Like lead, fluoride is a "cumulative poison." It builds up in your system over time with repeated exposure.
"Fluoride retention in bones increases as a person grows older."
Dr. John Yiamouyiannis, Fluoride the Aging Factor, Second Edition, Health Action Press, 1986, p. 45. "Fluorides accumulate in a linear fashion..." Barry Groves, Fluoride: Drinking Ourselves to Death?, Newleaf, 2001, p. 235.