According to the FDA, its 1ppm. However, they further contend that there is a safety factor of 10 built in to that figure.
Going further, The U.S. Environmental Protection Agency’s recommended reference dose, or level below which exposures are considered harmless, of mercury in the blood is 5.8 micrograms per liter.
Now coupled with say, an infant receiving 75 micrograms of mercury from three doses of DTaP, 75 micrograms from three doses of Hib, and 37.5 micrograms from three doses of hepatitis B vaccine - a total of 187.5 micrograms of mercury.
Considering that a baby has about a half liter of blood intheir bodies, I would say we were WELL beyond any responsible threshold of toxicity.