For the first time in 30 years, the Environmental Protection Agency has proposed a new limit for lead concentrations in the air. The agency is under court order to complete a new rule by Sept. 1, because of a lawsuit brought by environmentalists. Air, however, is no longer the most common source of major exposure to lead, which can cause I.Q. loss, kidney damage and other serious health problems. In most places, water and lead paint are more troublesome sources.