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Occupational limits are 0.05mg/m3.A note on those limits: The medical limit is zero, and that is not fear mongering but the result of ever more results over decades of research. In a little course I took from Tuft University about drinking water one of the four weeks was dedicated to the topic "lead". Two professors, one an engineer and one a medical professor taught the course. The medical professor made that point over several lectures.
Also, from personal experience of having been diagnosed with chronic heavy metal poisoning by lab data (university clinic, researcher specialist doctor) as well as by unexpectedly amazing success of chelation treatment using DMPS and DMSA, and continuing far beyond when excretion went way below "thresholds" I too can tell you from my own experience that effects are there for extremely small quantities, just as the often population-level and statistics based research shows. My "on/off" experience for years of using chelators let me have extremely uncommon experiences. First of all, this kind of stuff is almost never diagnosed (only pretty high doses of acute poisoning are easy to test for, what is stored in the body is inaccessible to tests unless you cut a piece off of organs to send to the lab - forget blood tests, they only show what's on the move outside cells right now). Second, when people get chelators they stop when only little is excreted. From my experience that is an error, treatment should continue until the patient says there are no effects (of the treatment) any more. What remains stored in the body comes out very very slowly. Chelators only have access to extracelluar space to begin with. Something that happens to (heavy metal) chronic poisoning patients quite often is that excretion levels at first remain pretty low, or they go down quickly - but after a few months of chelation the suddenly go up, and the body starts acting funny (it starts excreting on its own). Happened to me too, after half a year of linear decrease of after-chelation measurements it suddenly went up by a factor of three, and my body became very active, lots and lots of stuff happening. For example - and I felt that because the surrounding tissue was "active" for a few weeks - a nodule I had in an enlarged thyroid as well as the enlargement itself completely disappeared. The endocrinologist who just two years before had recommended surgery was left stumbling "I'm amazed" again and again - after checking the thyroid twice with ultrasound because he could not believe it. This phenomenon shows that the body may not excrete on its own when the burden is too high.
So for me the whole discussion is quite personal, but ten years ago I would have been like everybody else, I would not have taken this whole very abstract issue seriously at all.
That there is a limit is because it is not technically possible to get to zero when there are heavy metals, or for everybody to avoid it, so the government sets a limit as a compromise between cost and what is achievable.
By the way, I had my two Italian espresso portafilter machines tested for lead. The expensive one which had everything made of copper had no lead but elevated copper (which is no big deal, copper is essential and the body has pretty good transport mechanisms, see "ceruloplasmin"). The cheaper one with brass though exceeded the drinking water limit many times! I had tested it because I wanted to sell it. It was a Rancilio Sylvia, by the way.
What is also notable that several heavy metals at once may be far more (orders of magnitude) toxic than just one. There was a lethal dose study decades ago that tested lead and mercury individually. When they combined the two lethal dose dropped to a thousandth, meaning it was a thousand times more toxic. Mercury is in our environment, especially large predator fish, and in amalgam fillings for teeth. So if somebody who is exposed to "a little bit" of mercury is also exposed to lead you can forget those official thresholds even more.