Long-term exposure to diesel exhaust probably triggers a wide range of respiratory illnesses and causes lung cancer, according to a study released yesterday by the U.S. EPA. Based on decades of research, the study found “persuasive” evidence that the diesel engines operating on highways, farms, and construction sites around the country are hazardous to human health. The same conclusions have been reached before by various world health organizations and California agencies, but still, the new study lends urgency and credibility to efforts by the EPA and others to improve emissions standards for large trucks and buses under the Clean Air Act. In May, a federal appeals court upheld a Clinton-era regulation that would cut particulate matter from the diesel-powered vehicles by 90 percent and nitrogen oxides by 95 percent beginning in 2007. The Bush administration has largely stood by tougher emissions standards as well, despite heavy opposition from truck manufacturers and fuel refiners.