THERE MUST BE some people somewhere in the country who don't know what a skateboard is. But we bet most parents know all too well of these two-foot-long fiberglass boards with the polyurethane wheels on which presumably sane human beings propel themselves along sidewalks and thoroughfares at speeds approaching 50 miles an hour. In less than five years, what was once a California-based fad has attracted 20 million adherents and spawned a $500-million industry, national and international competitions and, in some countries, national skateboard teams. Most of us, however, have gauged the booming popularity of skateboards without resort to such facts. We see the evidence - youths of all ages on skateboards - on streets throughout the metropolitan area.

As you might expect, given the Evel Knievel derring-do with which many youths maneuver their skateboards, the number of skateboard-related injuries has also soared. The Consumer Product Safety Commission this month will release a report estimating that more than 375,000 such injuries will occur this year - twice the number reported in 1976. And most of these accidents will occur in public streets, with the added danger that means for vehicular and pedestrian traffic. Among all the others sports and recreational activities the commission studies, only bicycle-related mishaps occur more frequently (bicyclists outnumber skate boarders 4 to 1). Extrapolating from last year's figures, the CPSC estimates mean that at least 50,000 skateboarders this year will require some kind of hospital treatment; about 50 will die in skateboard-related accidents.

Federal officials, correctly we think, don't intend to use these statistics to make a case for federal regulation of the skateboard industry or of skateboard activity. They say their study conclusively shows that only a minuscule number of accidents are caused by defects in the product; most are due either to the skater himself or to the road surface he's riding on. What the officials hope to spur is increased public understanding that skateboarding has become not only an international sport (and a new mode of transportation for millions of youngsters), but also an enormous hazard to personal safety. That means that skateboarders, for maximum protection, should wear helmets, elbow pads, knee pads and athletic shoes and socks. Several police departments around the country, including that of Fairfax County, now incorporate such tips on skateboard safety into their public school safety programs.

Commission officials also are urging that communities consider positive ways of getting the skateboarders off the streets. Private and publicly build skateboard rinks are one alternative, though we would question whether public rinks should be a priority item in any municipally's budget. For many cities and towns a more feasible alternative would be to designate sections of existing parks for skateboard enthusiasts. Obviously, we can think of issues - non-proliferation of nuclear weapons, for one - more compelling than skateboard safety. But we also think that what's known about the hazards of skateboarding is a persuasive argument for greater attention to the skateboard-proliferation problem as well.