Typically chronic lead ingestion - which is usually unintentional (e.g. ingesting paint chips, incorporated into other substances and unknowingly ingested)
Can also occur if significant amount ingested acutely
Risk Factors
Pica - consumption of lead containing soil, paint (related to OCD, iron deficiency, other)
Working with/around/stripping lead based paints
Folks or herbal remedies; Aryuvedic medicine
Food stored in lead lined containers
Lead bullets (retained fragments, chronic inhalation @ firing range)
Work: using lead based glazes, welding/cutting lead paint covered materials, glassblowing w/leaded glass, other
Sometimes mixed w/substances of abuse (e.g. heroin)