San Francisco is a beautiful City. Many of the apartment buildings were built before 1979, and you should assume lead-based paint is present in one of the layers of paint.
Lead dust forms when two painted surfaced rub – like a door and frame. Microscopic dust falls on the floor below this friction point of the door and frame. A toddler crawls through the dust. Then the toddler puts tiny hands into his/her mouth.
Then the child’s pediatrician does a blood test and finds elevated levels of lead.