich countries are nitrogen dioxide (NO2, a brownish gas emitted by car exhausts, and particularly by diesels), ozone (a tri-atomic 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).
在富裕國家，最引人擔憂的三種汙染物分別是二氧化氮 (NO2，汽車——特別是柴油汽車尾氣中的一種棕色氣體) 、臭氧 (分子由三個氧原子構成，對肺部有危害) 以及截面直徑小於2.5微米的煙塵顆粒 (體積微小，可以深入肺部)。
These pollutants can cause a variety of medical difficulties, including asthma, heart disease, lung cancer and stunted lung growth in children.
這些汙染物會造成一系列的醫療難題，包括哮喘、心臟病、肺癌和兒童肺部的發育不良。
Levels of NO2 in London and Paris are routinely higher than World Health Organization (WHO) guidelines about what constitutes a long-term hazard, known as the annual average limit—and that goes, too, for particulate matter.
倫敦和巴黎的二氧化氮含量通常高於世界衛生組織 (WHO) 的參考值，二氧化碳濃度長期高於這一參考值就會對人體造成傷害，這個參考值即年平均濃度限值——這一標準同樣適用於細微顆粒物。
In London, during daytime, the concentration of NO2 exceeded the WHO’s limit by 41%, on average, over the 12 months examined.
根據這一年的資料記錄，倫敦白天的二氧化氮濃度平均要超出世界衛生組織限值的41%.
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.
在巴黎，國家空氣指數顯示，每五天之中有四天的空氣質量都是 “良好” 或者 “優質”，但是我們的分析結果表明，全天之內三種汙染物無一超標的天數屈指可數。