The Environmental Protection Agency on June 1 began to enforce a rule requiring refineries to cut on-road diesel fuel sulfur content to 15 parts per million, from 500 ppm. Construction trucks designed to run on ultra-low sulfur diesel (ULSD) go on sale later this year for federally mandated 2007 delivery. The next phase of EPA�s program takes effect in June 2007 when the sulfur limit for off-road diesel fuel, which currently isn�t regulated, is to be set at 500 ppm. In 2010, all diesel fuel must have no more than 15 ppm of sulfur. Because refiners need time to flush out contaminants, retailers are not required to sell ULSD until Oct. 15 this year. EPA says it expects a �smooth transition� and claims the rule will prevent 8,300 deaths and thousands of asthma attacks annually.