Child lead poisoning prevention programs in the United States rely on blood lead screening to identify sources of exposure. Thus, screening may generate externalities beyond the benefits to the screened child. This paper studies the determinants of the demand for screening and quantifies these externalities by analyzing almost 1.5 million Illinois birth records and over 1.7 million geocoded lead tests performed in the state between 2001 and 2016. First, demand for lead screening increases with the availability of funding for remediation, suggesting that inability to pay for treatment might lead parents to avoid seeking a diagnosis. Second, I estimate a latent exposure score for all children in my sample and I derive the willingness to pay of households with differential risk by analyzing the relationship between screening rates and distance to health care providers. This calculation suggests that parents internalize about 10.6% of the risk. Third, an instrumental variable approach that exploits seasonality in blood lead levels estimates that for each case of elevated blood lead level detected, the likelihood of a subsequent case at the same location decreases by 6.4-8 percentage points. Back of the envelope calculations imply that detecting all likely cases of elevated blood lead levels through higher screening rates or better targeting could decrease subsequent rates of lead exposure by two percent. Moreover, shifting screening to the summer months when lead levels are higher could improve detection rates by as much as 50 percent and decrease exposure rates by four percent.