THE ENVIRONMENTAL Protection Agency has built such a solid case for its new tighter limits on lead additives in gasoline that the only remaining question -- which EPA administrator Lee Thomas says he is still weighing -- is how soon leaded gasoline should be banned altogether.

Four years ago, Vice President George Bush's regulatory relief task force recommended that the EPA consider relaxing or rescinding restrictions on leaded gasoline. Since that time, however, agency policy makers and researchers have amassed evidence that the health costs to the public of leaded gasoline are so great -- and the benefits to vehicle users so slight if they exist at all -- that nothing but the practicalities of an orderly phaseout should stand in the way of eliminating lead from gasoline.

Lead has long been recognized as highly toxic to human beings. But only recently have scientists produced strong statistical and experimental evidence that lead from vehicle emissions can be absorbed into the body in sufficient quantities to cause serious health effects. Last summer, on the basis of studies showing that even minute amounts of lead can permanently reduce mental capacity in children, EPA proposed to cut lead in gasoline by more than 90 percent by next January. Now, armed with additional studies showing strong links between lead exposure and high blood pressure, EPA has stepped up the timetable for meeting that goal and is considering a total ban by 1988. EPA does not rest its case on its health findings alone, persuasive as they are. Its studies also show that increasing numbers of motorists have been illegally using leaded gasoline in newer cars, thus damaging the catalytic converters needed to reduce other automobile emissions. Leaded gasoline is slightly cheaper (2 to 3 cents a gallon less at the refiner level, though dealers magnify the savings to as much as 10 cents), and some motorists also believe that lead improves engine performance. But EPA has now demonstrated that most of these savings are offset by the need for more frequent replacements of engine oil and mufflers in cars using leaded gas.

EPA has even undermined the case for retaining some leaded gasoline for use in older vehicles. Lead additives were previously thought essential to prevent excessive valve-seal wear in heavy-duty trucks and in cars produced before the mid-'70s -- at least if they ran continuously at high speeds. But EPA has discovered that, as a practical matter, both the Pentagon and the U.S. Postal Service have been using unleaded gasoline in their extensive fleets without any indications of unusual valve wear.

A final thing that EPA has demonstrated -- a matter of no small importance -- is that its own sustained investment in good research and staff development pays off handsomely in sensible policy decisions.