This is a reasonable question which has some very good answers.
The data is from a program that aimed to mitigate problems from high lead. The program's primary aim wasn't to allow papers to be published about their research goal many years later. You make the best use that you can of the data that you've got. And don't insist on the perfect data that you don't have.
But even if this had been designed as a pure research program, the design that you suggest would never be approved on ethical grounds. We accumulated enough data many years ago on the negative impact of lead that we cannot in good conscience withhold treatment for the sake of collecting better data. Review the history of the Tuskegee Study for an example of why such ethical guidelines were put in place.
The data is from a program that aimed to mitigate problems from high lead. The program's primary aim wasn't to allow papers to be published about their research goal many years later. You make the best use that you can of the data that you've got. And don't insist on the perfect data that you don't have.
But even if this had been designed as a pure research program, the design that you suggest would never be approved on ethical grounds. We accumulated enough data many years ago on the negative impact of lead that we cannot in good conscience withhold treatment for the sake of collecting better data. Review the history of the Tuskegee Study for an example of why such ethical guidelines were put in place.