Despite 80 years of research, the toxicity of lead in children is still not fully understood. Therefore, exposures small enough to have been viewed as of little importance are now taken more seriously. In other words, we must consider the aggregate impact of multiple small lead sources in assessing health risk. For this mode of thinking to protect children, all forms of damage must be understood thoroughly, which they can't be. The effects of all sources of lead exposure must be understood, which they can't be.
Science has no way to calculate the cumulative effects of multiple exposures.