There is no threshold for safe mercury levels, just like safe (approved) levels of cyanide. Outside the pharmaceutical industry, it's not allowed; period.
If you care to, check out this link and it's discussion of mercury in vaccines - it's not what you think.
http://www.skepticalraptor.com/skepticalraptorblog.php/scary-flu-vaccine-ingredients/ If you'd rather not click, the relevant part:
Flu vaccine ingredients – thiomersal
So let’s move on to the other highlighted flu vaccine ingredients, and that’s the old canard, dangerous thiomersal (or thimerosal in the USA–my education is 100% US based, but for some reason I’ve used the non-USA spelling for 30 years). Let’s make some points clear right now. This is NOT mercury in its elemental form, which you might remember from old-style medical thermometers. So there isn’t a pool of mercury in the vaccine vial. Moreover, thiomersal is ONLY used, at least in vaccines, in multi-use vials, these days, only the flu vaccine.
Thiomersal is a toxic compound, there is no denying that. But let’s get back to math. The toxicity of compounds is measured through an analysis called the dose-response relationship, which describes the change in effect on an organism caused by differing doses of a compound after a certain exposure time. Table salt is tasty and safe in small amounts, but could kill you if taken in huge amounts. The dose-response relationship provides a graph that mathematically establishes what amounts of a compound causes what effects. This would seem to be a logical, and easily understood concept, but for many individuals, a bad substance is always bad.
First of all, the half-life of thiomersal in blood is around 2.2 days. That might seem long, but it means half is gone in a couple of days, cleared out by the kidneys. It does not accumulate.
But the math is even more telling. This flu vaccine, given once a year, has a maximum dose of 25 micrograms of mercury (but not elemental mercury). According to an NIH database, the LD50, that is, the approximate dose at which 50% of organisms will die (in this case a mouse), is 98 mg/kg body weight, if delivered subcutaneously, generally how vaccines are delivered.
A 20 kg child would get 25 micrograms of non-elemental mercury (this is important, thimerosal does not equal mercury) in one injection once a year. The theoretical LD50 dose for that same child would be around 2000 mg of thiomersal, or about 80,000 times higher than the amount of thiomersal in one vaccine dose–if vaccines used in children actually had thiomersal, which it doesn’t.
So, you would have to inject your child 80,000 times a day, every day, to make it potentially toxic. And no, dose-response relationships are not linear. That doesn’t mean that there’s some tiny risk of death from even a small dose of thiomersal–there is actually NO risk. And again, since there’s no thiomersal in pediatric vaccines this argument is ridiculous.
But more than all that, we have solid scientific data that show us that thiomersal is totally unrelated to autism, and is completely safe in vaccines. This illogical removal of thiomersal from vaccines makes it nearly impossible to have multi-use vials, so every vaccine has to be in a single-use prefilled syringe, which has rapidly driven up the costs of vaccines. Wait. That’s more evidence that antivaccination lunatics are in the pockets of Big Pharma. They pushed to get rid of thiomersal to make more profits for Big Pharma. That was an awesome move on their part.