Children get lead poisoning from many places. Old paint is the most common source, but lead also shows up in soil, water pipes, toys, and even certain spices. When a family sues a landlord or a manufacturer for a child’s lead exposure, they must prove where the lead came from. That sounds simple, but in practice it is often the hardest part of the case. This is the source problem, and it shapes every lead poisoning claim from start to finish.

To understand why, you need to know what a toxic tort claim requires. A plaintiff must show that the defendant exposed them to a harmful substance, that the exposure happened because of negligence or a defective product, and that the exposure caused the injury. For lead poisoning, the injury is usually brain damage, learning disabilities, or behavioral problems. But to link those injuries to a specific defendant, you need a chain of custody for the lead. You need to show that the lead in the child’s blood came from, say, the peeling windowsill in the apartment. That is rarely straightforward.

Lead is everywhere in older buildings. A child living in a house built before 1978 may have lead paint on the walls, doors, and baseboards. The soil outside might contain lead from decades of car exhaust when gasoline still had lead in it. The water pipes might be made of lead or have lead solder. If the family uses imported pottery or certain cosmetics, those can add even more lead. When a child tests positive for elevated blood lead, the doctor simply says “lead poisoning.“ The doctor does not say where it came from. That leaves the family’s attorney with a messy investigation.

In a lawsuit against a landlord, the basic claim is that the landlord knew or should have known about the lead paint hazard and failed to fix it. But the landlord will argue that the child could have gotten the lead from somewhere else. For example, the family might live near a busy road with contaminated soil. Or the mother might have used a lead-contaminated cosmetic during pregnancy. Or the child could have been exposed at a relative’s house where the family visited often. Each alternative is a crack in the plaintiff’s case. To win, the plaintiff must show that the landlord’s property was a substantial factor in the poisoning. This does not mean the property was the only exposure, but it must be a significant cause.

One powerful tool is a deteriorated paint inspection. An environmental specialist can take paint chip samples from the rental unit and compare them to dust wipes from floors and windowsills. If the lead levels in the dust match the lead levels in the paint, that creates a strong link. But what if the landlord painted over the old lead paint? Then the chips are sealed and no longer a hazard. What if the unit was recently renovated, but the construction spread lead dust from inside the walls? That is another route, but it requires proof of the renovation activities. Without such evidence, the link stays weak.

Timing is another issue. Lead exposure causes damage over months, not days. A child might live in one apartment for two years, but families often move. If the mother changes homes, the child could have been exposed to different lead sources at each place. Courts struggle with how to divide responsibility among multiple landlords. Some states use the “single substantial factor” test, where any landlord that contributed enough is potentially fully liable. Others require a precise allocation. This makes lead poisoning cases fact-intensive and expensive to litigate, which is why many lawyers only take the clearest ones.

There is also the question of what the landlord actually knew. Federal law requires disclosure of known lead hazards when renting. If the landlord handed over a pamphlet saying the property might contain lead, but did not inspect, they can claim ignorance of a specific peeling window. The family must show that the landlord had notice of deteriorating paint—maybe through a tenant complaint or a housing code violation. Without that notice, a negligence claim fails.

Then there are product liability claims against pigment manufacturers or paint companies. These are even harder because the lead paint on the walls was applied decades ago. To sue a maker of lead pigment, the family must prove that the specific company’s pigment was in that specific paint. Since lead pigment was a commodity sold to many paint companies, this is often impossible. Some courts allow “market share liability,“ where each pigment maker is responsible based on its share of the market, but not all states accept that.

In the end, the source problem dominates lead poisoning litigation. Many valid cases settle or fall apart because of it. Families must hire experts, run dust tests, and track every place the child spent time. It is not enough to show that lead is dangerous; you must show that this defendant’s lead got into this child. That is the hard truth. This is why experienced attorneys say lead poisoning cases are won or lost in the investigation, not in the courtroom. The science of lead’s harm is settled. The evidence of a specific source is stubborn.

So for anyone considering a lead exposure claim, the first step is not calling a lawyer. It is preserving evidence: keep paint chips, collect dust samples, and record the address and condition of every home the child lived in or visited. Because without a clear source, there is no case.