经济学人下载:饮用水危机--数百万美国人都在喝含铅水(2)

A clutch of newish studies on the effects of lead exposure has helped. The hypothesis that lead damage to developing brains causes violence later in life is one of the great mysteries of social science—widely believed by those who plot the decline in violence against the decline in lead exposure and note how the two track each other; widely mistrusted by researchers who mutter about correlation and causation. Newer studies are more nuanced. One, by James Feigenbaum of Boston University and Christopher Muller of Berkeley, tried to control for other factors by comparing cities where the PH of the water supply was below seven, making it acidic, and causing lead to leach into water. The authors found acidic water tallied with more crime. 

一系列关于铅暴露影响的新研究起到了帮助作用。铅对大脑发育的损害会导致日后的暴力行为，这一假说是社会科学的一大谜团。有些人普遍相信这一假说，他们同时也将暴力行为的减少与铅暴露量的下降进行了对比，并指出两者是如何相互作用的；而那些对相关性和因果关系喋喋不休的研究人员则普遍怀疑这一假说。新的研究更加细致。波士顿大学的詹姆斯·费根鲍姆和伯克利的克里斯托弗•穆勒开展的一项研究试图控制其他因素，将PH值低于7、呈酸性、且含铅的城市供水进行对比。研究提出者们发现酸性水与更多的犯罪相吻合。

Some cities flush chemicals—orthophosphates— into pipes, to coat them to stop lead getting into the water. Milwaukee spends $400,000 a year to do so. That helps, but disturbances—such as when mains pipes are replaced but service lines to homes are not—can shake out particulates that remain in water. Karen Dettmer, superintendent of water works in Milwaukee, says events in Flint spurred her city to stop all repairs of lead lines. They also found, in 2017, ten schools fed by lead lines that were promptly replaced. Nurseries run from private homes remain exposed. 

一些城市将化学物质正磷酸盐冲入水管中，以覆盖管道，防止铅进入水中。密尔沃基每年为此花费40万美元。这是有帮助的，但是也会起到阻碍作用，水中残留的颗粒物会渗出，比如，当总水管被更换，而到家庭的服务线路没有被更换的时候。密尔沃基市水利工程负责人卡伦·德特默说，弗林特市发生的事件促使她所在的城市停止了对铅线的所有维修。他们还发现，在2017年，有10所学校很快就更换了供水的铅线。私人住宅开办的托儿所仍然面临着铅暴露。

Milwaukee is trying to replace 1,100 lead lines each year—hoping to emulate cities such as Lansing, Madison and Green Bay which have recently replaced all their pipes. Pittsburgh, Newark and other cities also plan to do so. But the cost of replacing one pipe averages $11,000 in Milwaukee (it was lower elsewhere). And with service lines mostly on private land, the job involves negotiations and cost-sharing with owners. Doing it all “will take about 70 years, that’s not fast”, she says. Much housing stock is decades old and pipes inside homes may also be a source of lead. 

密尔沃基正试图每年更换1100条铅线，希望效仿兰辛、麦迪逊和绿湾等城市，这些城市最近都更换了所有的水管。匹兹堡、纽瓦克和其他城市也计划进行水管更换。但在密尔沃基，更换一根水管的平均成本为1.1万美元（其他地方的成本更低）。由于服务线路大多在私人土地上，这项工作需要与业主协商和分担费用。她说，彻底完成“需要70年左右的时间，这并不快”。很多房屋都有几十年的历史了，房屋内的水管也可能是铅的来源。