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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.
一天抽完一整包烟，一年这么抽个一两天，必然会让人感觉不适，但也许还不足以致命。
Smoking even one cigarette every day for decades, though, might do so.
但如果每天抽一支，数十年如一日下去，就可能会让你丧命了。
That is the difference between acute and chronic exposure, and it is a difference most people understand.
这就是急性暴露和慢性暴露的区别，大多数人都能理解这两者的不同之处。
What they may not understand is that the same thing applies to air pollution.
但是同样的情况放到空气污染上，他们可能就没弄明白了。
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.
大多数城市的每日天气预报里，只有当空气污染达到能让人当即感到不适的程度时，才会出现红色的警示。
That makes sense, for it lets people such as asthmatics take appropriate action.
这些警示有一定的作用，可以供如哮喘病一类的患者因时制宜。
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.
但也会让没有戒备心的人误以为，既然当地绝大多数时候空气质量状况指标为绿色，那么他们呼吸的空气基本上就是安全无害的。
This may well not be the case.
但事实很可能并非如此。
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.
比如在伦敦，国王学院的研究人员去年发表的一项研究称，空气污染会使伦敦居民的寿命减少9至16个月。
To investigate the matter, The Economist crunched a year’s worth of data collected from May2015 onwards in 15 big cities.
为了探明究竟，《经济学人》杂志用电脑处理了来自15个大城市于2015年5月开始收集、为期一年的数据量。
They were gathered by Plume Labs, a firm based in Paris, which uses them to produce a commercial air-quality app.
这些数据由巴黎的Plume Lab公司收集，并用于制作一款商业化的空气质量应用软件。
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).
在富裕国家，最引人担忧的三种污染物分别是二氧化氮 (NO2，汽车——特别是柴油汽车尾气中的一种棕色气体) 、臭氧 (分子由三个氧原子构成，对肺部有危害) 以及截面直径小于2.5微米的烟尘颗粒 (体积微小，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.
这些污染物会造成一系列的医疗难题，包括哮喘、心脏病、肺癌和儿童肺部的发育不良。
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.
伦敦和巴黎的二氧化氮含量通常高于世界卫生组织 (WHO) 的参考值，二氧化碳浓度长期高于这一参考值就会对人体造成伤害，这个参考值即年平均浓度限值——这一标准同样适用于细微颗粒物。
In London, during daytime, the concentration of NO2 exceeded the WHO’s limit by 41%, on average, over the 12 months examined.
根据这一年的数据记录，伦敦白天的二氧化氮浓度平均要超出世界卫生组织限值的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，国家空气指数显示，每五天之中有四天的空气质量都是 “良好” 或者 “优质”，但是我们的分析结果表明，全天之内三种污染物无一超标的天数屈指可数。