Lead or lead dust is predominantly a problem when it is:

Lead or lead dust poses a significant health risk primarily when it is ingested. This is because ingestion can lead to direct absorption into the bloodstream and can result in serious health complications, especially in children whose developing bodies are more sensitive to lead exposure. Lead is a toxic metal that can affect almost every organ and system in your body, and when ingested, it can cause neurological issues, developmental delays, and other serious health problems.

While the other options—smelling, seeing, or touching lead or lead dust—might indicate its presence, they do not directly cause harm in the same way that ingestion does. Smelling lead dust is not typically a primary concern, as lead is not volatile and does not produce harmful vapors. Seeing lead dust may signal an area that requires remediation, but visual detection does not equate to a health risk. Touching it can pose a risk of transferring lead to food, hands, or other surfaces, but again, it is the ingestion of lead particles that most directly leads to health issues. Hence, ingestion is the most critical aspect of lead and lead dust hazards.