The California Air Resources Board today will consider a first-of-its-kind crackdown on something usually not thought of as an environmental threat: small, portable gas cans. The cans are so leak-prone and ubiquitous — there are an estimated 10 million in California — that they are responsible for as much smog-forming pollution as 1 million cars, according to state air officials. Statewide, gas cans spew 87 tons of hydrocarbons per day, eight times more than the Los Angeles region’s dozen oil refineries. The board is considering a measure that would require cans sold in the state to have spill-proof devices and be thick enough to avoid most evaporation. If approved, it would eliminate more tons of pollution than all but a few major anti-smog rules adopted in recent years. In the 1990s, the board has turned its attention to lesser-known sources of pollution, such as paint, car wax, charcoal lighter fluid, and nail polish remover.