The legislative levels for lead come from several different sources. EPA, OSHA, HUD and so on. Those that concern children have mainly come from studies in urban low income areas in which children had eaten the dirt contaminated with lead. As of a few years ago (last time I did sampling for lead) there wasn't a firm guideline for inhalation exposure for workers - there are suggestive guidelines that you don't want to be over. As for surface contamination the most widely used guidelines are those set by HUD for a specified concentration per surface area - of course the intent here is to protect children, not adults who supposedly know what they're doing.
Incidentally, when you drive down the highway and see bridgework being done, many times the underside and sides of the bridge will be enclosed in plastic. You might think this is for worker protection but it's not. It was EPA mandated to protect the surrounding environment. Air concentrations of lead within that encapsulation were astronomical - too bad for the guys who have to work in there.
Essentially, if you don't work in a trade such as battery smelting or sandblasting painted surfaces which are fairly old then common sense should help. Don't eat lead contaminated dirt and wear a respirator if you're around heavily contaminated areas in which dust/fume may be present. And be sure to clean yourself up (change clothes, wash skin, etc...) when you leave the area to avoid taking the contaminant home.
Of course there's always room for the EPA to make some other arbitrary decision to decrease exposure levels of a substance without any scientific backing. They've already done this with DDT (currently over 3 billion additional deaths from malaria worldwide) and are well on the way with arsenic. Not to mention the absurdity of the proposed CalEPA guidelines in which a bakery can be fined for baking bread without a scrubber on the oven exhaust to get rid of that nasty "baked bread" smell.

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-livin' in the terminally flippant zone