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I’ve been saying for a while that (a) the elevated lead levels in Flint were fairly moderate and probably didn’t cause a huge amount of damage, and (b) the water is now safe to drink. A reader wants me to put my money where my mouth is:

@kdrum How about backing up your statements with references? Where are the studies? Who are the scientists? — David Lewis (@climateguy) May 5, 2016

OK. The exact data I’d like to have doesn’t seem to be available, but I can provide a rough sense of the landscape. Between 2013 and 2015, the number of children in Flint with elevated blood lead levels (above 5 m/d) rose from 2.4 percent to 4.9 percent. If you plot this out, it suggests that the average increase in BLL was somewhere between 0.2 m/d and 1 m/d. Increases in BLL are approximately associated with a loss of one IQ point per m/d, so this corresponds to an average loss of perhaps half an IQ point. However, most studies are based on children with elevated BLLs throughout their childhood. The elevated blood levels in Flint only lasted for about 18 months, which suggests that even half an IQ point is probably high. It’s more like a quarter or a third of an IQ point. That’s not even measurable.

Now, this is cocktail-napkin stuff, and I’m not an expert. All I’m trying to do is give you a rough idea of the magnitude of the problem. Anyone who has better data and knows how to analyze it more rigorously is welcome to set me straight if I’ve made a mistake.

That said, it’s unlikely that I’m off by a lot. What happened in Flint was a horrible tragedy, but it’s unlikely to have a major cognitive impact on the city’s children. However, this is on average. It could have a major impact on individual children, and this is why parents should have their kids tested for lead exposure. This is doubly true in areas of Flint that are known to have had especially high water lead levels.

As for the question about drinking the water today, that’s easier to answer: thousands of residential tests confirm that lead levels in Flint’s water are below the EPA’s action level of 15 parts per billion. What’s more, blood testing confirms that elevated BLLs have returned to their 2013 levels. All of this is strong evidence that Flint water is now safe to use.