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A further problem is that setting day-to-day limits is a local matter.
另一个问题是，每日的平均浓度限值是由当地定制的。
So, not only do they rarely take long-term risk into account, they also vary from place to place.
所以，这些标准不仅很少考虑长期风险，而且每个地区的标准都不尽相同。
In Britain’s index a concentration of NO2 up to five times the WHO’s annual average limit counts as “low”.
根据英国的空气质量指数，二氧化氮浓度高达世界卫生组织年平均浓度限值的5倍时才能算作 “差”。
America is more conservative.
美国相对谨慎些。
It draws the line at two-and-a half times the WHO limit.
他们把该值定在WHO限值的2.5倍。
Worse, in some cases there is no pretence of objectivity.
更糟的情况是，部分地区连客观现实都懒得粉饰了。
The website of Belgium’s BelATMO index, for example, warns that this is “a qualitative representation” of air quality that “has little scientific meaning”.
例如，比利时的BelATMO空气质量指数网站上，就有警告说世界卫生组织的年平均浓度限值仅仅是空气质量的一种 “定性描述”，而且 “科学意义不大”。
Cities also vary in the way they present pollution data.
每个城市呈现空气污染数据的形式也各不相同。
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.
大多数城市采用了十分制或百分制，将其划分成四到六个层次，分别代表着低质量、中等质量等等。
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.
在一些地方，以空气污染程度能否对人体健康造成迅疾危害来区分空气质量“低”和“中等”，等到几乎所有人都受到雾霾严重影响的时候才显示红色警告。
Others divide the scale into equal chunks, each representing the same additional daily risk of dying or being admitted to hospital because of pollution.
另外一些地区把空气污染等级均匀划分，每增加一级，空气污染造成的日死亡风险或者住院率相同增长。
Offcial indices also fail to capture patterns of variation within a day.
官方的空气质量指数也未能表现出一天之内的空气质量变化模式。
These can be important—and people might be able to modify their behavior if they understood them.
而这一变化模式十分重要，如果人们能明白这种模式，他们可能就会据此调整自己的生活习惯。
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.
举个例子，我们的分析表明，在巴黎，上午9点出门上班并且下午6点下班回家的人，如果往返都提前一个小时，那么他们日常通勤时吸入的二氧化氮可以减少16%。
Going two hours earlier would cut the intake by 28%.
如果提前两个小时，他们吸入的二氧化氮则会相比减少28%。
Weekly cycles also exist.
每周空气质量变化也有规律可循。
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.
在布鲁塞尔和巴黎，明智的家长会把孩子游泳课程这样的户内活动安排在周六，把踢足球训练之类的户外活动安排在周日。
That is because, during daytime hours, the concentration of NO2 in those cities was, on average, about 20% lower on Sundays.
这是因为在这两个城市的白天时段，平均下来周日的二氧化氮浓度要低大约20%，
In Amsterdam it was 16% lower.
在阿姆斯特丹则会低16%。
In all three places, fine-particle pollution also fell on Sundays, as did ozone in the summer months.
在这三个地方，周日的细微颗粒物污染也更少，就好像在夏季臭氧浓度也会减少一样。
The best pollution advice of all to people in these cities, though, is: move to America.
然而关于空气污染这一问题，给这三座城市的居民最好的建议是：搬到美国去。
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.
在纽约，二氧化氮浓度比世界卫生组织的标准低20%，这是美国的一典型，因为这里的柴油机比欧洲少得多。
As the inscription under the Statue of Liberty has it, “Give me your huddled masses, yearning to breathe free.”
就像自由女神底下的铭文说的那样：“让你那挤成一团的人民过来这里，他们渴望自由呼吸。”
翻译：木头
