The key word is accumulates. It accumulates in your body and the higher the level, the greater the chance of toxic side effects. You get a little exposure today and it doesn't seem to affect you. But you get more tomorrow and the next day and so on for years. Each exposure stays in your body and adds to the accumulated levels.
Nobody knows at what level of accumulation fluoride starts causing health problems.
Fluoride may cause or contribute to a host of health problems including weakening your immune system, structural damage to bone and muscle, genetic damage, cancer, and kidney and thyroid damage. It can cause nausea and vomiting,abdominal distress, diarrhea, stupor, weakness, tremors, convulsions, and even death. Other side effects of fluoride are skin eruptions, headaches, deactivates enzymes, eczema, increased thirst, decreased efficiency of white blood cells, acne-like eruptions around the mouth, disturbances of calcium metabolism, inhibition of DNA repair, delayed eruption of teeth, and calcified deposits on the aorta. Fluoride is used as a pesticide particularly for roaches, ants, and rats. In fact the U.S. is one of the most fluoridated countries but has one of the highest tooth decay rates in the world.